Thursday, November 17, 2016

From the Triassic Monster Manual

Alas, this is the biggest size I could find for this image.
We’ve already discussed some Triassic marine reptiles on this here blog: we talked about hupehsuchians HERE and HERE and then saurosphargids HERE and Atopodentatus HERE. Today, I want to talk about an emerging group of Triassic marine reptiles, one that only contains two genera (right now) and doesn’t even have an official name. I’m calling them "vancleaveans" and I expect full credit when that name is used later in actual paleontological literature.

Way back in the Year of Our Lord 1995, Long & Murry described a very fragmentary skeleton recovered from the famous Chinle Formation in Petrified Forest National Park. They named the animal after the man who collected the specimen in 1964: Philip Vancleave. Long & Murry believed this new critter, Vancleavea campi, was a terrestrial reptile of uncertain affinity but noted the tiny beast’s strange compliment of osteoderms which seemed comparable to those of thyreophoran dinosaurs. They write:

Vancleavea was a small quadruped with very primitive appendicular features at approximately the proterosuchian level. The hind limb elements and pelvis suggest a sprawling posture…the body appears to be have been rather long and slender and the hind limb elements were of delicate construction, though nearly solid. …the protective armature is more highly developed than that seen within primitive neodiapsids although the precise arrangement of the dermal armor cannot be reconstructed at present.
More specimens were described later. In 2002, Hunt et al. summarized the material referred to Vancleavea from numerous localities throughout the American southwest. Turns out it’s in Arizona, New Mexico, and Texas and is recognizable in all three states—mainly thanks to its characteristic osteoderms. In addition, they noted that Vancleavea is recovered from rocks spanning twenty million years. The authors also reference what I have to assume is a conference presentation by Small & Downs (also 2002) (no paper exists) that discusses a “Ghost Ranch” specimen and the idea that Vancleavea might be an aquatic animal due to its dorsally-facing nostrils, elongated body, short limbs, and tall “sculling tail.”

Well, isn’t that intriguing.  Hunt et al. are not totally convinced, however, noting that the taphonomic settings, faunal associations, and heavy body armor point to a terrestrial animal. “Ultimately,” they conclude, “there is a need for more study of the functional morphology of Vancleavea and its taphonomic settings.”

In 2005, Hunt, Lucas & Spielmann revisited Long & Murry’s holotype. They give it a proper description and again reference Small & Downs’ “Ghost Ranch specimen,” noting that its description will solve a lot of problems in trying to imagine its morphology and ecology. Hunt, Lucas & Spielmann’s pontificating of those topics, in their 2005 paper, is almost copied & pasted from Hunt et al. 2002. In fact, the whole paper is pretty much a retread of the 2002 effort, right down to summarizing the localities one finds Vancleavea in. There are, however, very nice photographs of the holotype material.

Bits & pieces of Vancleavea.
Finally, in 2008, we get the Ghost Ranch…wait, no, that’s still pending, sorry. In 2008, Parker & Barton publish some new material that includes a partial left dentary, some verts, and actually quite a few appendicular elements. Oh, and several osteoderms, of course. Importantly, however, the authors run Vancleavea through a phylogenetic matrix and it winds up in a polytomy with Doswelia, Chanaresuchus, Turfanosuchus, and Archosauria, but more derived (surprising) than Euparkeria.

At this point you’re probably thinking “Are they ever going to get around to describing this overhyped Ghost Ranch specimen?” And hey, I understand. In fact, I question why any of these authors bothered describing any material or making any presumptions about the animal’s morphology/ecology when they know this Ghost Ranch specimen—which answers all our questions—is out there, waiting for somebody to describe it. My assumptions are that (1) who knows when that will be; and (2) may as well report on new information in the meantime. As to the first point, I would remind you that Eoraptor was named (and described in a glorified press release) in 1993. It wasn’t fully—actually—described until 2013. That’s a gap of 20 years. It happens. I don’t know when that Ghost Ranch specimen was found, but it starts turning up by name in 2002 so the wait to describe it is maybe only…seven years.

Because in 2009, Nesbitt et al. published a big fat monograph detailing two, count ‘em, two beautiful specimens of Vancleavea campi, both of which are from the Ghost Ranch quarry.

Specimen GR 138
In a nutshell, Vancleavea is the kind of marine dragon you probably imagined and drew as a kid, possibly for your D&D campaigns. It has a long body, short limbs, and a deep tail. The tail is deep because of its elongated chevrons, sure, but also because it’s equipped with vertically-oriented osteoderms supporting (one assumes) a tail fin. Yes, osteoderms; not neural spines. In fact, Vancleavea is the only taxon to increase tail height in this manner.

Although small (about 4 inches long), the skull more or less exists primarily to support dentition that would terrify many full-sized dinosaurs. Oh, it just has two caniniform teeth on each side of its upper jaw and one gigantic one in its dentary. Nothing horrifying about that.

The skull of GR 138. Note the premaxillary fang is broken on this side (but intact on the other).
The entire body was covered in overlapping scales like some kind of reptilian pangolin. Scales of different sizes and shapes adorned different parts of the body. This is the complete opposite of other marine reptiles like mosasaurs, ichthyosaurs, and plesiosaurs, who shrank their scales to the point of maybe not having any in those last two groups. Not Vancleavea, though: he bumped his scale armor compliment up to 11.  And this isn’t without precedent. As Nesbitt et al. note, plenty of marine or aquatic reptiles have osteoderm coverings: turtles (obviously), but also phytosaurs, placodonts, and crocodilians are extensively armored. Just maybe not to this ridiculous extent.

