Recent Events: Physical Drivers and Marine Tetrapod Evolution – Symposium at the Society of Vertebrate Paleontology

It has been WAY too long since my last post, my sincerest apologies. I was consumed with a handful of projects, some of which are submitted, and some of which I presented at the Society of Vertebrate Paleontology meeting in Pittsbugh, PA this past October 10-13.

I fully intend to blog on some of this new material soon, but want to start with a couple of things. 1) A report on a symposium held at the recent Society of Vertebrate Paleontology meeting, and 2) a new series of blog posts reviewing recent literature and its relevance to the study of aquatic amniote evolution studies. I’ll get to the latter in my next post, but let me get started with this report on the recent (October 11, 2010) SVP Symposium titled, “Physical Drivers in Marine Tetrapod Evolution”. I’m keeping this brief, not only to save you from my usual boring wordiness, but also to protect the rights of authors of these presentations from having their unpublished material shared without their permission.

The symposium was organized by Neil Kelley (UC Davis) and R. Ewan Fordyce (University of Otago). Neil is a promising graduate student studying Triassic marine reptiles, and Ewan is one of the world’s leading experts on fossil cetaceans, and together it was a good match that brought lots of good minds together.

Neil Kelley and R. Ewan Fordyce, the organizers of the symposium, "Physical Drivers and Marine Tetrapod Evolution"

Among the minds brought together were (presented listed here only, though obviously many had co-authors that significantly contributed):

Neil Kelley (UC Davis, USA)

R. Ewan Fordyce (University of Otago, New Zealand)

Olivier Rieppel (Field Museum, USA)

Valentin Fischer (Royal Belgian Institute for Natural Science, Belgium)

Michael Polcyn (Southern Methodist University, USA)

Louis Jacobs (Southern Methodist University, USA)

James Parham (Alabama Museum of Natural History, USA)

Sanja Hinic-Frlog (Carleton University, Canada)

Tatsuro Ando (Ashoro Museum, Japan)

Brian Beatty (New York College of Osteopathic Medicine, USA)

Naoki Kohno (National Museum of Nature and Science, Japan)

Edward Davis (University of Oregon, USA)

Carolina Gutstein (Universidad de Chile, Chile)

Erich Fitzgerald (Museum Victoria, Australia)

Felix Marx (University of Otago, New Zealand)

Nicholas Pyenson (Smithsonian Institution, USA)

Topics covered included a variety of methods, details, and scales, though some common themes were:

• How aquatic tetrapod groups have and have not been affected by the changing coastlines, chemistry, and productivity of the world’s oceans. I found it dumbfounding that so many variables affect diversity and distributions of these groups, and controlling for them is the challenge we all faced. Some did so by looking at distribution correlations, others by ecological variables such as isotope geochemistry or dental microwear. Methods here included:
• Morphometrics of locomotor adaptations
• Dental microwear
• Stable isotope geochemistry
• Distribution patterns
• Bottom-up or top-down ecosystem design
• Sea level changes and its effect on available habitat, and the use of freshwater by some usually marine groups.
• Associations of taxa as implications of their ecology
• Spatial and temporal changes in diversity correlating with global climate events

• Some authors reviewed some unrecognized diversity, not only taxonomically but also in terms of ecological roles, that revise our understanding of how earth history may or may not have affected these groups. One compelling message of this was a clear reminder that we all need to come back to finding more fossils and describing them before jumping into complex analyses of existing records from databases alone.

• In a surprising, but great twist, some made a point of looking at the way that some of the diversity and distribution of groups were or weren’t affected by how the animals themselves dealt with the physical environment in terms of sensory perceptions. Thus, not only did we see distribution and ecological variables discussed, but aspects of sensory modalities in some groups and how that could tell us more about where and how they lived and dealt with the changing environment.

