Category Archives: systematics

June 14, 2017 · 12:33 pm

Do fossil exhibits have too many dinosaurs?

Reflexive discussion about the practice of communicating paleontological science to general audiences has become more common recently – there was even a two-day Popularizing Paleontology workshop in London last year. It’s about time – paleontology encompasses some of the most important questions about the world around us, from how life evolves to how ecosystems respond to planetary changes. Paleontology is the study of how the world came to be, and our understanding of the natural world is hopelessly incomplete without it. For the larger public, however, paleontology is synonymous with dinosaurs, and this can be a problem. Dinosaurs are awesome, but they are but one branch of the tree of life. And while their 160 million year dominance is significant, the era of non-avian dinosaurs is only a fraction of the 3.5 billion year history of life on Earth. Their story is not the only story worth telling.

Why the outsized fascination with dinosaurs? I suspect it’s the result of a self-perpetuating cycle. Human curiosity peaks somewhere between subjects an individual knows well and subjects that are completely new to them. In other words, people prefer to learn about things they are already familiar with. That means that museum visitors are drawn to dinosaurs like Tyrannosaurus and Apatosaurus because they already know something about them. Meanwhile, other fascinating creatures are bypassed precisely because visitors lack an existing mental framework to contextualize them. Somewhat paradoxically, in the sphere of informal learning, familiarity is king.

Generally, educators have been happy to indulge the public craving for dinosaurs*. In a must-read blog post resulting from the aforementioned Popularizing Paleontology workshop, Mark Witton describes dinosaurs as “one of the most important and potent tools at our disposal” because they are “gateways” to discussions about evolution, extinction, deep time, and even the nature of the scientific method. Witton then unpacks this conventional wisdom, highlighting several ways that relying on the built-in appeal of dinosaurs may not be as effective as traditionally assumed. It’s a fascinating discussion that I highly recommend reading.

Witton’s post got me thinking that if we’re going to consider easing up on dinosaurs in outreach efforts, we need some sort of baseline to firmly establish if (or the degree to which) they are being overused. One argumentum ad nauseum in these conversations is that museum exhibits are overstocked with dinosaurs. Allegedly, exhibit designers have responded to the popularity of Mesozoic dinosaurs by devoting an excessive amount of exhibit space to them, while relegating Paleozoic and Cenozoic specimens to the collections. This supposition can be (very, very crudely) tested by comparing the percentage of available exhibit space to the percentage of time non-avian dinosaurs dominated the planet. Assuming that exhibits should not be expected to allocate proportional space to pre-Phanerozoic life, I figure that the “Age of Dinosaurs” should cover 30-35% of an exhibit about life since the Cambrian (~160 million out of 541 million years).

To satisfy my own curiosity, I’ve gone and checked this figure against the three big paleontology exhibits with which I am most familiar. The slapdash maps below are traced from museum guides available online, with percentages calculated with the help of the Photoshop ruler tool. Green denotes dinosaurs, brown represents Cenozoic mammals, and blue encompasses everything else, including Paleozoic fossils, overviews of life over time, and non-dinosaurian Mesozoic life.

Field Museum of Natural History

Space allotment by subject in Evolving Planet at the Field Museum of Natural History. Dinosaurs: 31%; Mammals: 31%; Other: 38%.

Let’s start with the Field Museum, since it’s the most straightforward. The Evolving Planet exhibit (on view since 2006) occupies three elongated halls totaling 27,000 square feet. Evolving Planet is a classic “walk through time”-style exhibit, and the Paleozoic, Mesozoic, and Cenozoic are given remarkably equal amounts of floor space. Even though the central hall is larger than the other two, it is partially occupied by plants, marine animals, and early Triassic weirdos. At 31% of the total exhibit, dinosaurs are right about where they should be.

National Museum of Natural History

Space allotment by subject in the old fossil halls at the National Museum of Natural History. Dinosaurs: 15%; Mammals: 43%; Other 42%.

The old paleontology halls at the National Museum of Natural History (closed since 2014) demonstrate what happens when a museum goes without a dinosaur specialist for three quarters of a century. Cenozoic mammals and Paleozoic marine life were given room to spread out, while the dinosaurs were crowded into a paltry 15% of the available 31,000 square feet. It’s worth noting that unlike the Field Museum’s current fossil halls, which were designed from the ground up in the early 1990s, the NMNH paleontology wing was built up in a piecemeal fashion over the course of a century. The space was repeatedly carved into smaller sections to make room for new exhibits, and designers had to work around existing specimens that were too expensive or difficult to move. By the 1980s the halls had become something like a maze, and much of the available space wasn’t used very efficiently. Still, the consistently meager amount of space allotted to dinosaurs made it clear where the curators’ interests lay.

American Museum of Natural History

Space allotment by subject on the fourth floor of the American Museum of Natural History. Dinosaurs: 40%; Mammals: 30%; Other: 30%.

At the American Museum of Natural History, fossil exhibits are spread across six halls on the fourth floor. The last substantial renovation was completed in 1995, although a titanosaur skeleton was added to the Orientation Hall in 2016. This exhibit differs from its counterparts at FMNH and NMNH in that it’s arranged phylogenetically, rather than chronologically. It is also limited to vertebrate evolution, so plants and invertebrates are not included. With those caveats in mind, dinosaurs occupy 40% of the 65,000 square feet of exhibit space.

So, do museums have too many dinosaurs? Based on this exercise, these three museums have just the right amount (or even too few). The proportion of space allocated to dinosaurs closely matches the time span of their ecological dominance during the Phanerozoic. The percentage of dinosaur space at AMNH is on the high side, but if we also incorporated the square footage of the human evolution exhibit and the assortment of marine invertebrate fossils on display elsewhere in the museum, that percentage would decrease significantly. In fact, if this exercise has revealed anything, it’s that Cenozoic mammals get an awful lot of space, given that the “Age of Mammals” takes up only 13% of the Phanerozoic.

Again, this is an extremely crude way to measure dinosaur-themed engagement efforts. One might also look at the number of specimens on exhibit, or the newness of the displays (are dinosaurs getting updated more frequently, while other exhibits are left to languish?). And that’s to say nothing of outreach beyond the permanent exhibits. Still, I hope this is a helpful starting point. At the very least, it suggests to me that “are museums over-emphasizing dinosaurs?” is not the only question worth asking. We also need to tease out if audiences are ignoring non-dinosaur paleontology outreach efforts, and if there’s a way to counter that.

*It’s a tired but worthwhile point that comparatively few people can articulate what a dinosaur actually is. For many, anything big and dead (and displayed in skeletal form) is a dinosaur. This complicates the matter, because when people ask for dinosaurs they may actually mean prehistoric animals.

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Filed under AMNH, dinosaurs, exhibits, FMNH, mammals, museums, NMNH, systematics

December 21, 2016 · 9:30 am

Phylogenetics is Moon Man Talk

Phylogenetics is the study of the evolutionary history and relationships among extant and extinct organisms. More than any other organizational scheme, this is the way biologists think about the living world. In vertebrate paleontology in particular, an understanding of the evolutionary relationships of animals as identified via minute anatomical details is absolutely fundamental to our science. One might even argue that most new discoveries and inferences in this field are meaningless without some knowledge of the basic shape of the tree of life.

I’ve spent about eight years so far teaching science in museums, parks, and classrooms. And based on my anecdotal experience, most discussion of phylogeny comes across as incomprehensible babble to a plurality of people. For instance, one of the most commonly used definitions of “dinosaur” among paleontologists is “the most recent common ancestor of Triceratops and modern birds, and all it’s descendants” (there’s also the similar “most recent common ancestor of Megalosaurus and Iguanodon, and all it’s descendants”). This definition is not meaningful to most people. As evidence, I submit the following set of questions, all of which I have been asked by intelligent and well-meaning adults:

Did whales and dolphins evolve from marine reptiles?
Did giraffes evolve from sauropods?
Are [dromaeosaurs] related to cats?
Are dinosaurs related to sharks?
How can birds be dinosaurs if dinosaurs are reptiles?
Did the plant-eating dinosaurs evolve into mammals?
Are bats a kind of bird?
Are pterodactyls a kind of bird?

I don’t mean to ridicule or disparage people for asking these questions. Again, these all come from educated adults – museum and park visitors, undergraduate students, T.A.s, and at least one veterinarian! While these questions clearly show unfamiliarity with evolutionary relationships and how evolution works in general, they also show an effort to build a logical framework when none is available. For example, when a person asks if whales are descended from marine reptiles, he or she is hypothesizing that all large marine animals are related. This is incorrect, but it’s a sensible connection to make (and one that past naturalists have certainly explored).

For science communicators, this deficit of phylogenetic understanding is a serious problem which continuously undermines attempts to interpret zoology and paleontology. For example, think about how little meaning a statement like “Dimetrodon isn’t a dinosaur” has to somebody who can’t articulate what a mammal is or what a dinosaur is, much less the evolutionary distance between both groups. This is what we should expect from most of our audience, which means there is always a lot of catch-up work to do when explaining something as simple as the basic identity of a given organism. By the time you’ve satisfactorily defined “dinosaur” (good luck with that), explained the synapsid-diapsid split, discussed the tree of extinct stem-mammals, and positioned each of these things in deep time, you’re five minutes deep into a lecture when all you were asked was “what is it?”

USNM 8635, a handsome non-dinosaur. Photo by the author.

How can we solve this conundrum? The first step is to divide the issue into a number of smaller problems:

People don’t understand the fundamentals of how evolution works
People are unfamiliar with basic vertebrate classification
People lack knowledge of key evolutionary events through deep time
People don’t understand what traits are significant when assessing evolutionary relationships

The first problem is well known and has been discussed in-depth elsewhere (e.g. MacFadden et al. 2007, Spiegel et al. 2006, Spiegel et al. 2012), so I’m going to breeze over it and focus on the other three.

Basic Vertebrate Classification

It’s easy to toss out words like “mammal”, “reptile”, and “amphibian”, and take for granted that your audience will know what they mean. But even the most basic elements of vertebrate classification are specialized knowledge, and science communicators would do well to remember it. When I was teaching an undergraduate human anatomy course, I found that most of the class was familiar with the word “mammal”, and could name some examples. However, the students couldn’t articulate what sets mammals apart from other animals, and the relationship of mammals to other vertebrates within the tree of life was all new to them.

I think this is fairly typical, even among individuals with a background in biology. People are introduced to these categories in grade school, and you’d be hard-pressed to find somebody who couldn’t tell you whether (say) a cat is a mammal or a reptile. What is missing is what that actually means. We can’t assume that just because somebody knows a cat is a mammal, they know that fur and milk glands (much less auditory ossicles, a solid mandible, and heteromorphic teeth) are things to look for when categorizing mammals. They also may not know that “mammal” is an evolutionary group – that all the animals that fall under this banner are more closely related to each other than they are to anything else. No mammal is going to spontaneously become a bird or a fish. This is obvious to specialists, but not to most of our audience.

