Those Super-Cryptic Choristoderes

Dear Constant Readers,

The last time I talked about choristoderes was way back here, and a cursory glance revealed that I have a lot of updating to do. And now with newfound access to scientific journals (like a kid in a candy store) I finally have the materials I need to create a post on these enigmatic reptiles.

If the name "choristodera" hasn't been heard in your household, I won't hold it against you. They were first described by Cope (of the Bone Wars fame) in 1884* and for 100 years they were portrayed as moderately sized (2-2.5 meters) gavial-like reptiles that arose in the late Cretaceous and went extinct in the Eocene, apparently little affected by the massive K/T extinction event (Evens & Klembara 2005). Gao et al (2000) do an excellent job of summarizing the recent work done on this group. In the last 20 years, 9 new genera (only 2 known previously) were described from Western North America, and Eurasia from England to Japan ranging from the sub-tropics to the Arctic. They state that the choristoderes "are beginning to emerge from the shadows as a group of unexpected phylogenetic and ecological diversity"; and given my tendency to focus on emerging fields (see here and here) I could hardly resist.

One noticeable change is the temporal range choristoderes now occupy. Cteniogenys described in the late 80's pushed the choristoderans back to the latter half of the Jurassic and re-analysis of Pachystropheus (discovered in 1935) in the early 90's potentially pushed them 45 million years farther into the late Triassic. If choristoderes are considered basal Archosauromorphs** judging by cladistics this would mean their lineage could be another 50 million years older and originate in the late Permian (Storrs & Gower 1993). This would imply that choristoderes (or their direct ancestors) survived an even bigger extinction event earlier in the past only to go extinct later on. The range of choristoderes in the other direction is the story I'll focus on here.

* Date given by Evens & Klembara 2005. Other (older) sources (Storrs & Gower 1993, Gao & Fox 1998) confusingly give the date as 1876.

**DeBraga & Rieppel (1997) classify choristoderes as a sister group to the Archosauromorpha. Palaeos.com places them there as well, but don't exclude the possibility that they're closer to basal diapsids. Recent papers seem to avoid classification, perhaps for good reason.



Lazarussuchus

So when did choristoderes finally go extinct? In 1992 Lazarussuchus inexpectatus was described by Hecht (paper unavailable, dang) from France from the late Oligocene i.e. several millions of years later than the presumed Eocene extinction. But there's a problem Gao & Fox (1998) do not consider Lazarussuchus to be choristoderan! The authors define Choristodera as the last common ancestor of Cteniogenys and Champsosaurus and suggest that if Lazarussuchus is a sister group, another more inclusive name is needed for the new group. The authors also do not believe that choristoderes belong in the Archosauromorpha or Lepidosauromorpha and are possibly even outside of the Neodiapsida. I should mention that DeBraga and Rieppel's 1997 paper on reptile phylogeny is used frequently in these papers for clade definitions, but you can consult Palaeos.com for easier access and cheekier commentary. Back on subject, a position all the way out there would imply that Lazarussuchus belonged to a lineage that presumably evolved sometime in the Permian and didn't fossilize until over 200 million years later. Considering that freshwater animals should have a very good fossil record, this is a problem. It could be possible that they went through a marine phase, and it should be noted that gaps of lesser magnitude in freshwater animals are known (Storrs & Gower 1993). I haven't heard about any other ghost lineage of that magnitude, but of course I haven't read everything yet.

Before I write my own edition of Animal Facts and Feats, I should mention that the story of course does not end there. Susan E. Evans and Jozef Klembara described a new species of Lazarussuchus (L. dvoraki) from the early Miocene of the Czech republic in 2005. Differences between the taxa were slight, but if more of the skull and post-cranial skeleton is recovered, L. dvoraki may need to be reclassified (different genus?). These remains still complimented the previous species and helped resolve ambiguous damaged areas. Unlike Gao* and Fox the authors state that "the Merkur reptile is clearly a choristodere" although outside the Neochoristodera (including the more familiar Champsosaurs). Lazarussuchus shows a mixture of primitive and derived traits, and cladistic analysis in most cases puts it at the base of Choristodera. However, the authors also mention the possibility that since Lazarussuchus is so small (skull length 43-45 mm) it could have had a number of size-related character reversals. There is a lot of missing data from other basal choristoderes, so it looks like we'll have to wait to see what Lazarussuchus truly was.

