Skin Makes the Swimmer: Mosasaur Integument

There is a recent paper in PLoS ONE on mosasaur skin interpretation that falls within the realm of Paleobiomechanics, and which I found absolutely fascinating.  It is by Lindgren et al., and is entitled "Three-Dimensionally Preserved Integument Reveals Hydrodynamic Adaptations in the Extinct Marine Lizard Ectenosaurus (Reptilia, Mosasauridae)".  Because the paper is in PLoS it is open access (three cheers for open access science!) and you can find it here.

Here is the abstract:
The physical properties of water and the environment it presents to its inhabitants provide stringent constraints and selection pressures affecting aquatic adaptation and evolution. Mosasaurs (a group of secondarily aquatic reptiles that occupied a broad array of predatory niches in the Cretaceous marine ecosystems about 98–65 million years ago) have traditionally been considered as anguilliform locomotors capable only of generating short bursts of speed during brief ambush pursuits. Here we report on an exceptionally preserved, long-snouted mosasaur (Ectenosaurus clidastoides) from the Santonian (Upper Cretaceous) part of the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas, USA, that contains phosphatized remains of the integument displaying both depth and structure. The small, ovoid neck and/or anterior trunk scales exhibit a longitudinal central keel, and are obliquely arrayed into an alternating pattern where neighboring scales overlap one another. Supportive sculpturing in the form of two parallel, longitudinal ridges on the inner scale surface and a complex system of multiple, superimposed layers of straight, cross-woven helical fiber bundles in the underlying dermis, may have served to minimize surface deformation and frictional drag during locomotion. Additional parallel fiber bundles oriented at acute angles to the long axis of the animal presumably provided stiffness in the lateral plane. These features suggest that the anterior torso of Ectenosaurus was held somewhat rigid during swimming, thereby limiting propulsive movements to the posterior body and tail.


I found this paper to be a very exciting look at a feature of mosasaur anatomy which Justin and I have both recently developed an interest in, as well.  In terms of critiques, I thought that the general observations and conclusions were quite compelling, though I would have liked to see some consideration of how the specific scale patterns and integument reinforcement might have contributed to boundary layer control.  The authors almost get there - they discuss the importance of keeping a smooth body contour for reducing friction drag, but they never consider the possible effects (and advantages) of microturbulence generation, which is important in living sharks and some other "rougher skinned" swimmers.

For those that do not play with fluid mechanics on a regular basis: the boundary layer is the fluid adjacent to the solid body (the animal, in this case) that has very low velocity as a result of friction drag. Right at the interface, the fluid theoretically has no velocity, which is called the "no slip condition". Creating microturbulence in the layer just beyond the no-slip region creates a bit of extra drag initially, but it helps the rest of the water running along the animal to essentially "stick" to the boundary layer more effectively, so that major flow separation is reduced.  Because large scale separations add more to drag than the microturbulence, this is a net gain: by giving up a small amount of initial drag, the swimmer prevents a big jump in drag under more rigorous conditions.  This also reduces sound production in fluids, interestingly enough, which is probably how owls achieve silent flight (thanks to Dr. Andrea Prosperetti of Johns Hopkins University for point that out to me years ago).

In any case, that's my two cents for now.  A solid paper, all around, and well worth reading.

Cheers,

--MBH

Happy Thanksgiving: Microraptor ate birds

A recent paper in PNAS seems particularly worth mentioning on Thanksgiving (which, of course, is only really relevant to the U.S. readers of the blog, but so it goes), as it involves predation on birds:

http://www.pnas.org/content/early/2011/11/17/1117727108.abstract

The paper is entitled "Additional specimen of Microraptor provides unique evidence of dinosaurs preying on birds", and is authored by Jingmai O'Connor, Zhou Zhonghe, and Xu Xing.  The paper is based on an awesome little fossil that preserves an enantiornithine bird head-first in the gut of a microraptorine.  Because most enantiornithine birds seem to have been arboreal, the authors conclude that the Microraptor individual was likely hunting in the trees, and therefore potentially arboreal.  There are some potential gaps in this conclusion, particularly that modern arboreal birds are actually often predated on the ground (where they are more vulnerable) but it's an awesome specimen, regardless.  Direct evidence of feeding behavior is very rare as fossils, so this is a special case, indeed.

