Detour into Choristodera

This week a paper on unusual prehistoric aquatic reptiles was published. I am speaking, of course, of Brownstein (2022) on choristoderes. (If you thought it was going to be spinosaurs, you must be new around here!)

We have a soft spot for choristoderes here at Equatorial Minnesota Towers, even if they rarely rate so much as an "and also featuring Champsosaurus" credit in sci-pop culture. Brownstein (2022) includes the classic Champ but is more focused on the short(er)-faced choristodere Simoedosaurus, although under a fresh coat of taxonomic paint: the North American species S. dakotensis is moved to the new genus Kosmodraco on the grounds of anatomical differences and differences in time and place from the type species, European S. lemoinei. It also gets a new friend, K. magnicornis. Which said, sure... but the species of Kosmodraco still clade more closely to Simoedosaurus than to anything else, so if your genericometer was so tuned, you could still include them in Simoedosaurus with a clear phylogenetic conscience. (There are a lot of anatomical differences between the two genera. My only quibble with Kosmodraco is that it's an awfully pretty name for a choristodere; it sounds more like an an extravagantly crested pterosaur. Meanwhile, Champsosaurus itself is still secretly a taxonomic booby trap waiting to be sprung. Sooner or later someone is going to do something with the various species that is entirely legal by the rules of taxonomy and yet manages to displease everyone else.)

The type skull of Kosmodraco magnicornis. It'll take a moment to orient yourself: the bitey part is surprisingly short (see the "r. lacrimal"? The eye was behind that). What you're looking at is a modest snout attached to a greatly flaring right "cheek" (which comes with a scalloped fringe). Figure 1 in Brownstein (2022). CC BY 4.0.

Either way, the skull is worth a look. There's garden-variety "weird" and then there's "why is this all practically all post-orbital?" (It certainly wasn't for intellect, most of the posterior of choristodere skulls being a series of struts and bars. Note that the maxillary teeth of Kosmodraco only go back as far as the eyes.) Champsosaurus, as we've already seen, had a long, narrow toothy muzzle, with the rest of the skull being broad and low. Kosmodraco had a skull that was still low but much more wedge-shape in dorsal view, as if someone smooshed or amputated a Champsosaurus-like snout. The business end of a Kosmodraco skull is an interesting analog for the modern alligator gar. It's the back of the skull where things get different, as Brownstein (2022) notes. Kosmodraco has a lot more skull going on behind the eyes, which themselves are elevated on skull like those of an alligator. The posterior margin is also ornamented with a series of knobs, and the skull is quite low (Brownstein 2022).

A comparison of the palatal regions of choristoderes Kosmodraco and Champsosaurus to an alligator and an alligator gar also serves as a comparison of basic facial shapes for the four. Similarities between Kosmodraco and the gar aren't as great if you continue through the rest of the skull. Figure 13 in Brownstein (2022). CC BY 4.0.

I'd like to take the opportunity to note that there's another extinct group of aquatic tetrapods with flat, blunt, broad skulls and eyes relatively far forward on the skull, which has so far avoided comparisons to Simoedosaurus/Kosmodraco: the metoposaurid amphibians of the Triassic. Again, it's not a perfect comparison (metoposaur snouts are blunter), but it would seem to point to the long-term existence of a niche for freshwater predators with certain cranial adaptations.

One of the other points noted by Brownstein is there is more diversity in choristoderes than you might suspect by simply looking at the number of genera. For example, as of this writing Champsosaurus is composed of several species over about 20 million years. North American specimens have tended to be lumped with either the short-faced Simoedosaurus or the long-faced Champsosaurus.

References

Brownstein, C. D. 2022. High morphological disparity in a bizarre Paleocene fauna of predatory freshwater reptiles. BMC Ecology and Evolution 22: article number 34. doi:10.1186/s12862-022-01985-z.