* A later paper (2007) by Gao et al puts them as being choristoderan again. Evans was part of that "et al" interestingly enough.


Evans and Klembara finish up their paper by demonstrating that cooling and drying in the Eocene and Oligocene wiped out the large (neo-) choristoderes, leaving only this small branch. The next period of cooling occurred in the late Pliocene, so it is quite possible they survived until a couple million years ago. There are apparently other locations similar to Merkur and choristoderes in North America at this time have yet to be mentioned, so a lot about the final hour of this group can be untangled and learned from. If the final death of these extinction-busting super-stragglers was so close to the present, I suppose that can say a lot about where our planet is careening off to.

Since Evans' original paper covering the skeleton isn't available to me, I unfortunately can't try and make an illustration that wouldn't be horrendously speculative. Well, I could, but it wouldn't be much of a service to anyone. This little reptile probably looked like a moderately-sized aquatic lizard, nothing too exciting.



Strange Asian Choristoderes

While Lazarussuchus was conceptually interesting and conservative, Chinese fossils revealed that choristoderes could be surprisingly diverse. Champosaur-like Simoedosaurids (unpublished), bizarre Hyphalosaurids (keep reading), and another group all occupied the same area (Gao et al 2007b). It appears that all three were contemporary and occupied radically different niches.

The other group mentioned are the Monjurosuchids, which were surprisingly known since 1940. They were oddly classified as Rhynchocephalians in the family Sphenodontidae until 1995 without a firm basis (Gao et al 2007a). In other words, they were considered relatives of the tuatara. The original specimen was apparently lost in the Second World War, but Gao et al in 2000 described spectacular new specimens. They showed all of the diagnostic traits of choristoderes, but were clearly outside the familiar neochoristoderans. They were only about 30 cm (1 foot) in length with a tapering tail and webbed feet. The appearance was described as being surprisingly similar to Shinisaurus crocodilurus, the Chinese crocodile lizard which it probably resembled in niche as well. One spectacularly preserved specimen had a double row of ovoid keels which appear to resemble those on Shinisaurus. There was no discussion on what this curiously converged trait accomplished. Unlike the apparent scaly skin of the modern Shinisaurus, the choristodere had small scales (larger dorsally) and appeared to have fairly soft skin. The description of another genus (Phylidosaurus) in Gao et al 2007b illustrates this interesting pattern. There seems to be a trend of increasing number of life illustrations in these papers, which is a relief from the typical picture of a jumble of bones. Well, then again I'm not a worker on this and I'm more of the visual type.

When Philydrosaurus was described by (who else) Gao et al in 2005, there was a discussion of cladistics and the Monjurosuchids formed a tenuous clade with the enigmatic Hyphalosaurids (group defined for that analysis). Even if they aren't actually closely related, this group certainly is worth discussing. Two papers on the respective Chinese and Japanese genera are unavailable to me, so I'll have to make do. This group radically departs from the aquatic lizard, crocodile, and gavial-like bauplans then known from choristoderes; it has a long neck and superficially resembles a nothosaur (Gao et al 2000). I think that they resemble Plachypleurosaurs such as Keichousaurus even more strongly in size and shape despite one being marine and the other being freshwater. These choristoderes appear to have been fully aquatic and occurred in deeper-water environments (Gao et al 2007b). In the papers it isn't certain if choristoderes were aquatic or semi-aquatic, but the structure of this bizarre family probably would hinder them significantly on land. However, Hyphalosaurus was known to lay eggs (Gao et all 2007b). Hyphalosaurus recently made the news after a two headed juvenile was discovered, the oldest known case of polycephaly. This is an example of another article I'd like to read but again can't access.