Evolutionary History Through Deep Time

The situation is further complicated by the element of time. Somebody may know that a modern cat and lizard differ in several fundamental ways, but do they know that both groups still evolved from a common ancestor? Or that said ancestor lived more than 300 million years ago? Unfortunately, much of the public would appear to lack any knowledge of how the past is related to the present. I’ve had visitors insist on calling fossil turtles “dinosaur turtles” and Teleoceras a “rhino-saur.” For them, extinct animals (all labeled “dinosaurs”) are a category all their own, wholly independent from the categories that describe modern animals.

For specialists, it’s obvious that modern animals exist within a continuum that extends into the deep past. It’s also obvious that groups like “mammals” and “reptiles” had starting points, and are embedded within larger, more ancient groups. None of this can be considered common knowledge, but it’s critical to any discussion about the identity or categorization of a given taxon.

Box diagrams are a simple and intuitive way to ground students’ understanding of the diversity of life.

How can educators hope to cover so much ground without confusing, distracting, or alienating their audiences? One option is to use a cladogram, or evolutionary tree. Trees are absolutely the most precise and accurate way to portray relationships over time, but as Torrens and Barahona demonstrate, they are regularly misinterpreted by the public. When I’m dealing with a general audience, I prefer box diagrams like the one above. Boxes within boxes show tiers of relatedness in a way that is more intuitive and easily understood than a tree. Box diagrams allow educators to cover a lot of unfamiliar ground quickly, and it’s easy to test visitors’ comprehension by asking them to point to where an example taxon should be placed. While this visualization of vertebrate relationships lacks a time axis, people can at least grasp the relative order in which each group evolved (fish before amphibians, amphibians before reptiles and mammals, etc).

How Scientists Discover Evolutionary Relationships

Going back to the list of misguided questions at the top of this post, we can generally surmise the thought process that led to each inquiry. The person who asked if whales and marine reptiles are related was classifying based on shared habitat. The person who asked if giraffes evolved from sauropods was classifying based on similar body shape. We can also see classifications based on diet, and based on shared activities, like flight or attacking prey with clawed feet. All these questions reflect a misunderstanding of what kinds of traits researchers look for when working out evolutionary relationships. So how do we quickly and clearly explain which traits are relevant, and which ones are not?

This is a tricky problem, and one I have not found a perfect solution to. The most important distinction is between plesiomorphic and apomorphic traits: plesiomorphic traits are inherited from an ancestral form, while apomorphic traits are novel developments. Put simply, working out a phylogenetic tree is all about grouping organisms based on shared apomorphies. The more apomorphic traits between two species, the more closely related they are. Once introduced, this is a fairly intuitive distinction. You don’t even need to use the jargon – “old traits” and “new traits” will often suffice. Going back to our problem of defining Dimetrodon, we can clarify that the lizardy shape and general toothiness are “old traits” – so they don’t tell us much about what the animal actually is. Instead, scientists look at “new traits”, like the number of postorbital fenestrae, to work out Dimetrodon‘s evolutionary affinities.

All of this is a long-winded way of saying that relating phylogeny to the public is challenging, but very important. Too often, science educators assume visitors have more background than they do, and the discussion comes across as so much moon man talk. Alternatively, educators push past complicated parts too quickly, which leads to confusion or misunderstanding. Ultimately, being a good educator comes down to two things: knowing your content and knowing your audience. Both are equally important, and both need to be practiced and refined in equal measure to ensure successful communication.

References

Macfadden, B.J., Dunckel, B.A., Ellis, S., Dierking, L.D., Abraham-Silver, L., Kisiel, J., and Koke, J. 2007. BioScience 57:10:875-882.

Spiegal, A.N., Evans, E.M., Gram, W., and Diamond, J. 2006. Museums and Social Issues 1:1:69-86.

Spiegel, A.N., Evans, E.M., Frazier, B., Hazel, A., Tare, M., Gram, W., and Diamond, J. 2012. Changing Museum Visitors’ Conceptions of Evolution. Evolution: Education and Outreach 5:1:43-61.

Torrens, E. and Barahona, A. 2012. Why are Some Evolutionary Trees in Natural History Museums Prone to Being Misinterpreted?” Evolution: Education and Outreach 1-25.

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Filed under education, opinion, science communication, systematics

Tagged as education, paleontology, science communication, scientific literacy, systematics

March 17, 2015 · 3:55 pm

Framing Fossil Exhibits: Phylogeny – An Addendum

After I posted my slightly critical evaluation of the AMNH fossil halls last month, a reader suggested I take a look at Next of Kin by Lowell Dingus. Dr. Dingus was the project director for the 1995 renovation, and his book chronicles the decade-long process of overhauling these genre-defining exhibits. It also includes plenty of gorgeous photos of the AMNH fossil exhibits past and present. Although out of print, Next of Kin can be found online for next to nothing. If you find anything on this blog interesting, I would call this book required reading. I cannot recommend it enough.

Edwin Colbert designed this version of the Jurassic exhibit in 1956. This space is now the Hall of Saurichian Dinosaurs. Photo from Dingus 1996.

Next of Kin is full of fascinating information about the renovation, and the history of the halls in general. For instance, it was news to me that the original plan in 1987 was to modernize only the two fossil mammal halls. When William Moynihan took over as Director of AMNH the following year, however, he asked in a planning meeting why the dinosaur exhibits weren’t being renovated, and soon the project expanded to include all six halls on the 4th floor. Apparently the approaches to interpretation, aesthetics, and layout that characterize the exhibits today were already fully formed. The concept of a main pathway with branching alcoves representing individual clades was in place, so the exhibit team only needed to set the starting point back a ways to include the dinosaurs and the rest of the vertebrate family tree. Restoring the historic interior architecture, obscured since the 1950s, was also an early priority. Dingus relates how he wanted to eliminate the “black box” look of the midcentury exhibits and let natural light back into the halls. In my opinion, the well-lit, airy aesthetic is one of the standout features of the AMNH fossil halls, and one other museums might do well to emulate.

Dingus also points out a number of clever design choices that I missed during my last visit to the museum. For instance, the primate section was deliberately placed in the center of the mammal hall, to avoid the implications of directed evolution and human superiority that once marked the AMNH exhibits. Another cool feature is the use of minimalist metal armatures to suggest the size and shape of animals for which only limited material is available. This is an artful way to convey the dimensions of these species without resorting to fabricating most of the skeleton. Again, this is something I’d love to see more of at other museums.

Minimalist armatures suggest the size and shape of incomplete specimens. Photo by the author.

Still, I was most interested in reading Dingus’s rationale for the design and layout of the AMNH fossil halls. In my previous post, I argued that the phylogenetic arrangement was a worthwhile experiment, but in practice it may not be the most practical way to make the history of life meaningful to the museum’s primary audience. More than any other organizational scheme, phylogeny is the way biologists think about the natural world, and I applaud the effort to encourage visitors to look at fossils the way scientists do. However, even the most basic elements of evolutionary classification are specialized knowledge, and require a daunting amount of up-front explanation (especially when targeting multiple age groups). I don’t think this integrates well with the multi-entrance, non-linear exhibit space at AMNH.

During the initial planning stages of the AMNH renovation, Dingus and other staff toured several large-scale paleontology exhibits in North America and Europe. Dingus clearly did not like what he saw, lamenting that “some institutions rely heavily on easy-to-understand, anecdotal labels and robotic recreations of dinosaurs that appeal to the lowest common denominator of visitor intellect.” He rejected the “prominent contemporary school of exhibit design that advocates only giving the visitor what he or she asks for,” feeling strongly that his institution could do better. Referring to the renovation as a “scientific crusade,” Dingus was inspired to challenge his audience in a way that peer institutions did not. Dingus and his colleagues wanted to show visitors the real science behind paleontological reconstructions. The phylogeny-based arrangement was central to that goal, emphasizing rigorous anatomical analysis and empiricism in a field historically characterized by idle speculation.

The orientation hall is in the oldest of the 4th floor exhibit spaces. Until the 1960s, this space was occupied by the Hall of the Age of Man. Photo from Dingus 1996.

I agree wholeheartedly with all of this. There was a period in the 80s and 90s (I think the worst is behind us) when the trend toward visitor-focused, educational exhibits got mixed up with a push to make museums more competitive with other leisure activities. Customer enjoyment was valued above all else, even if it meant sacrificing the informative content and access to real specimens that made museums worthwhile institutions in the first place. The resulting displays were filled with paltry nonsense like simulators, pointless computer terminals, and the aforementioned robot dinosaurs*. These exhibits imitated amusement parks, but with only a fraction of the budget they quickly fell into disrepair and technological obsolescence. Despite being museums’ most important and unique resources, curators and research staff found themselves increasingly divorced from their institutions’ public faces.

*Fine, I admit robot dinosaurs are cool. But I’d prefer that they weren’t in museums.

Under these circumstances, a backlash is quite understandable. Nevertheless, it is a common mistake (which I am by no means accusing Dingus of making!) that a visitor-centered exhibit is the same as a frivolous one. When educators push for audience-focused exhibits, they have the same goal as curators: to communicate as much content as possible. Audience-focused exhibits aren’t about dumbing down or eliminating content. They’re about presenting content in a way that effectively reaches the museum’s diverse audience. The AMNH fossil halls would work well for an informed adult visitor with ample time to inspect every specimen and read every label. But this is not the typical audience for natural history museums, and unless AMNH is a major outlier, it’s not the core audience for these exhibits. Most visitors come in mixed-aged groups. The trip to the museum is a social experience, and interactions occur among visitors as much as they occur between visitors and the exhibits. The best museums anticipate and meet the needs of these visitors in order to provide a quality learning experience.

An updated version of the classic (and classically misleading) horse evolution exhibit. Photo by the author.

It’s admittedly fun to share horror stories about dumb comments overheard in museums. Who in this field hasn’t rolled their eyes at the parent who makes up an answer to their child’s question, when the correct information is on the sign right in front of them? And yet, some of the blame for this failed educational encounter should fall on the museum. Why was that parent unable to spot the relevant information with a quick glace? Can we design signage so that the most important information is legible on the move, or from across the room? Can we correct commonly misunderstood concepts in intuitive ways?

As Dingus argues, it’s important to aim high in the amount of information we want to convey. There’s nothing worse than a condescending teacher. But a carefully-honed message in common language will always be more successful than a textbook on the wall. Happily, this is the way the wind is blowing these days. In a strong reversal of the situation a decade ago, curators now work closely with educators on the front lines to produce exhibits that are both accessible and intellectually challenging. It’s been 20 years since AMNH opened the latest version of its fossil exhibits…perhaps a new and even better iteration is already on its way!

Reference

Dingus, L. (1996). Next of Kin: Great Fossils at the American Museum of Natural History. New York, NY: Rizzoli International Publications, Inc.