Me (Beatty) finishing my talk on dental microwear in the Sirenia (photo by Michael Ayoub)

In the end, the topics covered all had one common theme – that the evolution of aquatic amniotes is very closely linked with the evolution of the Earth. Despite what one might see as an interesting lesson in history, I think that many of these talks demonstrated that for many of the taxa still living today, we can only hope that these lessons learned will help us avoid reliving history, especially those parts that ended in extinction. These are the sorts of studies that make paleontology relevant to modern ecologists and conservationists in the face of global warming.

I think that the symposium was a massive success, particularly because it seems to have encompassed a huge diversity of methods, taxa, and times, and brought people together to share ideas and potentially collaborate. I know I’m already going down the road of starting some new projects with people I spoke with just after the symposium. That is, after all, the more proximate goal of these symposia, and I am glad to have been part of it. I would like to thank Neil and Ewan for inviting me, and thank you for sharing my interest in keeping current with what is going on among aquatic amniotes.

Until next time... which will be soon!

Tooth development in Trichechidae Part I

Alright, so this is not a complete post, but it's a start.
This Monday I will be visiting the Mammalogy Collection at the AMNH for one last data collection trip for the study of tooth development in the sirenian family Trichechidae. The Trichechidae is the family that includes modern manatees, as well as a number of fossil forms.
Aside from a comparatively abundant Pleistocene fossil record of manatees primarily found in Florida (subspecies Trichechus manatus bakerorum Domning 2005), most fossils of manatees are fairly scarce and poorly preserved, including the two Miocene taxa, Potamosiren and Ribodon, both from the Amazon River Basin of South America. Both of these taxa are known from little more than isolated teeth and some fragmentary maxillary or mandibular chunks.
But, thanks to two features of the skulls of fossil forms that pull their relationship closest to the modern manatees, there is a couple of oddball fossil forms from Belgium and Germany that ally closely with the Trichechinae (Potamosiren, Ribodon and Trichechus), the Miosirenae (Miosiren and Anomotherium)
Over the next several posts I will try to demonstrate some of the key features of all of these taxa, and how understanding some of the unique specializations in modern Trichechus (especially how those features in modern Trichechus vary inter- and intraspecifically), we can better grasp how these features evolved, possibly as a response to a rapid increase in abrasives in their mouths thanks to the uplift of the Andes.

Archosauriform dentitions possibly constrained by development differently than mammals

Hi folks, I'm glad to be back after something of a hiatus. I had some important personal reasons to be away, but now am back in business and have several posts ready to go for the next several weeks, with many more in the works so I can keep this going regularly. This has become a healthy outlet for my need to share ideas and stimulate my own internal discussions that make my work better (I hope), and it would not be worthwhile if it were not for you readers, so thank you for your time and comments. I read them all, and treasure the time you spend even contemplating my crazy ideas.

To start, I feel it is appropriate to do a brief report about a paper of mine that came out this past summer, coauthored with Andrew Heckert (Appalachia State University, NC) in Historical Biology titled, "A large archosauriform tooth with multiple supernumerary carinae from the Upper Triassic of New Mexico (USA), with comments on carina development and anomalies in the Archosauria"
Whew! Long title, I know, but it was intended to be descriptive.

To summarize, Andy had an unusual tooth and knew I have been focusing some of my energies on paleopathology, especially those of teeth, and suggested that we work together on describing this specimen he collected some years ago from the Triassic of New Mexico.


