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As I mentioned in a recent post a new crocodylian fossil has been discovered. Paleoblog and Hairy Museum of Natural History both have excellent posts on the subject. While reading the paper a couple of interesting things jumped out at me. Before I get to that, here is a picture of the fossil.
I’m doing it as a pop up because it is way too beautiful to reduce …
The first thing that jumped out at me was the prominent mention of Thomas Huxley. In particular, Salisbury et al focus on Huxley’s 1875 paper On Stagonolepsis Robertsoni and the Evolution of the Crocodilia (which doesn’t seem to available from the Huxley files – linked to above). In that paper Huxley laid out what he considered to be one of the strongest cases for the theory of evolution. According to Salisbury et all Huxley:
“…identified three main phases in the evolution of crocodyliforms: ‘Parasuchia’, ‘Mesosuchia’ and Eusuchia. His definition of Eusuchia was apomorphy-based, with referral to the group dependent on the combined possession of: (i) a fully developed bony palate, where the secondary choanae are fully enclosed mediorostrally by ventral laminae of the pterygoids; and (ii) procoelous vertebrae, in which there are synovial, semi-spheroidal articulations between adjoining vertebral bodies…”
“Although these features are now known to have evolved independently in other crocodyliforms, the presence of both features in combination with a sagitally segmented paravertebral shield has thus far proven to be limited to eusuchians and, therefore, phylogenetically informative…”
Over 130 years later we have a lot more fossils than Huxley could ever dream of and one wonders what he would come up with were he alive today.
A second point of interest concerns the evolution of the bony palate in crocodylians. Traditionally, the separation of the nasal cavity from the oral cavity was considered to be an adaptation for eating underwater (another idea of Huxley’s). Salisbury et al argue that it was really an adaptation to the biomechanical stresses related to a shift to a “bite and hold” style of feeding (think death roll) arguing:
Assuming that a urohyal valve existed in Isisfordia, enclosure of the secondary choanae by the ventral laminae of the pterygoids would have resulted in minimal respiratory advantage, given their position in the palate. Indeed, assuming the presence of a urohyal valve, the position of the secondary choanae in most neosuchians and many longirostrine mesoeucrocodylians is indicative of respiratory capabilities that are similar to those of extant crocodylians. It is hard to envisage how further caudal migration of the secondary choanae, commencing with Isisfordia and continuing into Crocodylia, would increase the efficiency of this system.
Structural reinforcement of the skull would therefore seem to be the most likely factor behind the progressive caudal migration of the secondary choanae in crocodyliforms, and the subsequent formation of an extensive bony secondary palate in eusuchians.
They then go on to relate this to several other features in Isisfordia duncani, but that is outside the scope of this post. What I found interesting about this part of the paper, leaving aside the Huxley angle, is how new finds can cast new light on current theories and suggest a different interpretation of that evidence.
Filed under: Paleontology |