Aquatic Mesozoic Mammals

I have been working on a Mesozoic mammal with some colleagues in Kansas (Michael Engel) and China (Dong Ren) and cannot help but comment on the strangeness that is the world of Mesozoic mammals, in particular those that are deemed "semiaquatic".

The poster-child for aquatic Mesozoic mammal is Castorocauda, the beaver-like docodontan from the Middle Jurassic Daohugou Beds of Liaoning Province in northern China. This is the same locality that has had the earliest gliding mammal, Volaticotherium, as well as some weird feathered dinosaurs like Epidendripteryx, and the mammal I am studying with Engel and Ren (I promise to share more about this critter when the paper is published). Castorocauda is an interesting animal, if anything because it shows few specializations for an aquatic life other than a beaver-like tail, unusual limb proportions, and larger than normal body size for a Jurassic mammal.

Back in 1994, Fred Szalay proposed that stagodontid marsupials, found in the Cretaceous of North America, were aquatic, based on the morphology of the bones in the ankle. In 2005 Nick Longrich presented an abstract at the meeting, Evolution of Aquatic Tetrapods in Akron, OH (hosted by Hans Thewissen at NEOUCOM), detailing how stagodontids might have been durophagous and semiaquatic, based on aspects of their dentition and postcrania, especially a caudal vertebra that resembled those dirsoventrally compressed caudal vertebrae of beavers which are also found in Castorocauda. Recent studies of stagodontids (Fox & Naylor 2006), however, have discredited these claims of being stagodontids as aquatic, although it would be interesting to see if some of Longrich's ideas can be further explored.

So, if semiaquatic mammals are rare in the Mesozoic, why? It has been fairly well documented that being semiaquatic (whatever that means - I'll rant on this some more in the future to be sure) is energetically more costly than being either fully terrestrial or fully aquatic (Williams 1999), so that may have been a hurdle impassible for them, but then why would so many other mammal groups manage it in the Cenozoic even strictly in freshwater, from a variety of body sizes such as desmans to beavers? Mesozoic mammals had been pegged as limited to smaller body sizes in the past, but it is increasingly evident that this was not the case.

I would suspect it is something altogether much less exciting, and much more mundane, expected, and depressing - the fossil record. The "pull of the recent" strikes again, and this time I wouldn't be surprised that because the fossil record of Mesozoic mammals is limited by exposures and the longer periods of time in which fossils may have been destroyed, we simply have fewer of them.

Plus, it is really hard to recognize some of the subtler aspects of adaptations for being semiaquatic in gorups which are still fairly rarely known from anything more than fragmentary teeth and jaws. Hell, if you had a river otter jaw in your hand, would you know it was semiaquatic? No, at least not until we start getting to understand the finer relationships of the skeletal and dental adaptations of aquatic and semiaquatic mammal mammals in a broader context.

Hmmm.... that is a tempting distraction from Mesozoic mammals, isn't it?