SVP Part 3: More than Looks?

A very brief post this evening.  One of the interesting talks at SVP 2011 was by Ryan Carney, who presented on his imaging study of the original Archaeopteryx holotype feather (it is now, of course, essentially an unidentified Jurassic bird element, as the holotype of Archaeopteryx has been transferred formally to the London specimen).

Most of the talk concerned using microstructure imaging to determine color in the feather, and it was determined that the feather was likely quite dark, possibly black.  This may seem to be purely a matter of appearance, but it turns out to be biomechanically relevant: melanin actually strengthens feathers, so black feathers are stronger than white ones, all else being equal (note that all else need not be equal; there are other ways of reinforcing light colored feathers).  It may be that incorporating pigment effects into structural analyses of basal bird wings will have interesting effects on the results.  I may be doing some of this myself over the next year, but look for similar analyses from multiple authors (Ryan, himself, mentioned the structural importance in passing during his presentation).

SVP Part 2: Water Launching Pterosaurs

I gave a second presentation at SVP this year, as well, in the form of a poster on pterosaur water launch.  Specifically, I presented a model that Jim Cunningham and I have worked out for a plausible water launch strategy in Anhanguera.  If you want to see what this might have looked like, turn your cursors here to Mark Witton's website.  The relevant illustration is on the far right.

I will not give too much detail on this presentation at the moment, as it is shortly bound for PLoS ONE.  However, here are some of the highlights:

- A bipedal water launch model appears to fail for Anhanguera (and other pterosaurs), just as the bipedal model fails for their terrestrial launch.

- A quadrupedal water launch model, in which the wings are the primary mechanism used to free the animal from the surface and to push along the surface to reach launch velocity, seems to check out for all of the parameters we can currently estimate with any confidence.

- Anhanguerids probably took multiple hops across the water surface to launch, but our calculations suggest that most of the actual energy expenditure was spent escaping the surface tension.

- Our model makes testable predictions about comparative anatomy of pterosaurs, which is important when building these kinds of models from fluid theory.  Our model predicts that water launching pterosaurs should have features such as: warped deltopectoral crests or dp crests with flared distal ends, enlarged scapulae, extreme disparity between forelimb and hindlimb lengths, and reinforced scapulo-notarial joints.  We have a more extensive list of features that can be shared a later date, but the primary note here is that these predicted features do indeed seem to show up mostly in marine pterosaurs, and less so in terrestrial taxa, so there is a least a loose, pattern-matching form of validation that can be applied to our hypothesis.

We hope to have animations and a full paper out on the topic of pterosaur water launch in the near future (next few months) so stay tuned!

SVP Part 1: Enormous, amazing, soaring birds

Over the next week I will be posting a bit about some of the talks and poster sessions I attended at SVP.  I am going to begin, however, with a bit about the presentation that Justin and I presented (because heck, it is our blog, after all).

Our talk was entitled "Flight Performance of Giant Pseudodontorn Birds".  As the title suggests, we were working with material from some of the largest flying birds in history.  Our results, in fact, may help explain how they obtained such sizes in the first place.

What are pseudodontorns?

Pseudodontorns were extraordinarily long-winged seabirds with a duration of nearly 55 million years in the fossil record (Paleocene/Eocene to Pliocene) - the wonderful image above is by Greg Paul (he owns the image; don't use without permission).  Some of the California specimens preserve feather impressions which allow us to make a quite good estimate of wing shape.  Typically, the long, narrow wings of pseudodontorns have been interpreted as adaptations to a soaring regime similar to albatrosses.  However, albatrosses have a rather specific specific soaring mechanism that is enabled by their high wing loading.  An alternative, looking at modern birds, would be a frigate bird type flight strategy, but the overall morphology of giant pseudodontorns (span, size, etc) seems at odds with this particular strategy (frigates are highly maneuverable kleptoparasites - they steal fish from other birds).  See the wonderful photo below of a frigatebird, taken by Chris Hobaugh (note the extremely broad wings).

