Monday, February 29, 2016

Tubby, Armored Sea Lizards of the Triassic

You may not remember, but I've briefly mentioned Saurosphargids before. In my hupehsuchian primer (that old chestnut), I tossed their name into the list of Triassic reptiles that were trying to make a name for themselves in a marine environment--perhaps to avoid becoming dinner for such the vicious rauisuchian pseudosuchians that were prowling the terrestrial environments. After doing some research, it turns out they are obscure to a fault--nobody's heard of them and there appear to be only four technical papers devoted to them. This should be an easy one, folks! Strap in and enjoy the ride. And stick around 'til the end for some fantastic art from Ethan freakin' Kocak of "The Black Mudpuppy" fame.

This Middle Triassic group has an embattled history that I’ll cover here to get it out of the way: Saurosphargidae takes its name from Saurosphargis volzi, named by Huene (1936) in a paper that was really more about Henodus, another animal that’s on the “Primer” docket. Unfortunately, the holotype of Saurosphargis was destroyed during World War II and the only record of its existence was in Huene’s 1936 figures. Almost seventy year later, Nosotti & Rieppel (2003) took this to mean that Saurosphargis was a nomen dubium and erected Eosaurosphargis dalsassoi from the Southern Alps, based on supposed similarities between that animal and the figures in Huene’s original paper.

As it turns out, there’s more Saurosphargis material known from the Netherlands—it just hasn’t been described. Furthermore, Eusaurosphargis isn’t a saurosphargid; it’s a helveticosaur. Some years later, Li et al. (2011) described Sinosaurosphargis as a saurosphargid by, again, comparing their new animal to Huene’s figures, but they also took the important step of resurrecting Saurosphargis as a perfectly legitimate taxon and anchor of the family (the holotype of Spinosaurus was also destroyed but the genus didn’t fall into disuse).

And then there’s Largocephalosaurus polycarpon, a fragmentary animal described as a eusauropterygian (that is, not a saurosphargid) by Chen et al (2012). Well, that is until Li et al. (2015) described a second species based on better remains, Largocephalosaurus qianensis, which turned out to be a saurosphargid after all!

So, to summarize: Saurosphargidae is made up of Saurosphargis volzi, Sinosaurosphargis yunguiensis, Largocephalosaurus polycarpon, and L. qianensis. You’ve probably never heard of any of them. To call this group “obscure” would be a compliment.

Pelvis and limbs of Largocephalosaurus quianensis (GMPKU-P-1532-B): (a) Pelvic region, showing large number of osteoderms; (b) Right forelimb; (c) Left foot; (d) Right hindlimb.  

These are smallish, fat-bodied, armored animals with pachypleurosaur-like heads and flipper-like limbs. Not all genera are armored—Saurosphargis may lack armor, while Largocephalosaurus has several distinct rows of osteoderms along its body, and Sinosaurosphargis has a full carpace made of thousands of tiny osteoderms, as in derived placodonts, which even extend onto the limbs. Our picture of saurosphargids has radically improved in the last few years. As I said, the original Saurosphargis specimen was destroyed, but really only preserved some vertebrae and ribs. The key, however, is that the ribs were laterally expanded like those of turtles or hupehsuchians. In 1936, Huene was reminded of sea turtles (hence the name) but hupehsuchians were unknown at the time.

When Sinosaurosphargis was described in 2011, it was the expanded ribs that made the connection to Huene. Sinosaurosphargis is known from three specimens, each preserving a different section, or view, of the body. Aside from its impressive body armor, Sinosaurosphargis has relatively short limbs, very broad ribs (which contact each other) and matching expanded gastral ribs, enclosing the body in a “rib basket,” as the authors write. Hupesuchian researchers would say a “bony body tube.” I think the comparison to an actual turtle shell might be more appropriate here, skeletally anyway, because Sinosaurosphargis (and its cousins) would have had immobile torsos.

This probably means they had to solve the problem of how to actually breathe without intercostal muscles, just like turtles and (I assume) hupehsuchians did.

Type specimen of L. quianensis (IVPP-V-15638) in ventral view. 

Largocephalosaurus polycarpon was described in 2012, not as a saurosphargid, but a relative of Wumengosaurus. Oddly enough, the authors (Chen et al.) don’t include saurosphargids in their analysis at all, despite the existence of Sinosaurosphargis. The only known specimen of L. polycarpon comprises the skull in lateral view, the upper third or so of the trunk (in dorsal view) and the left forelimb—which is quite long. There doesn’t appear to be any armor present, but the specimen is incompletely prepared, poorly photographed, and very poorly figured. 

The figures are so poor and low-resolution that my efforts to duplicate and enlarge the images for this blog post proved a fool's errand. So you're not getting L. polycarpon here. Li et al. (2013) report that their own examination of the L. polycarpon specimen has revealed osteoderms and a “rib-basket.” They are prepping a “full description” of the species.

Referred specimen of L. quianensis in dorsal view.
In the meantime, we do have Largocephalosaurus quianensis, described by Li et al. in 2013. It is known from three gorgeous specimens that paint a more or less complete picture of a saurosphargid—the best look we currently have of one of these animals. One specimen in particular (IVPP-V-15638) provides virtually the entire skeleton in ventral view. Another specimen (GMPKU-P-01532) provides the skeleton in dorsal view. Largocephalosaurus is a barrel-chested marine reptile with a relatively short neck, long forelimbs, and seemingly shorter hindlimbs. The tail is not particularly long, but is wide at the base. Its largest osteoderms run down the spine, while smaller, “granular” osteoderms are arranged “in lines or rows on the dorsal surface of the trunk” except for near the pelvis and tail, where they are arranged in sheets.