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Filed under AMNH, education, exhibits, fossil mounts, mammals, museums, opinion, reviews, science communication, systematics

Tagged as amnh, exhibits, fossils, museums, paleontology, science education

February 26, 2015 · 11:51 am

Framing Fossil Exhibits: Phylogeny

This is the third part of an on-again, off-again series about organizational and interpretive approaches in large-scale paleontology exhibits (see the introduction and walk through time entries). This time, I’ll be discussing exhibits arranged according to phylogenetics – that is, the evolutionary relationships among living things. Natural history museums have displayed specimens according to their place on the tree of life since the days of Charles Wilson Peale, and more than any other organizational scheme, phylogeny is the way biologists think about the living world. Perhaps unsurprisingly, this arrangement was more common in the past, when exhibits were typically designed by and for experts. Examples of these old-school displays include the fossil mammal gallery at the Peabody Museum of Natural History and the paleontology halls at the University of Kansas Natural History Museum (neither has been thoroughly overhauled since the 1950s).

The jargon-heavy signage in the Peabody Museum’s classic fossil mammal exhibit is probably ignored by most visitors. Photo by the author.

Modern natural history museums rarely attempt phylogenetic exhibits. In vertebrate paleontology, an understanding of the evolutionary relationships of animals as identified via minute anatomical details is fundamental to our science. However, most people simply don’t think about the world in this way. For example, I was halfway through my first semester teaching an undergraduate anatomy course when I realized that most of the class didn’t really understand what a mammal is. The students were familiar with the word “mammal” and could provide some examples, but they couldn’t articulate what sets mammals apart from other animals, and the relationship of mammals to other vertebrates within the tree of life was all new to them. It’s easy to forget that even the most basic elements of evolutionary classification are specialized knowledge, even among biology students.

Describing the history of life on Earth chronologically is relatively easy—museum visitors intuitively understand the forward progression of time. But scientific classification (as opposed to colloquial categorization) requires a lot of explanation up front, and it’s easy to overwhelm an audience with jargon. While not impossible (see Neil Shubin’s masterful Your Inner Fish), it is very difficult to explain phylogeny to a general audience in a relatable and approachable way.

In 1995, the American Museum of Natural History attempted to do just that with the most recent renovation of its historic 4th floor fossil halls. This evolutionary arrangement was a major change for AMNH, since this space had a “walk through time” layout for most of the 20th century. In the accompanying book Discovering Dinosaurs in the American Museum of Natural History, curators Norell, Gaffney, and Dingus explain that phylogenetics (and the cladistic methodology in particular) is the only way to approach the study of prehistoric life in an objective way. Patterns of evolutionary relationships revealed by cladistic analyses are hard evidence in a field of study traditionally characterized by idle speculation. Norell and colleagues argue that the new exhibit arrangement shows visitors the credibility and scientific rigor behind modern paleontology.

Map of the fossil halls on the 4th floor of AMNH. Source

Communicating the rigorous and trustworthy nature of scientific conclusions is a worthy goal, and the choice to ground the AMNH exhibit in this way seems almost prophetic given the litany of speculation-heavy paleontology “documentaries” that have proliferated in the years since it opened. Scientific rigor is definitely a running theme here – sign after sign explains that popularly depicted dinosaur behaviors like parental care and pack-hunting are largely untestable speculation. To a degree, this label copy takes the fun out of an undeniably fun subject, but I can appreciate the effort to legitimize paleontological science in the public eye. Overall, the AMNH exhibits represent an attempt to train visitors to look at fossils the way scientists do, and the phylogenetic layout is central to that goal.

In the exhibit, visitors are meant to walk through a cladogram of chordates. You’ll pass through large halls dedicated to broad groups like saurischian dinosaurs and advanced mammals, while visiting smaller cul-de-sacs that represent narrower clades like ornithomimids and testudines. A central black path guides you through the evolution of life, and centrally-situated pillars along your route identify major evolutionary innovations, such as jaws or the ability to reproduce on land. The insanely comprehensive vertebrate fossil collections at AMNH make this institution uniquely capable of putting so much diversity on display (although non-tetrapods are woefully underrepresented). Meanwhile, an open floor plan allows you to spend as much or as little time in each area as you wish, and ample natural lighting goes a long way toward making it possible to study specimens in detail.

Pillars mark major evolutionary milestones in the Hall of Vertebrate Origins. Photo by the author.

The evolutionary pathway becomes considerably less obvious among the dinosaurs. Photo by the author.

Nevertheless, I agree with Riley Black that the AMNH fossil halls don’t do the best job communicating the story of vertebrate evolution to their core audience. The underlying purpose of any exhibit structure is to provide meaning and context for objects – to help visitors see them as more than neat things to look at. According to visitor surveys, the default mode of understanding for most people passing through a paleontology exhibit is what I’ve been calling “dinosaur pageantry.” After seeing the exhibit, most visitors will recall a list of cool skeletons they saw. A few might consider which ones are meat-eaters and which ones are plant-eaters, but without further prompting that’s all we can usually expect from non-specialists. It’s the museum’s job to give visitors the intellectual tools to contextualize those fossils in a more sophisticated way, but there’s a fine line to walk. Provide too little information and nobody learns anything, but provide too much and the content is ignored. Unfortunately, the AMNH exhibits fall into the “overkill” category.

As discussed, phylogeny is complicated, often counter-intuitive, and largely unfamiliar to many visitors. To overcome this, the AMNH designers rely on a fairly long orientation film, which introduces the concept of categorizing organisms based on shared derived characteristics. There are a few problems with this. First there’s the film itself, which dives right into the traits that characterize different groups – like the stirrup-shaped stapes of derived mammals and the temporal fenestrae of archosaurs – without explaining why these traits are significant. To a layperson, these probably seem like really inconsequential things to hang a whole group on. The video also presents a cladogram of vertebrates without explaining how to read it. As Torrens and Barahona demonstrate, interpreting a phylogenetic tree is a specialized skill that many natural history museum visitors lack. Second, I saw no incentive or instruction to actually start my visit to the 4th floor in the orientation hall. There are no less than four entrances to the fossil exhibits, so many visitors won’t know there is an orientation film (I sure didn’t) until they’re halfway through the galleries. Finally, there’s the reliance on media in general: do we really want visitors to spend even a portion of their time in an exhibit full of real fossils watching a video in a darkened room? Telling visitors what to think in a narrated video is easy, but it’s not nearly as meaningful as showing them the same concept with specimens (or better yet, coaxing them to reach conclusions themselves).

Hall of Saurischian Dinosaurs, American Museum of Natural History. Photo by the author.

Iconic mounts in the Hall of Saurischian Dinosaurs are iconic. Photo by the author.

Within the actual fossil halls, interpretation remains stubbornly unapproachable. For example, the sign introducing proboscidians tells visitors that this group is defined primarily by eye sockets located near the snout. An observant visitor might wonder why scientists rely on such an obscure detail, as opposed to the obvious trunks and tusks. There’s a good teaching moment there concerning why some characteristics might face more selection pressure (and thus change more radically) than others, but instead visitors are only offered esoteric statements. Relatedly, the exhibit does little to prioritize information. Most label text is quite small, and there’s a lot of it. Compare this to Evolving Planet at the Field Museum, where there is a clear hierarchy of headings and sub-headings. Visitors can read the main point of a display without even stopping, and parents can quickly find relevant information to answer their charges’ questions (rather than making something up).

Evolving Planet also compares favorably to the AMNH fossil halls in its informative aesthetics and spatial logic. At FMNH, walls and signs in each section are distinctly color-coded, making transitions obvious and intuitive. Likewise, consistent iconography – such as the mass extinction zones – helps visitors match recurring themes and topics throughout the exhibit. AMNH, in contrast, has a uniform glass and white-walled Apple Store aesthetic. It’s visually appealing, but doesn’t do much to help visitors navigate the space in a meaningful way.

Phylogenetic interpretations change quickly – Edentata is no longer considered a natural group. Photo by the author.

The phylogenetic layout introduces a number of other unique interpretive challenges. Since there is no temporal axis, it’s often unclear whether the lineage in a particular cul-de-sac cluster went extinct, continued on, or gave rise to another group elsewhere in the exhibit. Visitors that want to know which animals lived contemporaneously are out of luck. Meanwhile, the exhibit sometimes uses modern animal skeletons to fill out displays where fossil examples are limited, such as bats and primates. While these are labeled, the text is too small to be seen from a distance. The evolutionary organization is also burdened by the fact that phylogenetics is a fast-moving and often changing field of study. While the order of geologic time periods will never change, the 20 year-old displays at AMNH are already out of date in several details. For example, there is a cul-de-sac devoted to edentates, which is now considered polyphyletic, and a cladogram in the Hall of Saurischian Dinosaurs incorrectly places tyrannosaurids among the carnosaurs.

Glass architecture lets visitors see through displays and get a sense of what lies beyond. Photo by the author.

Neat comparison of mammal teeth. Too bad there's no obvious label.

This display is a great example of the diversity in mammal teeth, but it’s a confusing centerpiece for the Hall of Primitive Mammals. Photo by the author.

The AMNH fossil exhibits excel in many respects, chiefly in the amazing diversity and quantity of specimens on display. The exhibit throws a lot of good science at visitors, but falters in explaining why it matters. The point of all this is not to nit-pick the design choices at AMNH, but to reiterate that phylogenetically-arranged fossil exhibits are really hard to pull off. This is not the most intuitive way to introduce the history of life, or even the process of evolution. With so much background to cover, perhaps a more structured and linear layout would be better. In fact, a lot of my issues with the AMNH fossil exhibits seem to stem from a disconnect between the phylogenetic interpretive content and the wide-open aesthetics. Open exhibits can be great, but in this case it hinders the learning opportunities for self-guided groups of visitors. It’s difficult to imagine a typical visitor, arriving with their family or another mixed-age group, having the patience to make sense of it all. Regrettably, such visitors default to the dinosaur pageantry level of understanding, making all the work invested in creating a meaningful exhibit space for naught.

References

Norell, M, Gaffney, E, and Dingus, L. (1995). Discovering Dinosaurs in the American Museum of Natural History. New York, NY: Alfred A. Knopf, Inc.

Torrens, E. and Barahona, A. (2012). Why are Some Evolutionary Trees in Natural History Museums Prone to Being Misinterpreted? Evolution: Education and Outreach 1-25.

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Filed under AMNH, dinosaurs, exhibits, fish, FMNH, fossil mounts, mammals, museums, opinion, reptiles, reviews, systematics

Tagged as amnh, dinosaurs, evolution, exhibit reviews, exhibits, fossil mounts, fossils, mammals, museums, natural history museums, paleontology, science communication

October 30, 2014 · 10:39 am

Bully for Camarasaurus

Note: This post was written in 2014. It predates Emanuel Tschopp and colleagues’ landmark paper which, among other things, resurrected the genus Brontosaurus. I’ve attempted to update the taxonomy where appropriate, but it may still be a bit of a mess.