If you are familiar with archosauriform dentitions like this one, it is ziphodont (mediolaterally appressed, with carinae along the mesial and distal edges) and resembles the tooth of a large number of taxa. The age, locality, and size of the tooth suggests it belongs to a semiaquatic, crocodile-like reptile known as a phytosaur, although it could be from a large rauisuchian or other similar animal.
What matters more is that in addition to the normal carinae on the mesial and distal edges, there are two extra carinae, running parallel to the long axis of the tooth, but found on the lingual surface of the tooth (the tongue-side). Split carinae have been noted in theropods for some time, including tyrannosaurids (Currie et al 1992, Erickson 1995), carcharodontosaurids (Candeiro & Tanke, 2008), and even a split carina was noted in a phytosaur previously (Hungerbuhler 2000). This was the first time anyone had recognized something that was not a split carina, but a duplication of one, including one so far lingually displaced.
This is odd, considering that unlike the tooth deformities found in some sharks that can result from traumas when feeding (Becker et al 2000), carinae are structures of the tooth that are formed during development within the jaw as the tooth forms, and are only surface features. There is no plausible way to consider alterations or duplications of carinae in these teeth as anything but developmental anomalies, for this reason.
But dental variation is quite common in many animals, so it should be such a surprise, should it? This is where it gets interesting... Most dental variation studies are focused on mammals, where tooth development is very well understood, and that understanding centers around the fact that much of the variation is a result of small regional differences in the thickness of enamel in different areas of the tooth. The gene expression aspects of this are now well understood for mammals in general (references abound on this: 1, 2, 3, 4), but less is known about archosauriform tooth development.
Considering that the external morphology of interest is ultimately the surface of the enamel, knowledge about which tissue influences tooth morphology most can be derived from looking at the end product of tooth development, the adult tooth itself. It is rather difficult to observe the stages of development of a fossil organism, but if one can derive development from an adult form, then that's a start. So, if one looks at enamel thickness, one might get at how tooth morphology in this and other archosaurimorphs works.
Thankfully, P. Martin Sander (a hero of mine for all the creative ways he has tried to tackle paleobiological conundrums) has done a significant amount of work in describing the enamel microstructure of a large number of fossil non-mammalian amniotes, including rauisuchians, phytosaurs, and many others (Sander, 1999). It turns out that like our specimen, many non-mammalian amniotes maintain a mostly uniformly thin enamel, including the enamel covering the carinae and their denticles (this is not always the case for the denticles, but is fairly common). This would in turn suggest that their crown morphology is the result of the control of the morphology of the underlying dentine, not the enamel.
This has huge potential implications for the limitations that non-mammalian amniotes have in developing complex crown morphologies like those seen in mammals, and may explain why only a few rare crocs here or there (Simosuchus, etc) have anything but cone-like dentitions. Ornithischian dinosaurs are a whole other group that seem to overcome this hurdle, though at present I don't think anyone is considering the question of how their teeth developed, or even how they are structured in much of a histological sense. If anyone is up for tackling this issue, I am certainly game :-]
All in all, this one tooth led to some tantalizing, if speculative, thoughts on how archosauriform teeth develop, and how that is limited. The paper goes much further into the evo-devo of tooth development and what we know of it in amniotes in general, as well as some coverage of the curisou fact that the serration density of the extra carinae are uniform and identical to that of the normal carinae, suggesting that even with the new carinae, serration density seems to be stable and conserved, perhaps supporting its utility as a phylogenetic character and suggesting a developmental independence between the location of carinae and the morphology of the denticles they have.
I hope you find it as stimulating as I did.

If you want more discussion about this, see coverage on the blog, Chinleana.

Next up:
Sirenian tooth development

New, regular posts are coming, I promise!!!

Adventures in prehistoric animal reconstructions - A preview of a new reconstruction by Carl Buell of a new desmostylian from Vancouver Island!

