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