Our answer is a third strategy: Justin and I suggest that giant pseudodontorns, such as Pelagornis, were "globe trotters" - that is, our analysis indicates that they were adapted to extreme flight range.  This update comes, in part, from using new mass estimates from an exceptionally complete specimen found in Chile.  It turns out that pseudodontorns had such elongate wings that mass estimates extrapolated from other seabirds produce overestimated body masses.  The new, lower, body masses suggest that Pelagornis and kin combined a very high aspect ratio wing shape with rather low wing loading.  This would enable highly efficient flight (glide ratio of 27:1 - the highest for any bird) and the ability to use micro-lift sources.  Such an animal would not be able to soar fast like an albatross or turn on a dime like a frigate, but the potential time in the air and maximum range could have been quite extraordinary.

The new mass estimates also modify our thoughts on the limb strengths of pseudodontorns.  It turns out that they had rather strong hindlimbs compared to, for example, a living albatross.  Because launch is hindlimb driven in birds, this might mean that pseudodontorns had more juice over short takeoff sprints, and therefore a larger maximum size for effective takeoff.

All told, a fascinating group of birds that is sorely understudied.

SVP and PLoS ONE

Greetings everyone!

There has been a long hiatus from posting here on H2VP.  Justin and I are back with a vengeance, however, and we have just returned from the annual meeting for the Society of Vertebrate Paleontology.  SVP 2011 was held in Las Vegas, Nevada, and was an exceptionally good time.  We attended some excellent talks (puls some weaker ones) and will be blogging about SVP extensively over the next week.

In the meantime, however, I am a co-author on a new paper in PLoS ONE.  The citation is:

Dyke GJ, Wang X, Habib MB, 2011 Fossil Plotopterid Seabirds from the Eo-Oligocene of the Olympic Peninsula (Washington State, USA): Descriptions and Functional Morphology. PLoS ONE 6(10): e25672. doi:10.1371/journal.pone.0025672

The paper concerns relatively early members of an extinct seabird group call plotopterids.  Some of these grew quite large (up to 6 feet in length, potentially), and all of the known taxa appear to have been flightless, wing-propelled divers.

As some of you may know, I have a bit of a thing for wing-propelled swimming birds, and I published a biomechanical analysis of them in 2010 entitled "The structural mechanics and evolution of aquaflying birds".  One of the primary points in the paper is that the biomechanics and swimming dynamics of penguins are quite different from other living wing-propelled birds.  In particular, penguins have a mirrored stroke, meaning that the upstroke and downstroke produce similar propulsive forces.  In contrast, the other living aquaflying birds use mostly the downstroke to propel themselves through the water (though the upstroke does add some thrust).  The mirrored stroke system appears to be reflected in the particularly broad forelimb bones of penguins (photo above copyright by L. Henricks, 2009).

This raises the following question: are penguins unique in this way because they are flightless?

If so, then other flightless aquaflyers should have similar bone mechanics.  As such, we compared plotopterids to both living alcids and penguins.  We found that plotopterids, while they had similar hindlimb mechanics to living penguins (suggesting lots of walking), did not actually have similar forelimb shape to penguins.  Instead, plotopterid forelimbs are more comparable to alcids, which we take to suggest that plotopterids did not use the penguin-style swimming stroke.

If we are correct, then penguins are unique for reasons beyond simply being flightless wing-propelled swimmers.  The full story on the acquisition of the unique penguin swimming mode will likely be revealed by careful biomechanical study of their fossil history...