Scales of the belly (A), neck (B), top of the tail (C), and side of the tail (D). 
Nesbitt et al. hypothesize that Vancleavea was a semi-aquatic lunge predator. It would have moved through the water like a marine iguana, undulating its body and tail to propel it forward. Its femur is pachyosteosclerotic, basically meaning heavy, with a reduced marrow cavity. This is a common strategy in aquatic animals for dealing with buoyancy (also present in—surprise—Spinosaurus). Vancleavea’s nostrils opened upward, not to each side, which is a feature only seen in aquatic animals (consider crocodilians and, to a much greater degree, whales).

Vancleavea skeletal, minus body-covering osteoderms. Note the tail-supporting osteoderms on the tail.
Finally, to Hunt et al. (2002) and Hunt, Lucas, and Spielmann’s (2005) point about taphonomy and faunal associations, Nesbitt et al. point out that every Vancleavea locality indicates a fluvial floodplain. Some of the localities even preserve bones of lungfish and other fish remains. So while terrestrial animals are found in these localities (drepanosaurids, rauisuchians, etc.) they may not have been permanent residents or particularly indicative faunal components.

One other thing that Nesbitt et al. do is a little cleanup: regarding the question as to whether all the Vancleavea material throughout the southwest really does belong to V. campi, the authors conclude that:

It is unclear whether the differences in these specimens are of systematic or ontogenetic importance when compared to the smallest specimens (GR 138 and GR 139). We conclude that the present evidence is ambiguous with regard to whether differences among the specimens are ontogenetic or taxonomic. Therefore, we refer all of the Vancleavea-like specimens to V. campi.
This is somewhat surprising given that Vancleavea apparently lasted twenty million years (Hunt et al. 2002) but when results are ambiguous, I agree that caution is the best approach. But it does raise a question—does Vancleavea exist in a vacuum? Who are its ancestors or even closest relatives? Nesbitt et al. ran a new phylogenetic analysis based on their excellent new specimens and found that Vancleavea is more derived than Erythrosuchus but basal to Chanaresuchus and Tropidosuchus, who themselves are basal to Euparkeria. So everybody's still an archosauriform. Still, that doesn’t tell us much about its immediate cousins. There’s a pretty big morphological gulf between Erythrosuchus and Vancleavea.

Well, it took seven more years, but there’s finally another…vancleavean in town.

The paper (by Li et al. 2016) doesn’t say when this new guy was unearthed, but DAMN is it a nice-looking fossil. Litorosuchus somnii is from (reads directly from the paper) "the Zhuganpo Member of the Falang Formation in Fuyuan County, Yunnan Province." Interesting, plenty of other marine reptiles are found there, like nothosaurs, placodonts, thalattosaurs, and others. This in itself is an important differentiator: while Vancleavea favored freshwater habitats, Litorosuchus is decidedly near-shore marine.

Litorosuchus holotype. Notice the long, deep tail and long neck.
 Approximately two meters long, Litorosuchus is larger than the Ghost Ranch specimens of Vancleavea and differs in important ways. The animal is proportionately longer and more slender, with a coelophysoid-like skull complete with premaxillary “notch” and a tail that just doesn’t quit.

Seriously, the animal is like half tail. The limbs are beefier than in Vancleavea—this was clearly an animal that would have been more comfortable on land, but it had webbed toes (and probably webbed fingers) like a crocodilian. Like its North American cousin, Litorosuchus is covered in osteoderms—four of which are uniquely shaped compared to Vancleavea. While there are osteoderms along the top of the tail, they are triangular, not wedge-shaped and do not form a “tail fin” as in the Ghost Ranch specimens. Rather, the tail looks something like a crocodile’s tail.

Line drawing of the skeleton (blue areas are osteoderms) and reconstructed skeleton, minus body osteoderms. Note the differences in proportions between Litorosuchus and Vancleavea.
The skull is definitely odd. It’s longer than Vancleavea’s, of course, but there are actually three caniniform teeth in the upper jaw and one big on at the front of the dentary. The skull actually looks (superficially) a whole lot like that of Spinosaurus, including an elongated notch between the maxilla and premaxilla. As in Vancleavea, the nostrils open upwards, not sideways.

The very toothy skull of Litorosuchus, which looks weirdly spinosaurian.
Taking the aquatic adaptations displayed by Vancleavea and Litorosuchus, the authors turn to other archosauriforms that have similar characters, which may indicate similar habits: certainly Qianosuchus and Diandongosuchus, two other Chinese critters, fit the bill. One specimen of the latter actually has fish vertebrae in its belly, as well as an undescribed specimen with a small marine reptile in there. Certainly, these aquatic archosauriforms were doing something right—as stated before, vancleaveans (heck, V. campi by itself) were around for the majority of the Triassic. 

It will be interesting to see if more vancleavean taxa are found in the coming years, and exactly how diverse they were. If they conquered both freshwater and marine habitats, I expect that vancleaveans were far more diverse than we currently know. The purpose of this post was to highlight them--like so many Triassic hellasaurs, these critters don't get as much time in then spotlight as they deserve.

Now if you’ll excuse me, I have a D&D game to run.

2 comments:

  1. beaut of an animal...makes me think of a sea snake for some reason.

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  2. The Ghost Ranch Vancleavea specimen was excavated in 1982 with Coelophysis material (Rinehart et al., 2009), and first reported as a choristodere in an SVP abstract by Downs and Davidge (1997). Small and Downs (2002) is also an SVP abstract.

    Note Ezcurra's (2016) archosauromorph analysis recovered Vancleavea as a proterochampsian (10 more steps needed to move it to Nesbitt et al.'s position). I'd like to see Litorosuchus added to that analysis.

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