 Skull and mandible of L. quianensis (IVPP-V-1563) in dorsal view

I asked Ethan Kocak—who writes and illustrates my third or fourth-favorite “Aztek-god-meets-salamander” webcomic (it’s a crowded field) to illustrate Largocephalosaurus and he came up with this brilliant image of a bottom-walking critter. Perhaps, like basal turtles, saurosphargids expanded their ribcages and gastralia for ballast. Osteoderms may have also helped in this effort.

Largocephalosaurus as a bottom-walked by Ethan Kocak.
The osteoderms of Largocephalosaurus do not, however, collectively form the carpace-like structure seen in Sinosaurosphargis.

That carpace-like structure, by the way, is impressive in Sinosaurosphargis. It appears to cover the entire back and the osteoderms even extend to the upper arms. Osteoderm covering of this type—thousands of tiny bones forming an armor covering—is actually pretty common among animals both terrestrial and marine. Among landlubbers, the armadillos, glyptodonts and pampatheres* achieved their ridiculous body-enveloping shells the same way. Turtles probably went through a similar process, but their shells are made up of far fewer, much larger, osteoderms. The closest analogue to Sinosaurosphargis, however, is another group of Triassic marine reptiles: derived placodonts.

Type specimen of Sinosaurosphargis yunguiensis (IVPP-V-17040) in dorsal view.

We’ll discuss placodonts in detail in another post, but this is yet another marine clade that developed increasingly-complex protection. Early representatives were barrel-bodied shellfish-crunchers without extensive armor, but later examples (like Cyamodus and Placochelys) had turtle-like carpaces made up of hundreds of smaller and larger osteoderms. In one subfamily, the “shells” were divided between the upper body and pelvis, while in another subfamily, a single squarish shell covered the entire back. Sinosaurosphargis appears to mimic the latter condition.

Ribs and gastralia of the paratype of S. yunguiensis (IVPP-V-16076): (b) Dorsal verts & ribs in ventral view; (c) gastral ribs--note they are bordered by tiny osteoderms.

But the similarity with turtles, in both cases, is merely convergent. The rib basket thing, however, is intriguing, and Li et al. (2011) ran a couple of phylogenetic analyses just to see if saurosphargids ever grouped with turtles, but they never did. Instead, saurosphargids grouped with thalattosauriformes and possibly ichythopterygians (which include hupehsuchians).

Skull and mandible of the type specimen of S. yunguiensis.

Ethan took pen to paper once more to illustrate Sinosaurosphargis as more water-adapted than Largocephalosaurus—the shorter neck, shorter limbs, and extensive osteoderm armor all point to a critter more at home under the water than out of it. I approve of his interpretation of the carpace essentially being part of the soft tissue rather than extending beyond the soft tissue as in placodonts. But honestly, there haven’t been a lot of restorations of saurosphargids—this is pretty much an open field. I do expect his illustrations to become embedded paleoart memes, though.

Illustration of Sinosaurosphargis yunguiensis interpreted as dedicated swimmer by Ethan Kocak.

So if they’re not turtle relatives, what are they? It’s a question Li et al. (2013) set out to answer. Based on their beautiful Sinosaurosphargis and Largocephalosaurus material, the authors found that saurosphargids form an immediate outgroup to the Sauropterygia, no muss, no fuss. This gels nicely with Li et al. (2011).

So there you have it, kids. Saurosphargids—amaze your friends and befuddle your enemies!

Oh, and I can’t leave without this third picture from Ethan: a derpy version of Largocephalosaurus because, I mean, look at that thing. I asked Ethan for these pictures specifically because he’s good at portraying derpy sauropsids.

Largocephalosaurus as a derpy half-wit by Ethan Kocak.

By the way, I fully endorse the inevitable Cartoon Network show that develops from this illustration, featuring all manner of cartoony Triassic marine reptiles. This character could be Derpy Flippers or something. Make it a cross between My Little Pony: Friendship is Magic and SpongeBob Squarepants.

You’re welcome.

Big thanks to Ethan Kocak for the illustrations. Read his comic! It's being rebooted in April and it's really good!

*Exciting aside: a new study shows that glyptodonts (and, presumably, pampatheres) are actually crown-group armadillos, not a sister group to armadillos! How cool is that?

References (there aren't many!)

Li, Chun; Olivier Rieppel; Xiao-Chun Wu; Li-Jun Zhao; Li-Ting Wang (2011). A new Triassic marine reptile from southwestern China. Journal of Vertebrate Paleontology 31(2): 303–312.

Chun Li, Da-Yong Jiang, Long Cheng, Xiao-Chun Wu and Olivier Rieppel (2014). A new species of Largocephalosaurus (Diapsida: Saurosphargidae), with implications for the morphological diversity and phylogeny of the groupGeological Magazine 151(1): 100–120.

Long Cheng, Xiaohong Chen, Xiongwei Zeng and Yongjian Cai (2012). A new eosauropterygian (Diapsida: Sauropterygia) from the Middle Triassic of Luoping, Yunnan ProvinceJournal of Earth Science 23(1): 33–40.

No comments:

Post a Comment