Monday, July 17, 2017

The Saurosphargid That Wasn't

Saurosphargid by Ethan Kocak
You might all remember that, some time ago, I introduced you all to an obscure group of Triassic marine reptiles called saurosphargids. This strange group of tubby critters includes just three genera: Saurosphargis, Largocephalosaurus, and Sinosaurosphargis. Confusingly, Nosotti & Reippel (2003) named and described Eusaurosphargis under the assumption that the original genus—Saurosphargis—was a nomen dubium as the original (and only) specimen of that taxon was destroyed during WWII.* Nosotti & Reippel named it Eusaurosphargis even though—according to their own analysis—Eusaurosphargis was not a saurosphargid, but a sister taxon to Hovasaurus, a very different marine reptile from the Permian. They could not decide whether Eusaurosphargis was aquatic or terrestrial, but since it was routinely found to be closely allied to Triassic marine reptiles, though, a life aquatic was the more obvious choice.


Now we seem to have a definitive answer, although it’s not what you’d expect.

Scheyer et al. (2017) recently described a complete juvenile specimen of Eusaurosphargis from Switzerland. It’s a tiny, short-tailed, fat little critter that reminds me of a horny toad (Phrynosoma). It’s also very clearly terrestrial.

Notice the shingle-like osteoderms along the sides of the body.
Notably, Eusaurosphargis has several rows of osteoderms along its body. There’s a central row of small osteoderms which reaches from the neck to the tail, another row of small osteoderms on either side of the central row, and a lateral row of much larger, overlapping, single-shaped osteoderms on either side of the body, between the fore- and hindlimbs. More osteoderms cover the forelimbs and the pelvis was also covered with small plate-like osteoderms. This arrangement reminds me of Largocephalosaurus qianensis, really, although that animal lacks the shingle-shaped lateral osteoderms.

Fantastic reconstruction by Beat Scheffold
The authors compare the (disarticulated) skull of Eusaurosphargis to basal placodonts like Palatodonta and Paraplacodus. The juvenile’s ischium is apparently similar to Pararcus (a placodont) and Largocephalosaurus qianensis; however, the presumably adult holotype’s ischium has a very different shape. After some discussion on the pros and cons of running phylogenies of Triassic marine reptiles with or without obvious marine adaptations, Scheyer et al. find Eusaurosphargis as the sister taxon to (Placodontiformes + Sauropterygia). Helvicosaurus, thalattosaurs, and ichthyopterygians form successive outgroups of this relationship.

But wait, you say, Eusaurosphargis is terrestrial, but it’s surrounded by fully marine taxa. This incongruity does not go unnoticed by the authors, who write that the “inferred terrestrial lifestyle of E. dalsassoi…would thus represent a reversal from aquatic habitats rather than retention of an ancestral terrestrial condition.”

Well, I mean, that’s not the only option. Look at your outgroups—ichthyopterygians and thalattosaurs almost certainly took to the water independently. Aside from its crazy tail, Hovasaurus looks perfectly at ease both in and out of the water. Why assume that your marine superclade families were all ancestrally marine to begin with? I think it’s also interesting that Eusaurosphargis turns out to be the sister group of placodontiformes (which are armored) and sauropterygia, since saurosphargids are also armored. Is it possible that Eusaurosphargis simply represents the terrestrial ancestor of (Placodontiformes + Sauropterygia)?

So there you have it. Eusaurosphargis is not a saurosphargid and also isn’t aquatic.


*Hilariously, Scheyer et al. agree with Nosotti & Reippel that ‘Saurosphargis’ is a nomen dubium, and for largely the same reasons. They disagree with Li et al. (2011) that Huene’s 1936 description is sufficient to hang on to the genus, instead alleging that the description and its accompanying photographs lack any diagnostic characters. If accepted, that means “Saurosphargidae” must be abandoned as well, right?

2 comments:

  1. Saurosphargidae is defined as whatever family contains Saurosphargis. Is Saurosphargis undiagnostic even at the "family" "level", or is it only undiagnostic at the "genus" "level" but it can still be determined which "family" it belonged to?

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  2. "If accepted, that means “Saurosphargidae” must be abandoned as well, right?"

    No, no it does not. That's a myth started by Wilson and Upchurch (2003) who stated "the genus Titanosaurus
    is invalid and co-ordinate suprageneric Linnean
    taxa must likewise be abandoned" without justification from the ICZN. Indeed, you can read the ICZN and find it never states an indeterminate genus cannot have eponymous family-level taxa. Unfortunately, some in the dinosaur community have fallen for this, and used it as a rationale for changing the type or type species of e.g. Stegosaurus, Allosaurus, Diplodocus.

    Like Marjanovic says above, it all depends on whether the genus is a member of the family in question.

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