The story of the mismatched head of Brontosaurus is one of the best known tales from the history of paleontology. I think I first heard it while watching my tattered VHS copy of More Dinosaurs—scientists had mistakenly mounted the skull of Camarasaurus on an Apatosaurus skeleton, and the error went unnoticed for decades. The legend has been repeated countless times, perhaps because we revel in the idea that even experts can make silly mistakes. Nevertheless, I think it’s time we set the record straight: nobody ever mistakenly placed a Camarasaurus skull on Apatosaurus. The truth is a lot more nuanced—and a lot more interesting—than a simple case of mistaken identity.

Intrinsically related to the head-swap story is the replacement of “Brontosaurus” with “Apatosaurus” in the popular lexicon. This is well covered elsewhere, so I’ll be brief. Scientific names for animals are governed by the International Code of Zoological Nomenclature, which includes the principle of priority: if an organism has been given more than one name, the oldest published name is the correct one. Leading 19th century paleontologist O.C. Marsh named Apatosaurus ajax in 1877, based on a vertebral column discovered in the Morrison Formation of Colorado. Two years later, Marsh introduced Brontosaurus excelsus to the world, from a more complete specimen uncovered in rocks of the same age in Wyoming. Like many of Marsh’s publications, these descriptions were extremely brief, offering a scant two paragraphs for each taxon. However, Marsh did provide a longer description of Brontosaurus in 1883, complete with the first-ever restoration of the complete skeleton.

Come play with us, Brontosaurus…forever and ever and ever. Photo courtesy of the AMNH Research Library.

In 1903, Elmer Riggs of the Field Museum of Natural History underwent a survey of sauropod fossils held at various museums and concluded that Brontosaurus excelsus was too similar to Apatosaurus to merit its own genus. The name “Brontosaurus” was dropped, and the species became Apatosaurus excelsus for most of the 20th century. However, a substantial re-evaluation of diplodocoid sauropods by Emanuel Tschopp and colleagues in 2015 reversed Riggs’ decision. So the name Brontosaurus is back, but keep in mind that the species excelsus never actually went anywhere—it was just hidden under the Apatosaurus umbrella. Following Tschopp et al., Apatosaurus and Brontosaurus were distinct animals that lived in the same environment.

So how does the mismatched head fit into all of this? The short answer is that it doesn’t. The fact that some Apatosaurus mounts had incorrect heads for much of the 20th century has nothing to do with which name was being used at any given time, although the two issues have often been conflated in popular books. I suspect the two stories got mixed up because paleontologists were pushing to correct both misconceptions around the same time during the dinosaur renaissance.

Marsh’s second and definitive Brontosaurus reconstruction, first published in 1891.

Let’s go back to Marsh’s 1891 Brontosaurus reconstruction*, pictured above. The Brontosaurus type specimen did not include a head, and many have reported that Marsh used a Camarasaurus skull in this illustration. However, this would not have been possible, because the first complete Camarasaurus skull wasn’t discovered until 1899. What Marsh had instead was a few fragmentary bits of Camarasaurus cranial material, plus a snout and jaw (USNM 5730) now thought to be Brachiosaurus (more on this at SV-POW). Although these pieces were found far from the Brontosaurus quarry, Marsh extrapolated from them to create the best-guess skull that appears in his published reconstruction.

*Note that this is the second of two Brontosaurus reconstructions commissioned by Marsh. The first drawing, published in 1883, has somewhat different skull, but it still does not resemble Camarasaurus.

Although Stephen Gould states in his classic essay “Bully for Brontosaurus” that Marsh mounted the Brontosaurus holotype at the Yale Peabody Museum, Marsh never saw his most famous dinosaur assembled in three dimensions. In fact, Marsh strongly disliked the idea of mounting fossil skeletons, considering it a trivial endeavor of no benefit to science. Instead, it was Adam Hermann of the American Museum of Natural History, supervised by Henry Osborn, who built the original Brontosaurus/Apatosaurus mount (AMNH 460), six years after Marsh’s death in 1899.

Clockwise from top: AMNH sculpted skull (Source), Peabody Museum sculpted skull, real Apatosaurus skull (Source), and real Camarasaurus skull.

To create the mounted skeleton, Hermann combined fossil material from four separate individuals. All of the material had been collected by AMNH teams in Wyoming specifically for a display mount—and to beat Andrew Carnegie at building the first mounted sauropod. Like Marsh, however, they failed to find an associated skull (a Camarasaurus-like tooth was allegedly found near the primary specimen, but it has since been lost). Even today, sauropod skulls are notoriously rare, perhaps because they are quick to fall off and roll away during decomposition. Instead, Hermann was forced to make a stand-in skull in plaster. Osborn explained in an associated publication that this model skull was “largely conjectural and based on that of Morosaurus” (Morosaurus was a competing name for Camarasaurus that is no longer used).

Was it really, though? The sculpted skull is charmingly crude, so the overt differences between the model and a real Camarasaurus skull (top and bottom left in the image above) might be attributed to the simplicity of the model. Note that there isn’t even an open space between the upper and lower jaws! Still, Hermann’s model bears a striking resemblance to Marsh’s illustration in certain details, principally the elongate snout and the very large, ovoid orbit. It’s reasonable to assume that Hermann used Marsh’s speculative drawing as a reference, in addition to any actual Camarasaurus material that was available to him. At the very least, it is incorrect to say that AMNH staff mistakenly gave the mount a Camarasaurus skull, since Osborn openly states that it is a “conjectural” model.

A young Mark Norell leads the removal of the sculpted skull from the classic AMNH Apatosaurus. Source

In 1909, a team led by Earl Douglass of the Carnegie Museum of Natural History finally discovered a real Apatosaurus skull (third image, lower right). They were working at the eastern Utah quarry that is now Dinosaur National Monument, excavating the most complete Apatosaurus skeleton yet found (CM 3018). The skull in question (cataloged as CM 11162) was not connected to the skeleton, but Douglass had little doubt that they belonged together. Back at the Carnegie Museum, director William Holland all but confirmed this when he found that the skull fit neatly with the skeleton’s first cervical vertebra. As he wrote at the time, “this confirms…that Marsh’s Brontosaurus skull is a myth.”

The Carnegie team prepared and mounted the new Apatosaurus, and Holland initially planned to use the associated skull. However, when Osborn heard about this he threatened to ruin Holland’s career if he went through with it. You see, the new skull looked nothing like the round, pseudo-Camarasaurus model skull on the AMNH mount. Instead, it was flat and broad, like a more robust version of Diplodocus. Osborn wasn’t about to let Holland contradict his museum’s star attraction, and Holland backed down, never completing his planned publication on the true nature of Apatosaurus. Meanwhile, the mounted skeleton at the Carnegie Museum remained headless until Holland’s death in 1932. After that, museum staff quietly added a Camarasaurus-like skull. This was an important event, as it would be the first time an actual cast skull of Camarasaurus (as opposed to a freehand sculpture) would be attached to a mounted Apatosaurus skeleton. While I’ve had no luck determining precisely who was involved, Keith Parsons speculated that the decision was made primarily for aesthetic reasons.

Carnegie Museum Brontosaurus circa 1934. Source

Carnegie Museum Apatosaurus alongside the famed Diplodocus, sometime after 1934. Source

Elmer Riggs assembled a third Apatosaurus mount (FMNH P 25112) at the Field Museum in 1908. Riggs had recovered the articulated and nearly complete back end of the sauropod near Fruita, Colorado in 1901, but was unable to secure funding for further collecting trips to complete the mount. Riggs was forced to mount his half Apatosaurus as-is, and the absurd display stood teetering on its back legs for 50 years. Finally, Riggs’ successor Orville Gilpin acquired enough Apatosaurus fossils to complete the mount in 1958. As usual, no head was available, so Gilpin followed the Carnegie Museum’s lead and gave the mount a cast Camarasaurus skull.

The completed mount as it stood in the 1970s, Camarasaurus head and all.

Orville Gilpin finally completed the FMNH Apatosaurus in 1958.

The last classic apatosaurine mount was built at the Yale Peabody Museum of Natural History in 1931, using Marsh’s original Brontosaurus excelsus holotype (YPM 1980) and a lot of plaster padding. The skull this mount originally sported (third image, upper right) is undoubtedly the strangest of the lot. A plaster replica sculpted around a small portion of a real Camarasaurus mandible, this model doesn’t look like any known sauropod. The overall shape is much more elongated than either Camarasaurus or the AMNH model, and may have been inspired by Marsh’s hypothetical illustration. Other details, however, are completely new. The anteorbital fenestrae are thin horizontal slashes, rather than the wide openings in previous reconstructions, while the tiny, forward-leaning nares don’t look like any dinosaur skull—real or imaginary—I’ve ever seen. The sculptor is sadly unknown, but this model almost looks like a committee-assembled combination of the Marsh drawing, the AMNH model, and CM 11162 (a.k.a. the real Apatosaurus skull).

During the mid-20th century, vertebrate paleontology lapsed into a quiet period. Although the aging dinosaur displays at American museums remained popular with the public, these animals came to be perceived as evolutionary dead-ends, of little interest to the majority of scientists. The controversies surrounding old mounts were largely forgotten, even among specialists, and museum visitors saw no reason not to accept these reconstructions (museums are, after all, one of the most trusted sources of information around).

The Peabody Brontosaurus with its original head. Note that the Camarasaurus in the foreground also has a sculpted skull.

This changed with the onset of the dinosaur renaissance in the 1970s and 80s, which brought renewed energy to the discipline in the wake of new evidence that dinosaurs had been energetic and socially sophisticated animals. In the midst of this revolution, John McIntosh of Wesleyan University re-identified the real skull of Apatosaurus. Along with David Berman, McIntosh studied the archived notes of Marsh, Douglass, and Holland and tracked down the various specimens on which reconstructed skulls had been based. They determined that Marsh’s restoration of the Brontosaurus skull, long accepted as dogma, had in fact been almost entirely arbitrary. Following the trail of guesswork, misunderstandings, and scientific inertia, McIntosh and Berman proved that Holland had been right all along. The skull recovered at Dinosaur National Monument along with the Carnegie Apatosaurus was in fact the only legitimate skull ever found from an apatosaurine up to that point. In 1981, McIntosh himself replaced the head of the Peabody Museum Brontosaurus with a cast of the Carnegie skull. AMNH, the Field Museum, and the Carnegie Museum followed suit before the decade was out.

Remounted Apatosaurus at the Carnegie Museum. Photo by the author.

Given the small size of the historic community of dinosaur specialists, it may have been particularly vulnerable to the influences of a few charismatic individuals. To wit, Marsh’s speculative Brontosaurus skull was widely accepted despite a lack of compelling evidence, and Osborn was apparently able to bully Holland out of publishing a find that contradicted the mount at AMNH. What’s more, the legend of the mismatched Brontosaurus skull somehow became distorted by the idea that either Marsh or Osborn had accidentally given their reconstructions the head of Camarasaurus. This is marginally true at best, since both men actually oversaw the creation of composite reconstructions which only passingly resembled Camarasaurus. Nevertheless, the idea that the skull of Camarasaurus was a passable substitute for that of Apatosaurus was apparently well-established by the 1930s, when Carnegie staff hybridized the two sauropods for the first time. Even today, there are numerous conflicting versions of this story, and it is difficult to sort out which details are historically accurate and which are merely assumed.