Last summer (2008) I went to Victoria, British Columbia, to work on a new specimen (collected on my birthday in the summer of 2007) of a desmostylian that came from a locality similar to the type locality of Cornwallius sookensis. Joan Kerik, the Collection Manager (an extraordinary one at that) at the Royal British Columbia Museum in Victoria had contacted me a couple of year before regarding Cornwallius, which was something I was working on at the time (just now published in JVP). Eventually when this new material came up which we all expected would be Cornwallius, she connected me with Thomas Cockburn, a Research Associate that specializes in the Sooke Formation invertebrate fauna.
Tom took me to the locality, as well as the type locality of Cornwallius sookensis, Muir Creek. We had an extraordinary time, saw deer eating algae on the rocky intertidal and a bear eating something dead in the intertidal as well. At the new locality we came across only a little new specimen, this time a caudal vertebrae of a cetacean (remember this is Late Oligocene, so it could be from any number of weird cetaceans of the time). But no more desmostylian. Still, I happily had spent the prior two days studying this partial skeleton already in the museum, which includes half a skull and its teeth (except the incisors), a partial scapula, most of a humerus, and most of the vertebrae and ribs. The skull was quite interesting, because it looked very much like the skull of Cornwallius, yet the teeth looked like a smaller version of Behemotops. Behemotops is not known from much of its skull other than the posterior portion (B. katsuiei from Japan - Inuzuka 2001), and the new specimen does is throw much of that out, revising the relationships of the two main clades of Desmostylia and suggesting that Behemotops is more like Cornwallius and Desmostylus than previously asserted.
So, Tom and I are currently working on the manuscript and I asked my friend, Carl Buell if he could draw this new animal so we could submit it as a possible cover for JVP. He is still not finished with the final illustration with a background and all, but I thought it would be nice to share the final draft of what we came up with of what the animal should look like (with a neutral background). Carl said I could post this, but if you have any questions about its use, I would contact him. While you are at it, you should check out his Flick'r page. The only thing that trumps the quality of Carl's illustrations is how wonderful and generous a friend he is.
I hope you enjoy this - for more you'll just have to wait for the paper to come out (here's where I shouldn't, but will, suggest that you urge JVP editors to move fast - I'm just kidding!!!).

the problem with microwear #1 (of many to come)....or... "if seacows eat seagrass, why can't we consider them grazers?"

Although I haven't published as much as I would like on the subject yet, many of you that know me know that I have spent an inordinate amount of my research life focused on the study of microscopic damage to tooth surfaces, known as dental microwear. Most of this has been focused on marine mammals, particularly members of the Sirenia and Desmostylia, although most members of these groups are extinct. I presented one of the more thorough studies of dental microwear in modern and some fossil Sirenia at the Society of Vertebrate Paleontology meeting in Bristol, UK this year.

But, to give some insight into the reality of such work, the sort that makes me pull my graying hair out, I thought I would share here. WARNING, I may rant about the flawed science of many microwear studies, but only to highlight the complexity of the problem, I do NOT intend this as a criticism of my valued colleagues endeavoring to get to the answers of a VERY complicated, messy bit of science. They deserve credit for having the guts to put it out there, knowing that in the end they will inevitably fall short of the full story - that's the way science goes, sorry everybody. I learned this perhaps a bit too late, but just get used to it and publish!

See, I digressed already!

Ok, so the problems with dental microwear are many, many, many.... but among other things, many folks attempt to apply systems of ecology to many groups of animals across large fields of diversity (and hence, morphology, physiology, and evolutionary background). The classic is the idea of the hippo-ecomorph. There are many fossil mammals with large bodies and short limbs, such as Teleoceras, Coryphodon, etc., that commonly get lumped into being hippo-like in their morphology (which is superficially true) AND lifestyle (which is rarely, if EVER supported by data). The anecdotal comparisons with hippos that most paleontologists make usually only serve to demonstrate their ignorance of modern hippo ecology (being noctural grazers with little to no social system, only found in groups because of their reliance on a scarce resource - water). But nonetheless, you will still find references of Teleoceras as a hippo ecomorph in textbooks and such, even though several thorough studies have shown that the only evidence potentially telling about this indicates that they were very much NOT like hippos.

So how does this relate to dental microwear? In the strange world of classic microwear (excluding some more elaborate confocal microscope-using methods), there is:

1. an SEM method that visualizes very small portions of the tooth at a high magnification
2. a light microscopy method that visualizes a larger area of the tooth at a lower magnification

The differences in these methods are great, but the basic idea is that teeth incur damage from what they eat (or more likely, the dirt that is on whatever they are eating), and the small bits of damage can be characterized in different ways that roughly correlate to different diets of grass, browse, or a mix of both.