I of course can't pretend that this is anywhere near a complete treatment of the choristoderes; there are of course 3 other families and some controversies (e.g. Pachystropheus) that I just barely touched upon. There's always the chance of there being another post, especially if more papers and discoveries are published. I think for now this illustrates the great potential for this poorly known group. Their ability to occupy surprisingly different niches is certainly something to look into, and doubtless more oddballs will show up. It would also be interesting to see if the ghost lineages can cut shorter, or if there are genuine limitations to the fossil record. Are there marine groups? Their potential super-late survival is of course most interesting to me. How does such an adaptive and resilient group go extinct? Unfortunately, I haven't heard even the slightest indication of strange little crocodilian/lizard-like still swimming about. Dang.




References:

These are unfortunately not freebies but only available to those of you fortunate enough with access. I'd like to thank Brown University for indirectly supplying me with this newfound ability. I am not certain how to cite the Chinese family name of one frequent author (Gao) which varies from paper to paper, so I'm tenuously putting it last i.e. the one being alphabetized. According to the vast knowledge of my father, Brits apparently do the whole thing in reverse. Just FYI.


DeBraga, Michael & Rieppel, Oliver. 1997. Reptile phylogeny and the interrelationships of turtles. Zoological Journal of the Linnean Society 120: 281-354. Available: Here

Evans, Susan E. & Klembara, Jozef. 2005. A choristoderan reptile (Reptilia: Diapsida) from the lower Miocene of Northwest Bohemia. Journal of Vertebrate Paleontology 25: 171-184. Available: Here

Gao, Keqin & Fox, Richard C. 1998. New choristoderes (Reptilia: Diapsida) from the Upper Cretaceous and Palaeocene, Alberta and Saskatchewan, Canada, and phylogenetic relationships of Choristodera. Zoological Journal of the Linnean Society 124: 303-354. Available: Here

Gao, Kequin & Evans, Susan & Ji, Qiang & Norell, Mark & Ji, Shu'an. 2000. Exceptional fossil material of a semi-aquatic reptile from China: The resolution of an enigma. Journal of Vertebrate Paleontology 20: 417-421. Available: Here

Gao, Ke-Qin & Fox, Richard C. A new choristodere (Reptilia: Diapsida) from the Cretaceous of western Liaoning Province, China, phylogenetic relationships of Monjurosuchidae. Zoological Journal of the Linnean Society145: 427-444. Available: Here

Gao, Ke-Qin & Li, Quanguo. 2007a. Osteology of Monjurosuchus splendens (Diapsida: Choristodera) based on a new specimen from the Lower Cretaceous of western Liaoning, China. Cretaceous Research 28: 261-271. Available: Here

Gao, Keqin & Ksepka, Danial & Lianhai, Hou & Dongyu, Hu. 2007b. Cranial morphology of an Early Cretaceous Monjurosuchid (Reptilia: Diapsida) from Liaoning Province of China and evolution of the choristoderan palate. Historical Biology 19: 215-224. Available: Here

Smith, J.B. & Harris, J.D. 2001. A taxonomic problem concerning two diapsid genera from the Lower Yixian Formation of Liaoning Province, China. Journal of Vertebrate Paleontology 21 (2): 389-391. Available: Here

Storrs, G.W. & Gower D. J. 1993. The earliest possible choristodere (Diapsida) and gaps in the fossil record of semi-aquatic reptiles. Journal of the Geological Society, London 150: 1103-1107. Available: Here

Addendum: Later the same day

Here's a little drawing of a fairly generalized Monjurosuchid (top) and Hyphalosaurid (bottom), approximately to scale. I could only find dorsal views of the fossils, so their lateral nature is speculative. The Monjurosuchid is based off of an illustration of Philydrosaurus with the proportions of Monjurosuchus from fossils. The Hyphalosaurid was imagined as being unarmored and flippered, and the lateral head view is an imagined adult form of this. For a sense of scale, it measures about 2 feet long. I think if you click on it, it should be around life sized...that was not intentional.