I’d like to close by pointing out that while the head-swap story is often recounted as a scientific gaffe, it is really an example of science working as it should. Although it took a few decades, the mistakes of the past were overcome by sound evidence. Despite powerful social and political influences, evidence and reason eventually won out, demonstrating the self-corrective power of the scientific process.

References

Berman, D.S. and McIntosh, J.S. 1975. Description of the Palate and Lower Jaw of the Sauropod Dinosaur Diplodocus with Remarks on the Nature of the Skull of Apatosaurus. Journal of Paleontology 49:1:187-199.

Brinkman, P. 2006. Bully for Apatosaurus. Endeavour 30:4:126-130.

Gould, S.J. 1991. Bully for Brontosaurus: Reflections in Natural History. New York, NY: W.W. Norton and Company.

Osborn, H.F. 1905. Skull and Skeleton of the Sauropodous Dinosaurs, Morosaurus and Brontosaurus. Science 22:560:374-376.

Parsons, K.M. 1997. The Wrongheaded Dinosaur. Carnegie Magazine. November/December:38.

Tschopp, E., Mateus, O., and Benson, R.B.J. 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3:e857. https://doi.org/10.7717/peerj.857

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Filed under AMNH, CMNH, dinosaurs, field work, FMNH, fossil mounts, history of science, museums, reptiles, sauropods, systematics

Tagged as apatosaurus, brontosaurus, dinosaurs, Henry Osborn, museums, O.C. Marsh, paleontology, sauropod

March 14, 2014 · 4:27 pm

What’s the deal with Astrodon?

In Laurel, Maryland, a trail of banners depicting a herd of the sauropod dinosaur Astrodon johnstoni leads the way to Dinosaur Park, the site of a historically significant fossil deposit. At the Maryland Science Center in Baltimore, a life-sized Astrodon sculpture towers over the “Dinosaur Mysteries” exhibit. And since 1998, Astrodon has been the official state dinosaur of Maryland, joining other state symbols like the black-eyed susan and Baltimore oriole. In short, Astrodon is a sort of mascot for mid-Atlantic paleontology. Named in 1858 for fossils found in a Prince George’s County iron mine, the appeal of Astrodon for Marylanders is obvious: it’s a home-grown dinosaur in a region that is not widely recognized for its fossil resources, and the story of its discovery also calls attention to the state’s industrial heritage.

But what sort of animal was Astrodon, and how much do paleontologists truly know about it? Compared to many other extinct animals found around the world, the fossil record for Astrodon is and always has been fairly poor. The name Astrodon was first bestowed upon nothing more than isolated teeth, and although other fragmentary remains attributed to Astrodon have been uncovered over the past 150 years, reconstructions of the Maryland sauropod are mostly derived from the fossils of relatives found elsewhere. What’s more, the name Astrodon has a convoluted history, having been applied haphazardly to fossils found across the country and even around the world. For these reasons, some paleontologists would prefer that the name Astrodon not be used at all.

Lacking a scientific consensus on what sort of animal the Maryland sauropod was or even what it should be called, I find myself in a difficult position as an educator. How can the messy and contentious taxonomy of Astrodon be condensed into something teachable? Is simplifying or downplaying this controversy doing our audience a disservice, and to what degree?

The taxonomic history of Astrodon

The first scientifically recognized North American dinosaur fossils were found in the Mid-Atlantic region, a scant 17 years after dinosaurs were first recognized as a biological group in 1842. Joseph Leidy’s Hadrosaurus from the New Jersey coast is credited as the first American dinosaur to be described, but Astrodon was a close second. During the mid-19th century, iron mining was big business in central Maryland. Miners extracted large boulders of siderite, or iron ore, from open pit mines throughout Prince George’s County, and these miners were the first in the region to discover dinosaur bones and teeth. The siderite was being mined from clay deposits now known as the Arundel Formation, part of the larger Potomac Group that extends throughout Maryland (the Potomac Group was laid down during the Early Cretaceous period, between 125 and 113 million years ago). Members of the Maryland Academy of Sciences recognized the fossils from the Arundel clay as similar to the English fossil reptiles that Richard Owen had recently unified as Dinosauria. In 1858, academy member Christopher Johnston published a description of a set of teeth from the iron mines in the American Journal of Dental Science, which he named “Astrodon” (Joseph Leidy turned this informal name into a proper binomial, Astrodon johnstoni, in his 1865 review of North American fossil reptiles).

Today, most paleontologists consider it poor judgment to name a new taxon based only on teeth. When scientists describe a newly discovered organism, they designate a type specimen, which is used to define that taxon in perpetuity. But when the type specimen is especially fragmentary, or only consists of a small part of the organism, it poses a problem for future researchers. In the case of Astrodon, no newly discovered fossils other than teeth can be confidently referred to the same species. In 1858, however, paleontological norms were very different. All dinosaur fossils known at the time were exceedingly incomplete: scientists knew that dinosaurs were reptiles and that they were very big and not much else. Any new fossils, even teeth, represented a major addition to our understanding of the life appearance and diversity of these extinct animals. For modern paleontologists, Johnston’s published description of the Astrodon teeth is vague and uninformative, but in his day, these fossils were distinct from anything else yet known.

Astrodon teeth are on the lower left.

In December of 1887, famed paleontologist Othneil Charles Marsh sent his best fossil hunter, John Bell Hatcher, to search the area in Prince George’s County where Astrodon was discovered. Judging from Hatcher’s journal entries, he didn’t have a great time. It rained and snowed almost constantly, and on several days his team didn’t bother to show up for work. Although Hatcher managed to find numerous dinosaur, crocodile and turtle fossils, these finds did not match the quality of the fossils Hatcher had been finding in the western states, and no return trips were made. Nevertheless, Marsh saw fit to name two new dinosaur species from the material Hatcher collected: Pluerocoelus altus and Pluerocoelus nanus. Neither taxon was named for material that would be considered diagnostic if found today: P. altus was based on a tibia and fibula, while P. nanus was based on four nonadjacent vertebrae.

By this time, more complete dinosaur fossils from the American west were beginning to reveal a clearer picture of dinosaur diversity. Based on the shape and size of the fossils collected by Hatcher, Marsh determined that they belonged to sauropods, the group of long-necked herbivores that includes Diplodocus and Apatosaurus. More specifically, Marsh recognized that the Arundel sauropods were similar to “Morosaurus” (now called Camarasaurus) from Colorado. Today, the lineage of stocky, broad-nosed sauropods that includes Camarasaurus and its closest relatives are called macronarians. Unfortunately, by modern standards Marsh’s descriptions of P. altus and P. nanus are rudimentary in nature, and no distinguishing characteristics not common to all macronarian sauropods were offered.

Pleurocoelus elements. Image from NMNH Backyard Dinosaurs.

Pleurocoelus (or Astrodon?) fossils collected by Hatcher. Image from NMNH online exhibit Backyard Dinosaurs.

Contra Marsh, Hatcher suspected that there was only one sauropod in the Arundel Formation. P. altus and P. nanus were probably growth stages of one species, and the Astrodon teeth, now recognized as typical of macronarians, probably came from the same animal, as well. Since the International Code of Zoological Nomenclature decrees that the first published name given to a taxon has priority, Astrodon would take precedence over Pluerocoelus. Later, Charles Gilmore published a review of the Arundel fossils, in which he concurred with Hatcher that P. altus was a junior synonym of Astrodon, but retained P. nanus as a separate species.

Then things started getting really complicated. While paleontologists were still debating how many sauropod species existed in the Arundel clay, Marsh and others had started naming lots of new species of Pluerocoelus. Fossils found in Texas, Oklahoma and even the U.K. were all thrown into the Pluerocoelus bucket, including P. montanus, P. valdensis, P. becklesii and P. suffosus. For much of the 20th century, Pluerocoelus was a classic wastebasket taxon, into which any and all sauropod fossils from early Cretaceous strata were casually thrown. Since the Pluerocoelus type specimens designated by Marsh were insufficient to define the taxon based on morphology, the name became little more than a temporal marker. Adding to the confusion, researchers continued to disagree over whether all these new Pluerocoelus species should be sunk into the earlier genus Astrodon.

In recent years, some progress has been made toward untangling this mess of early Cretaceous sauropods. There is a general consensus that fossils not found in Maryland’s Potomac Group differ substantially from the Arundel sauropods and should never have been referred to Pluerocoelus or Astrodon. New names have been proposed for the midwestern sauropods, including Astrophocaudia and Paluxysaurus. However, removing the non-Maryland fossils from the discussion merely returns us to the original set of problems: how many sauropods are represented in the Arundel clay, what were they like in life, and what should we call them?

Creating a coherent picture of Astrodon

Unfortunately, the answers to these questions depend on who you ask. The most thorough review of Arundel sauropods from the last decade was published by Kenneth Carpenter and Virginia Tidwell in 2005. Carpenter and Tidwell reaffirmed Hatcher’s conclusion that Pluerocoelus is synonymous with Astrodon, and that as the earliest published name, Astrodon has priority. This decision is apparently based only on the fact that the fossils came from the same stratum, however, since the Astrodon holotype cannot be compared to anything besides other teeth. For this reason, Michael D’Emic proposed in 2012 that the names Astrodon and Pluerocoelus are nomen dubia and should both be dropped entirely. Ultimately, neither solution is practical for identifying the sauropod fossils that continue to be collected from the Arundel Formation. Either we blindly refer any and all sauropod fossils to Astrodon, even though we lack a usable holotype, or we have no label available at all. One solution would be to establish a new type specimen (called a neotype) for Astrodon, but this has yet to be done.

Both camarasaur and brachiosaur shaped Astrodon reconstructions are equally reasonable.

Both camarasaur and brachiosaur-shaped Astrodon reconstructions are reasonable. Artwork by Dmitry Bogdanov, via Wikipedia.

While many more sauropod fossils have been found in the Arundel clay since Hatcher’s 1887 expedition, we do not have enough material to fully elucidate what these animals looked like. Size estimates have varied enormously, from as little as 30 feet to as much as 80 feet in length. The assortment of fossil bones and teeth that have been found tell us we have a macronarian sauropod, and we can reconstruct its general shape based on more completely known relatives. However, macronarians were a fairly diverse bunch, ranging from the comparatively stocky camarasaurs to high-shouldered, elongate brachiosaurs. Carpenter and Tidwell describe the Arundel sauropod fossils, particularly the limb bones, as being fairly slender, but still more robust than those of Brachiosaurus. They do recognize, however, that nearly all known Arundel sauropod fossils come from juveniles, which may vary proportionally from adults. Because the precise affinities of Astrodon are unclear, artistic reconstructions vary substantially. The National Museum of Natural History’s Backyard Dinosaurs exhibit and website shows a camarasaur-shaped sauropod, while the life-sized sculpture at the Maryland Science Center is based on the brachiosaur Giraffatitan. At Dinosaur Park in Laurel, meanwhile, both versions are on display. More fossils, ideally cervical vertebrae or more complete adult material, are needed to clarify what the Arundel sauropod looked really like.

