An Update on Stem Turtles

The beaky noggin of Eorhynchochelys by...IVPP?

Couple things I need to talk about. First, it's been a very long time since I've blogged, which was not my intention (it never is). I hit a rough patch of writer's block, which was followed up by a 9-day stint at the hospital where I got a CF-related tune-up. Taxonomy Tuesday is not proving to be the rich well of inspiration I was hoping for, so while I still intend to write up Taxonomy Tuesdays, they almost certainly won't be weekly. Now then, right before I was admitted, I wrote this short post about a new stem turtle. I wrote about turtle origins way back in 2015 and I'm always excited when a new one pops up. Thankfully, in addition to this post, I'm halfway through essays about mesosaurs and tanystropheids, so regular blogging will commence soon.

In my previous stem-turtle post, we went through the basalmost stem-turtles: Eunotosaurus, Pappochelys, and Odontochelys. Since that post, a full description of Pappochelys has seen print (Schoch & Sues, 2017) that confirms its diapsid status. It also has a large, and ventrally-open, lower temporal fenestrae like lizards and thalattosaurs. To be fair, Eunotosaurus also has this condition but it’s constructed a bit differently. Here’s their reconstruction of the whole skeleton of Pappochelys:

Cool, right? In particular, the gastralia are starting to take on a larger role, and the enlarged interclavicle doesn’t look all that different from what you see on the anterior end of normal turtle plastron. Notably, it doesn’t seem to have any overtly aquatic adaptations, although Schoch & Sues (2015) suggest that its enlarged ribs and gastralia were adaptations to provide ballast in an aquatic setting. It is apparently the most common fossil reptiles in the Velberg lake deposit, “which suggests that it either lived along the lakeshore or frequently entered the lake.” 

Lyson etal. (2016) write that Pappochelys is found in lacustrine sediments “associated with fully terrestrial animals” while Eunotosaurus, is apparently commonly found in terrestrial floodplain deposits, so perhaps stem turtles were primarily terrestrial but not above foraging underwater from time to time. In this scenario, Odontochelys would be an early pioneer into the marine sphere.

A NEW CHALLENGER APPROACHES

SMMP 000016, aka Eorhychochelys sinensis

This week saw the publication of another stem-turtle, Eorhynchochelys sinensis (Li et al., 2018), from the lower Carnian of China’s Guizhou Province. Interestingly, it comes from beds approximately 7.5 meters below where they found Odontochelys. It’s the earliest turtle with a beak, although that beak is restricted to the tips of the upper and lower jaws.

Unlike its cousins, Eorhynchochelys is enormous, about 2.3 meters long (7.5 feet). Aside from its very wide body, this stem-turtle has a surprisingly long tail, a short neck, and a bafflingly small skull—just 9 centimeters long (about 3.5 inches). This reminds me of the body-to-head proportions of Cotylorhynchus, the pin-headed caseid. As I said, the small beak is restricted to the premaxillae and anterior ends of the dentaries. The teeth are similar to those in Pappochelys and Odontochelys. Note that Odontochelys is younger than Eorhynchochelys but does not have a beak. Li et al. write the fully-toothed jaw of Odontochelys is likely a reversal, as is Eorhynchochelys having twelve dorsal vertebrae (every other stem turtle has nine). Like in every other clade, there seems to be a good deal of homoplasty at the base of “Pantestudines” (a clade name I really don’t like).

The hilariously small skull of Eorhychochelys (snout pointing up)

Eorhynchochelys is also the earliest stem-turtle with a “puboischiadic plate,” a turtle feature which represents yet another wholesale rearrangement of part of the skeleton to deal with having an oversized, rigid torso. It does not, apparently have something called a “separate hypoischium” which is present in Odontochelys and Proganochelys.

The authors note that Eorhynchochelys’ broad ribs, stout limbs, and large claws point to a fossorial ecology, and that it was most likely “predominantly terrestrial.” However, the fact that it occurs in marine sediments and has a separated astragalus & calcaneum in adulthood suggests at least some aquatic adaptation. Merely being found in marine sediment doesn’t mean that Eorhynchochelys was primarily marine, of course. As the authors state:

The new form may have been an inhabitant of coastal waters foraging on land as well as in the water, searching the mud along the shore using its powerful limbs in a way that many living pond turtles also do.

In fact, in the supplementary information, Li et al. state that the skeleton was probably transported from coastal waters to its final resting place by currents and “does not represent” part of the local ecosystem.

Image credit: IVPP again (do they just not credit artists over there?)

So of course every piece of paleoart I’ve seen that accompanies popular science articles about Eorhynchochelys shows it as a marine animal, swimming around as though that’s obviously what it was adapted for. I have yet to see a piece of paleoart depicting it as a beach-combing digger, which is disappointing. In fact, this excellent picture of Eunotosaurus by Andrey Atuchin (from 2016) is the only stem-turtle restoration I’ve seen which advocates a fossorial ecology. It bothers me.

Maybe it's digging its way towards the sea.

It still boggles my mind to consider that our knowledge of turtle origins has exploded in the last 10 years. Prior to 2008 (when Odontochelys was described), Proganochelys was still the best-known fossil turtle, and turtles were a relic group of anapsids related to pareiasaurs or millerettids (molecular evidence notwithstanding). Welcome to the club, Eorhychochelys--I have no doubt that more of your cousins are waiting in the ground or in museum collections to be found and described.