BUT, as anyone that has ever raised an herbivorous mammal, had a garden, or even made a salad understands, not even the simplest diets can be broken down that simply AND not a single animal on this planet (except for maybe the koala) can ever be described as being a strict consumer of a single plant type. Plants come in all shapes and sizes, as well as all sorts of material properties and abrasiveness. The general notion that the silica nodules, known as phytoliths, that are found to surround vascular bundles in grasses are the cause of the scratchy wear in grazers itself is an example of this issue. Not only have phytoliths been demonstrated to not all have the hardness needed to wear enamel (Sanson, 2007), but many plants that do not have phytoliths wear enamel in very similar patterns. The best example I know are seagrasses and the wear found on the teeth of manatees, Trichechus manatus. Manatees eat a lot of seagrass, yet not a single seagrass has phytoliths inside, so what causes the wear? My research on this of late has pointed in the direction of substrate, specifically siliclastic substrate that some seagrasses like to grow in. In the end, the simple answer to the question of what causes wear is that for seacows, it isn't phytoliths. For all animals in general, it might be better put as - consider all the options before you rule any single thing out, and consider the system at hand. In the end, the data for one ecosystem may ultimately NOT be comparable to another for just this reason.

I will try to continue these rants to cover other aspects of microwear, including issues with methodologies, assumptions, dietary interpretations, and the ever-persistent attempts to apply microwear to fossil organisms, including dinosaurs, despite clear differences in mastication, ecology, etc that should act as BIG warning labels to most people that microwear should not, could not, and cannot be applied in the same way for every animal that ever wore a tooth. The simple notion of using data from one study and comparing it with that of another is a complex matter that needs addressing as well, so I will try to bring it up here as well.

So much to do, so little time! Thanks for your patience and time.
Brian

Misadventures in prehistoric animal reconstructions - The many faces of the Desmostylia I:

This is Desumon, a cartoon desmostylian that was created by a natural history museum in Japan to help introduce the Desmostylia to the public. Desmostylians are very popular in Japan, which is just another sign of their cultural advancement, in my opinion. I mean, there has to be a correlation between the quality of their science and math education and the preponderance of desmostylians in popular culture.

In my anticipation of seeing what marvelous work comes from an illustration by Carl Buell of a new desmostylian I am working on, I thought it would be fun to have a romp through the strange history of the reconstruction of desmostylians. What follows are simply those images I have on my computer at present, although I'll try to make sure and get some of the older images (including ones that compare desmostylians with multituberculates) scanned for future posts. In general, I have several future posts planned to explore the history of the ways people have reconstructed aquatic amniotes. It is not only illustrative of how our perceptions of those animals have changed over time, but also how we approach reconstructions from a scientific standpoint and why so many paleoartists these days are also such excellent anatomists.

Enough blabbing, here are some Desmostylia!



To start, it is a good idea to present a skeleton to get an idea of what we're working with. This is a nice generalized reconstruction of a desmostylian skeleton. Aside from some large feet, funny angles in their ankles, and slightly short limbs, they are a fairly mundane large mammalian herbivore shape -think Anrsinoitherium or Coryphodon (also animals considered semiaquatic - see future posts for critiques of that!)

But, partly because they have these wide, thick sternebrae and short limbs, just how close to the ground they were has been debated.

Most of all, it is because of the finds of the Utanobori Desmostylus skeletons (such as Utanobori I), and the way they were preserved with their limbs sprawled out, some paleontologists have interpreted the Desmostylia as having a "herpetiform" posture, with limbs sprawled laterally like what is seen in most modern squamates. The debate over this matter became even a series of papers, back and forth between Norihisa Inuzuka and L. Beverly Halstead.

So, many of the following reconstructions are not only coming from an attempt to reconstruct an animal that spends much of its time in the water, but also a very specific interpretation of their limb posture. This is not the normal way in which most people would reconstruct mammalian limbs, which is why I think some of them look a little awkward.

Rather than blab on and on, I'll let them speak for themselves. The pics are in order of most aquatic/herpetiform to most terrestrial.... I'll let you decide what you think is most realistic. For fun, I've numbered them so you can refer to them in comments. Please, tell me which ones you think are most realistic, or most of what your opinion of Desmostylia has been thus far.
Have fun!





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This is just a start, I'll try to have more on this soon!