Teaching Astrodon

When I show people the teeth and partial bones attributed to Astrodon during public programs, I am almost always asked, “if that’s all you’ve found, how do you know what the whole animal looked like?” As demonstrated by this post, it takes 1,700 words and counting to give a proper answer, which is too much for all but the most dedicated audiences. Nevertheless, to do anything less is to skip crucial caveats and information. Scientists are choosy about the words they use, filling explanations with “probablys” and “almost certainlys”, but they do so with good reason: when one’s job is to create and communicate knowledge, there is no room for ambiguity about what is and is not known. It is therefore just a bit dishonest to say that a large sauropod called Astrodon that was related to Brachiosaurus lived in Maryland, and yet I do so every week. How can I possibly sleep at night?

I’ll admit it can be difficult, but I get by because using one proviso-free name for the Maryland sauropod seems to be informative and helpful to my audience. I only have people’s attention for so long, and I’d rather not spend that time on tangents about how Astrodon should really be called Pluerocoelus or why my use of either name is imprecise and problematic. I want visitors to walk away understanding how paleontologists assemble clues from sedimentary structures and anatomical comparisons to reconstruct ancient environments and their inhabitants. I’d like for visitors to practice making observations and drawing conclusions, and understand how paleontology is a meticulous science that can be relevant to their lives. “Paleontologists are weirdly obsessed with changing names” is not one of the most important things to know about paleontology.

Taxonomy, the science of naming and identifying living things, is unquestionably valuable. Biologists would be lost without the ability to differentiate among taxa. From my perspective, however, the public face of paleontology tends to overemphasize taxonomic debates in lieu of more informative discussions. There will always be somebody willing to argue whether Tarbosaurus bataar should be sunk into Tyrannosaurus, or to give incorrect explanations for why we lost “Brontosaurus.” In the end, though, these debates have more to do with people’s preferences than the actual biology of these animals. Astrodon may not be a diagnostic taxon in the strictest sense, but we need to call our fossils something, and taxonomic labels exist to be informative and useful. If asked, I’m always happy to provide the full story. But for the time being, Astrodon seems to be working just fine.

References

Carpenter, K. and Tidwell, V. 2005. Reassessment of the Early Cretaceous sauropod Astrodon johnstoni Leidy 1865 (Titanosauriformes). In Carpenter and Tidwell (eds.), Thunder-Lizards: The Sauropodomorph Dinosaurs. Bloomington, IA: Indiana University Press.

D’Emic M.D. 2012. Revision of the sauropod dinosaurs of the Lower Cretaceous Trinity Group, southern USA, with the description of a new genus. Journal of Systematic Paleontology, iFirst 2012, 1-20.

Gilmore, C.W. 1921. The fauna of the Arundel Formation of Maryland. Proceedings of the United States National Museum. 59: 581-594.

Kranz, P.M. Dinosaurs in Maryland. 1989. Published by Maryland Geological Survey, Department of Natural Resources, Educational Series No. 6.

Marsh, O.C. 1888. Notice of a New Genus of Sauropoda and Other New Dinosaurs from the Potomac Formation.

Please note that the usual disclaimer applies: views or opinions expressed here are mine, and do not reflect any institution with which I am affiliated.

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Filed under citizen science, Dinosaur Park, dinosaurs, field work, history of science, reviews, sauropods, systematics

Tagged as astrodon, dinosaur park, dinosaurs, education, fossils, pluerocoelus, science communication, taxonomy

January 1, 2013 · 4:00 pm

Beating the orthogenetic horse

According to the rad personalized 2012 review provided by WordPress, the top search engine terms leading people here over the last year were dinosours, horse evolutionary tree, horse evolution tree, horse phylogenetic tree and Daspletosaurus. It’s not too difficult to pick out the pattern there – horse evolution seems to be a major draw, even though I only mentioned it in a single post back in June. I aim to please, so I suppose a more detailed discussion of horse phylogeny is in order. First off, let me recommend Brian Switek’s thorough and thoughtful take on the subject. If you stick around here, you’re going to get more of a tirade.

Most depictions of horse evolution available online, including the one I posted a few months ago that is luring people to this site, are terrible. The typical linear presentation of horses progressively increasing in size from Eohippus to modern Equus, losing toes along the way, misrepresents not only what we know about horses as a group, but how evolution works in general.

This didn’t happen.

Evolution is, of course, neither linear nor progressive: it is primarily the result of populations adapting to thrive in their particular environments. As environments change over time species may evolve or go extinct, but there is no predetermined goal that lineages are reaching for. Modern Equus is not the most “highly evolved” horse – this is, in fact, a misleading if not meaningless concept, because a species’ success is dependent on its ability to thrive in that specific time and place. A modern horse is well adapted for grazing and running fast on open plains, but relocate one to the Eocene cloud forests where Eohippus thrived and it would do very badly.

Furthermore, it has been known for over a century that horses as a group did not consistently grow larger over time or otherwise become more Equus-like. Instead, horses diversified into a variety of forms over the group’s 55 million year existence, each group adapting to different environmental niches across the northern hemisphere. Large and small, forest-dwelling browsers and plains-dwelling grazers, these and all manner of other horses overlapped in time and space over the course of the Cenozoic. As J.W. Gidley of the American Museum of Natural History had worked out as early as 1907, horse evolution was not a linear progression but a tangled bush (just like the evolution of most other clades).

A modern horse phylogeny. From MacFadden 2005, via Laelaps.

So where did the orthogenetic depiction of horse evolution come from, and why is it still with us today? The answer highlights the importance of museum exhibits and specimen provenance in the public’s understanding of paleontology, with a dose of jealous personalities for good measure.

In 1859, Charles Darwin published On the Origin of Species, in which he articulated the process of evolution by natural selection virtually exactly as we understand it today. Darwin’s book incited a whirlwind of debate in both scientific and public circles because of its implication that the diversity of life could be attributed to natural forces, rather than an unknowable divine power. Within a decade, however, the vast majority of the scientific community was convinced by the soundness of Darwin’s theory, and to this day billions of individual observations of the natural world tell us that evolution is assuredly true.

One of the many lines of evidence covered in On the Origin of Species is the fossil record, with which we can trace the evolution and extinction of organisms over time, including the ancestors of modern life. However, Chapter 9 of Darwin’s book, “On the Imperfection of the Geological Record” (full text pdf) reads like like a lengthy apology for the incomplete nature of fossil preservation. Today, the use of organized, cladistic methodologies allow paleontologists to piece together detailed phylogenies from fossils, but in Darwin’s day, the evidence was patchier, and he opted to de-emphasise the fossil record’s usefulness to avoid such criticism. As Darwin put it, “we have no right to expect to find in our geological formations an infinite number of of those fine transitional forms.” Unfortunately for paleontology specialists, this led other biologists to believe that fossils could not make any independent contribution to the understanding of evolution. Largely shut out of the biggest biological discovery of all time, paleontologists became stewards of a “second-class discipline” (Sepkoski 2012, 9).

Paleontologists in the late 19th century.

Since biologists interested in evolution considered paleontology mostly irrelevant, late 19th-century paleontologists were left with three options. They could support evolution as best they could and accept that other biologists might not take notice, they could ignore theoretical discussion entirely and focus on purely descriptive studies of morphology, or they could be spiteful and seek alternatives to Darwinian evolution. The second course of action was the most popular well into the 20th century. E.D. Cope seems to be an example of the third approach, favoring an odd sort of neo-Lamarckism in his book The Origin of the Fittest. Such conceptions of directional change, such as Cope’s Law, are counter to evolution as proposed by Darwin and as understood today. However, a handful of paleontologists stuck with it and endeavored to provide meaningful fossil evidence for evolutionary theory.

Throughout the 1860’s, paleontologist O.C. Marsh amassed an impressive array of fossil horses from Wyoming and elsewhere in the American west. Horse fossils had been found in Europe much earlier, but Marsh’s horse collection was much more complete, and was probably the best fossil record compiled for any vertebrate group at the time. In 1870, the influential British naturalist Thomas “Darwin’s Bulldog” Huxley visited Marsh in New Haven and was suitably impressed: Marsh’s fossils ranged from the Eocene up until the Pleistocene, providing a clear picture of how the horse family had evolved over time. While Darwin had been hesitant to make too big a deal about the fossil record as evidence for evolution, the horse fossils were blatant examples of animals changing over time.

During the same visit, Huxley gave a lecture in New York in which he cited the horse fossils as a fantastic new line of evidence in support of evolution. Unfortunately, Huxley’s lecture (while admittedly aimed at a general audience) tread into some severely teleologic territory. As quoted in The Gilded Dinosaur (Jaffe 2000, 162), Huxley told his audience that “the horse is in many ways a most remarkable animal in as much as it presents us with an example of one the most perfect pieces of machinery in the animal kingdom.” He went on to explain how horse ancestors, from the little four-toed Hyracotherium in the Eocene to increasingly large horses like Merychippus and Pliohippus, gradually perfected the design of the modern horse. According to Huxley, over the course of the Cenozoic horses got bigger, faster, leggier, and generally better at being horses as we know them today. Problematically, this essentialist narrative rather misses the point of evolution as described by Darwin.

Marsh, like Huxley, was an early advocate of evolution, but his narrative of horse evolution was more on the mark. Marsh concluded that the smaller early horses with brachydont teeth were well suited for life in the rainforests that covered the western United States 50 million years ago. Horses like we know them today emerged as a direct result of the Earth getting cooler and drier over the course of the Cenozoic, and by the end of the Pleistocene the lineages of forest horses were completely extinct. Equus is with us today not because it is the best horse for any circumstance, but because it was most successful during the ice ages that shaped the modern flora and fauna (it also helped that humans figured out that horses are useful and ensured their survival through domestication).

Unfortunately, Marsh was never enthusiastic about public education, and so the progressive view of horse evolution was the one that made it into the public sphere. The history of horses remained a popular example of evidence for evolution, trotted out over the years by prominent biologists like George Simpson and Stephen Gould. Indeed, it was the first good evolutionary story known from fossils, although by no means the last or the best. In the earliest 1900s, Henry Osborn had a major role in solidifying the orthogenetic horse evolution story in the public eye when he curated the exhibit on the subject at the American Museum of Natural History. It is on the basic premise of this exhibit that the textbook, museum, and web descriptions of linear horse evolution that persist to this day are based.

The fossil horses of AMNH. Photo by the author.

After the modern biological synthesis, paleontologists realigned with the rest of biology, and the odd pseudo-evolutionary ideas that persisted in paleontological circles began to fall by the wayside. However, orthogenetic ideas remain common in natural history exhibits on horse evolution to this day (in about 62% of them, according to MacFadden et al. 2012). The reason these exhibits have stuck around isn’t entirely clear. MacFadden and colleagues suggest suggest a lack of inertia or funding for the renovation of exhibits is a factor, but they also point out that even some newer exhibits fall back on linear horse evolution.

The biggest problem is that orthogenetic evolution makes more intuitive sense to non-specialists. We often use the word “evolution” to imply improvement, so it would follow that horses should get bigger and better over time. This is an important misconception to overcome, because, as if we need a reminder, only 15% of Americans believe humans evolved from other animals via strictly natural processes, and an even smaller number can correctly articulate how evolution works. Evolution is the fundamental principle underlying everything we see in the natural world, and it is imperative that a correct understanding of how it works is the basis of any biology education. With the proper background, the real story of horse evolution is a great example of how changing climates effect organisms and ecosystems over time. This is helpful for interpreting the ever-important subject of climate change, but it won’t click until the linear horse evolution story is trampled out for good.

References

Jaffe, M. 2000. The Gilded Dinosaur: The Fossil War Between E.D. Cope and O.C. Marsh and the Rise of American Science. New York, NY: Three Rivers Press.

MacFadden, B.J., Oviedo, L.H., Seymour, G.M. and Ellis, S. 2012. “Fossil Horses, Orthogenesis and Communicating Evolution in Museums.” Evolution, Education and Outreach 5:29-37.

Sepkoski, D. 2012. Rereading the Fossil Record: The Growth of Paleobiology as an Evolutionary Discipline. Chicago, IL: University of Chicago Press.

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Filed under AMNH, history of science, mammals, museums, science communication, systematics

Tagged as Cope and Marsh, evolution, fossils, horse evolution, horses, paleontology, science communication, science education

June 20, 2012 · 9:41 am

Communicating Systematics, Part 2

In the previous post, I discussed how phylogenetic trees, while powerful and informative to trained eyes, can be misinterpreted by lay audiences. These misunderstandings are easy to diagnose, but actually finding solutions to the problem is challenging.

In a perfect world, every child would be introduced to evolutionary theory early and often in their obligatory science education, and everybody would be able to interpret phylogenetic trees the way scientists do. This is unlikely to happen anytime soon, especially in the United States, so educators are going to have to get creative. One option is to provide additional information to aid in the interpretation of the diagram. On the surface, adding more information is always an attractive prospect, but unfortunately it does not always work as intended. Attention spans are perilously short, and the goal of a visual representation should be to make the content immediately intuitive and easier to understand.

If conventional shapes and symbols in evolutionary trees are not getting the intended message across to target audiences, then perhaps we need to rethink how we are structuring these trees. I don’t have a catch-all solution, but the following might be enough to at least start a conversation.

Change the shape of the tree

Torrens and Barahona argue that many misinterpretations of trees stem from ideas of essentialism and teleology that are deeply ingrained in and continually reinforced by western culture. Likewise, equating up with good and down with bad is a recurring, internalized motif. Therefore, trees that illustrate evolution and diversification proceeding upward or to the right only encourage presuppositions of linear, goal-oriented evolution.

One solution that has been experimented with (at AMNH, for example) is to draw trees as circles (see below). This eliminates the problem of associating up with good and bad with down, or upward movement with progress. A circular “tree” has no orientation, and thus does not imply any taxa to be better than the rest. Personally, I find circle diagrams confusing to read, but I appreciate what they are intended to accomplish. A diagram of evolutionary relationships could theoretically take any shape, since the crucial information is in the branching order, not the nature of the lines.

A circular tree. From eplanetscience.com.

Be careful with representation of ancestors

Many phylogenetic diagrams place specific fossil taxa at nodes along the tree in order to illustrate the course of evolution. This is informative of general evolutionary trends, but it can also be confusing. As a case in point, I just did an image search for a horse evolution diagram to use as an example, and found that many of the top results came from creationist websites. These sites aren’t worth linking to (although they are easy enough to find), but they erroneously assume that fossil taxa are thought to be directly ancestral to modern Equus caballus. Evolutionary scientists think no such thing, but looking at the image below I can see how that conclusion could be reached.

This diagram of the evolutionary history of horses can lead to the mistaken assumption that earlier species are thought to be directly ancestral to later ones. That polytomy that leads to three unlabeled nodes doesn’t help either.

In a proper cladogram, taxa are only placed at the ends of branches. Direct ancestry is (almost) never inferred, because the scarcity of the fossil record prevents us from ever knowing exactly what evolved into what when. The cladogram below shows the relationships between the seven modern-day species of Equus. Systematists have determined a series of branching relationships based on anatomical and molecular data, and even provide a suggestion of when these divergences occurred, via the time scale. Each node represents a common ancestor that definitely existed, but we will probably never find or identify their fossils.

A cladogram of modern horse species. From Hooper Virtual Natural History Museum.

In this case, I would prefer if books or exhibits for popular audiences nixed images like the first one and instead went with cladograms that do not suggest specific ancestor-descendant relationships. Obviously the cladogram could be spiced up with colors and illustrations, but it is important to use a format that represents precisely what scientists do and do not know.

Always clarify orientation

Proboscidea phylogeny from academic.reed.edu.

Individuals well-versed in evolutionary science automatically read trees from the basal node out to the tips. Typically, and in the elephant phylogeny above, that would be from the bottom up. It can therefore come as a surprise (it certainly did for me) that non-specialists frequently attempt to read phylogenetic trees from left to right. Viewers may assume that the horizontal order of taxa across the top is significant, representing either the course of evolution or time. Neither would be correct, as Mammut on the far left and Mammuthus on the far right were roughly contemporaneous, and Loxodonta africana and Elephas maximus in the middle are the only extant elephants. Although it may not occur to specialists, it is a simple and necessary precaution to label the orientation of the tree and avoid such confusion.

Avoid calling anything “more evolved”

This is more of a nomenclature issue than a visual one, but poor graphics can exacerbate this misconception. All contemporary species, from sponges to frogs to humans, have been evolving for the same amount of time. An amphibian or reptile is not “primitive”; it is just as adapted to its environment as we are. Using this sort of terminology is attractive as a shortcut when referring to less-diverse sister groups to more-diverse clades, but it misrepresents the nature of evolution and should be discouraged.

References

MacDonald, Teresa E. “Communicating Phylogeny: Evolutionary Tree Diagrams in Museums.” 2010. Paper presented at the NARST (National Association for Research in Science Teaching).

Torrens, Erica and Barahona, Ana. “Why are Some Evolutionary Trees in Natural History Museums Prone to Being Misinterpreted?” 2012 Evolution: Education and Outreach 1-25.

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Filed under mammals, museums, reptiles, science communication, systematics

Tagged as cladistics, evolution, science communication, science education, systematics

June 8, 2012 · 9:55 am

Communicating Systematics

In case you forgot, only 15% of Americans polled by Gallup accept that human beings evolved from other animals through natural processes*. This statistic has not changed meaningfully since Gallup started asking this question in 1982. This fact should be in the back of the mind of every science educator, and for that matter, every scientist, each and every day we go to work. It is a scientifically well-established fact that all life has evolved over long periods of time, and that all forms of life are related to each other. This fact is fundamental to our understanding of life on Earth. The goal of both educators and scientists is to expand our knowledge and awareness of our world, and it is therefore disconcerting that so few people are willing (or have had to opportunity to) acknowledge the wealth of information that an understanding of evolution provides.

*A couple complaints about that link. First, the phrasing of the question, “human beings evolved over millions of years from less advanced forms of life” (emphasis mine) is poor, read on for reasons why. Second, belief that humans evolved “with God’s guidance” does not seem like a meaningful distinction to me, and does not suggest a proper understanding of evolution.

The overwhelming number of people who do not accept evolution is intimidating. The fact that our politicians and leaders are often among this number is even more troubling. It can be tempting to retreat into academia and whine about the problem to our peers, or perhaps ignore it entirely. However, 30 years of unchanging results on the Gallup poll indicate that the issue is not going to go away. Both educators and scientists need to take the offensive and directly address misconceptions and misunderstandings about evolution, as well as find effective means to mitigate them.

Phylogenetic Trees

In the world of science education, one of the trickiest issues is supplying appropriate context. Although all good science can be explained in clear, readily-understandable language, most research still requires some background on the Big Ideas in science. Two huge examples are evolution by natural selection and the scientific method, which I briefly discussed here and here. Without an understanding of how scientific ideas or generated or how evolution works, discussing the finer points of, say, feeding strategies of tyrannosaurs is quite pointless. Unfortunately, even among people who accept the fact that evolution is a real phenomenon, this background all too often does not exist.

Educators need to supply the public with the context they need to understand current science, and one good area to focus is the reading of phylogenetic trees. A phylogenetic tree is a branching diagram that depicts inferred evolutionary relationships among organisms. A tree implicitly shows that included organisms descended and diversified from a common ancestor. As such, phylogenetic trees are a visual embodiment of evolutionary theory, and provide an informative narrative of the history of life.

As is often the case, David Hone has already provided a wonderful explanation of how scientists construct trees and how to read them correctly, so I’ll just drop that link and move on. The problem is that although evolutionary trees are often used to convey ideas in museum displays and general interest science articles, many lay-viewers are interpreting them inaccurately. Reading a tree requires practice and expertise that shouldn’t be taken for granted, because misinterpretations only provide fodder for the anti-evolution/anti-science lobby. Let’s go through the common misinterpretations one at a time (many of these are discussed in Torrens and Barahona 2012, a few are my own additions).

Evolution is goal-oriented. In fact, evolution is not progressive, but is the product of organisms adapting to their specific environment. When that environment changes, taxa that were once well-adapted often die out. Being “well-evolved” is therefore fluid and transitory state. The misconception of directed evolution is probably related to ingrained western religious views of human superiority over nature. Rather annoyingly, cultural anthropologists often buy into the erroneous idea of progressive evolution, and attempt to use it as evidence that science is but one of many equally correct world-views.

There is a “main line” of evolution. This is largely the product of late 19th century drawings of trees of life which used literal trees as the basis of the diagram. Most famously, German natualist Ernst Haeckel illustrated the Systematischer Stammbaum des Menschen in his book Anthropogenie in 1874. In this drawing, the diversity of life is overlaid on a tree, which has a thick trunk running straight up to humans and other primates at the top. Again, this plays into concepts of human superiority and inevitability that have nothing to do with biological evolution.

Some contemporary species are more or less evolved than others. All contemporary species, from sponges to frogs to humans, have been evolving for the same amount of time, and are just as adapted to their environments as we are. Unfortunately, placing humans or mammals at the top or the right of phylogenetic trees seems to be an unshakable habit, even for systematists, which only encourages the notion that these taxa are somehow better.

Similarity among taxa always implies relatedness. Determining evolutionary relationships is a complex process. Modern systematists use huge matrices of independent characters to calculate the most parsimonious trees. Furthermore, Hennigean cladistics requires that relationships only be determined using synapomorphies (shared derived traits) rather than plesiomorphies (shared primitive conditions). Although the salmon and the lungfish below superficially appear more like one another than the cow, similarities like a fishy shape and a lack of a neck are primitive conditions, not specializations. The synapomorphies shared by the lungfish and cow, such as jointed limbs and the ability to breathe air, inform us that they shared a more recent common ancestor than either did with ray-finned fish.

A counter-intuitive cladogram. Subjective similarity does not always mean relatedness.

Change only occurs at nodes. The nodes in a phylogenetic tree do not represent literal evolutionary events. Rather, evolution is a continuous process. This is a case where I like to ask people who make this misconception, “how could we know that?” This can get people thinking about what evidence is available to scientists, what conclusions can be reached from these data, and what isn’t known.

Example taxa illustrated lower in the tree represent direct ancestors of taxa higher in the tree. It can be helpful to use fossil species to illustrate the general state of an evolutionary lineage at varying points in time (this is done all the time with diagrams of horse evolution). However, with few exceptions, the incomplete nature of the fossil record makes it impossible to know exactly which species were directly ancestral to others.

Traditional Linnean categories are directly applicable to trees. In fact, most (sensible) modern systematists prefer the cladistic methodology, which requires that all groups be monophyletic (that is, made up of all descendents of a common ancestor, with no exclusions). For example, the traditonal Linnean definition of reptiles, which includes turtles, lizards, snakes, tuataras and crocodiles, is not monophyletic, because any cladistic unification of these taxa would also have to include birds.

The traditional definition of reptiles, which excludes birds, is paraphyletic.

This went on a bit longer than I expected, so I’m going to leave these issues hanging for the time being. But do not fret, I will finish this train of thought soon with a discussion of potential solutions to these misinterpretations that have been attempted, and some that may be attempted in the future.

References

MacDonald, Teresa E. “Communicating Phylogeny: Evolutionary Tree Diagrams in Museums.” 2010. Paper presented at the NARST (National Association for Research in Science Teaching).

Torrens, Erica and Barahona, Ana. “Why are Some Evolutionary Trees in Natural History Museums Prone to Being Misinterpreted?” 2012 Evolution: Education and Outreach 1-25.

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Filed under museums, reptiles, science communication, systematics

Tagged as evolution, museums, pictures, science communication, science museums, systematics

Aside

Medullary Bone and the Dinosaur-Bird Link

One of the coolest lines of evidence that birds are extant dinosaurs is the presence of medullary bone in multiple dinosaur species. Medullary bone (hereafter referred to as MB, to avoid confusion with the medullary surface) is a temporary tissue that forms on the interior surfaces of the long bones of birds. MB is identified by unique collagen organization: it is both densely mineralized and strongly vascularized. This structure helps MB serve its purpose as a readily retrieved source of calcium for use in forming eggshells, and prevents incapacitating bone resorption during this process. Among extant animals, MB is only found in mature female birds in the process of producing eggs. Its creation is triggered by hormones during the onset of ovulation, and it disappears during the laying process. Among extant animals, MB is only known in birds. However, in 2005 Mary Schweitzer and colleagues reported their discovery of medullary bone in a Tyrannosaurus rex individual. Lee and Werning followed up on this research in 2007 by reporting MB in the theropod Allosaurus and the ornithopod Tenontosaurus.

Medullary bone in modern Gallus and fossil Tyrannosaurus. From http://www.abc.net.au/science.

Since MB is unique to reproductively active females, most popular coverage of dinosaur MB has focused on its potential use for determining the sex and life stage of individual dinosaur specimens. We shouldn’t, however, lose sight of the fact that MB is an independent line of evidence supporting a close phylogenetic relationship between dinosaurs and birds. Nearly all paleontologists agree that the evidence that birds are dinosaurs is overwhelming, and MB is but a drop in the ocean of shared characters between birds and dinosaurs. Nevertheless, it is noteworthy that few authors have attempted to challenge Schweitzer’s initial publication.

The only work I have found that disputes Schweitzer and colleagues is the dissertation of Dr. Devon Quick (.pdf link), in which Dr. Quick and Dr. John Ruben investigated the reliability of the methods used to recognize MB in the fossil record using extant animals. This is not, incidentally, the only work by Quick and Ruben challenging the dinosaur-bird connection. As a doe-eyed student, I’d like to take a shot at reviewing this paper. And since I’m posting it publicly, I of course welcome anyone who’d be so kind as to call me out for being wrong.

Quick and Ruben looked at cross-sections of the femora and tibiotarsi of a crocodilian (Alligator mississippiensis) and several birds. Scanning electron microscopy revealed that the medullary surfaces of the tibiotarsi of reproductively active birds displayed the highly contoured and floccular texture that is characteristic of MB. Likewise, the male and non-reproductively active female birds displayed smooth medullary surfaces. In this regard, Quick and Ruben are in agreement with previous work. However, the authors also reported that the medullary cavity of the alligator femur contained “material superficially similar to…avian medullary bone” (Quick and Ruben 2008). This material was limited to the immediate diaphyseal side of the metaphysis, making it much less extensive than what was observed in birds. Since the alligator individual used in the study was a juvenile male, it was almost certainly not producing reproductively-specific MB. From this observation, the authors conclude from these data that a floccular texture may indicate early-stage bone mineralization and is not a reliable indicator of MB.

Quick and Ruben’s results are unconvincing in part due to a weak experimental design. Their conclusions are dependent on observations gleaned from a single alligator specimen, which is not an adequate sample. The authors’ conclusions would carry more weight if they had looked at multiple individuals. It would also be beneficial to compare males, females, adults and juveniles. Ideally, additionally crocodilian species ought to be included in the study, as well. Schweitzer and colleagues carried out a similar investigation, in which they looked for evidence of MB in multiple alligators, including gravid females, males and juveniles (Schweitzer et al. 2007). Schweitzer and colleagues found no evidence of MB, even with estrogen stimulation, and their larger sample size allows their study to carry more weight than that of Quick and Ruben. Furthermore, although Quick and Ruben assert that that “histological aspects of Tyrannosaurus tissues that are supposedly consistent with an avian-style reproductive physiology were not analyzed carefully”, they did not look at the Tyrannosaurus material as part of their study. Accordingly, no evidence is provided that the structures the authors observed on their alligator were synonymous with those observed by Schweitzer and colleagues on Tyrannosaurus. Finally, Quick and Ruben’s observations are focused on the floccular texture used to identify MB, when in fact Schweitzer and colleagues used several other indicators, including extensive vascularization, to identify MB in Tyrannosaurus. It is notable that the structure, thickness and texture of MB in modern birds varies considerably based on the specifics of the animal’s reproductive biology and the size of the taxa. Given that Tyrannosaurus is several orders of magnitude larger than most extant birds, some structural difference is to be expected (wow, that sentence had some serious science snark).

Quick and Ruben suggest that the floccular texture on the alligator bone may be the result of early-stage mineralization, which would be consistent with the sub-adult status of the individual they used in the study. The authors go on to speculate that a similar explanation might account for the evidence of MB in Tyrannosaurus. Again, it would have been helpful if the authors had amassed more examples of sub-adult archosaurs undergoing skeletal mineralization, and compared them directly to the Tyrannosaurus material in question, rather than merely speculating. If the Tyrannosaurus was forming MB, this would be consistent with information from lines of arrested growth in Tyrannosaurus and other dinosaurs, which indicates that dinosaurs became reproductively active before reaching adult size.

Having reached the somewhat tenuous conclusion that texture is not a reliable indicator of MB, Quick and Ruben go on to argue that even if MB is present in dinosaurs, the fact that it has been reported in both saurischians and ornithiscians “offers no particular insight into the phylogenetic origins of birds.” On the contrary, MB is an independently observable feature that unites the crown group Dinosauria with Avialae, and therefore supports the consensus that Avialae is bracketed by Dinosauria. At the very least, MB suggests marked similarity in reproductive strategies employed by birds and dinosaurs. As demonstrated by Schweitzer and colleagues, MB is not known in crocodilians. Quick and Ruben freely admit this, which makes their statement that MB “may well be a plesiomorphic trait that first evolved in basal archosaurs” nonsensical (Quick and Ruben 2008). The authors could theoretically argue that MB production is primitive but was lost in modern crocodilians, but there is no evidence for this.

Overall, Quick and Ruben’s work is hindered by weak experimental design and vague, unsupported conclusions. Given that a similar but more rigorous study regarding MB in crocodilians has already been carried out by Schweitzer and colleagues, Quick and Ruben’s interpretations are not convincing. Even the broadest interpretation of the available evidence indicates that MB originated after the divergence of crocodilymorphs from the main archosaur line. The phylogeny postulated by Schweitzer and colleagues remains most tenable, in which MB originated in early dinosaurs, and was inherited by ornithiscians, tyrannosaurids and modern birds (Schweitzer et al. 2005).

References

Lee, A. H. and Werning, S. “Sexual maturity in growing dinosaurs does not fit reptilian growth models.” 2007. PNAS 105:2:582-587.

Quick, D. E. and Ruben, J. A. “Amniote bone structure and longbone histology in birds, alligators and the theropod Tyrannosaurus rex.” 2008. Oregon State University.

Schweitzer, M. H., Elsey, R. M., Dacke, C. G., Horner, J. R. and Lamm, E. T. “Do egg-laying crocodilian (Alligator mississippiensis) archosaurs form medullary bone?” 2007. Bone 40: 1152-1158.

Schweitzer, M. H., Wittmeyer, J. L. and Horner, J. R. “Gender-Specific Reproductive Tissue in Ratites and Tyrannosaurus rex.”2005. Science 308: 1456-1460.

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January 20, 2012 · 6:26 am

Category Archives: systematics

Do fossil exhibits have too many dinosaurs?

Field Museum of Natural History

National Museum of Natural History

American Museum of Natural History

Phylogenetics is Moon Man Talk

Basic Vertebrate Classification

Evolutionary History Through Deep Time

How Scientists Discover Evolutionary Relationships

References

Framing Fossil Exhibits: Phylogeny – An Addendum

Reference

Framing Fossil Exhibits: Phylogeny

References

Bully for Camarasaurus

References

What’s the deal with Astrodon?

The taxonomic history of Astrodon

Creating a coherent picture of Astrodon

Teaching Astrodon

References

Beating the orthogenetic horse

References

Communicating Systematics, Part 2

Change the shape of the tree

Be careful with representation of ancestors

Always clarify orientation

Avoid calling anything “more evolved”

References

Communicating Systematics

Phylogenetic Trees

References

Medullary Bone and the Dinosaur-Bird Link

References

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Category Archives: systematics

Field Museum of Natural History

National Museum of Natural History

American Museum of Natural History

Basic Vertebrate Classification

Evolutionary History Through Deep Time

How Scientists Discover Evolutionary Relationships

References

Reference

References

References

The taxonomic history of Astrodon

Creating a coherent picture of Astrodon

Teaching Astrodon

References

References

Change the shape of the tree

Be careful with representation of ancestors

Always clarify orientation

Avoid calling anything “more evolved”

References

Phylogenetic Trees

References

References

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