Monday, April 24, 2017

Thalattosaurs: Doin' Things With Snouts

The holotype of Thalattosaurus
When I started this humble blog in (dear lord) late 2014, I did not intend for it to becomes a blog dedicated to Triassic hellasaurs, but here we are. The more I read about the Triassic, the weirder it gets. For example, it seems like half of the reptiles alive during the Late Triassic were marine or at least semi-aquatic. We’re already covered hupehsuchians (twice) and those wonderful but woefully obscure saurosphargids. There was that recent post about underappreciated vancleaveans. Placodonts will come later and will probably spread across several posts—it was a big group. Today we’ll be talking about a diverse, well-known family of Triassic marine reptiles that still aren't well-known outside the paleo community: thalattosaurs.

Thalattosaurs were around during the Middle and Late Triassic, with a wide distribution that currently favors the Northern Hemisphere: Alaska, California, Nevada, British Columbia, Italy, Switzerland, Austria, and China. Particularly in China, thalattosaurs made up a significant portion of the fauna in successive marine formations, where they are present in the Xingyi (Middle Triassic, Landian) and Guanlling (Late Triassic, Carnian). These animals were anguilliform swimmers, using lateral undulations of the body, combined with a deep, laterally-flattened tail, to move through the water like marine iguanas.

Askeptosaurus skeleton

Askeptosaurus skeletal restoration from Müller (2005)
Thalattosauria (or Thalattosauriformes) was split into two large clades. The more plesiomorphic of these two were the Askeptosauroidea. Although there are currently only four askeptosauroid genera known, they demonstrate considerable disparity in terms of skull shape and dentition. Askeptosaurus had a long snout filled with sharp, recurved teeth. Anshunsaurus was similar but had a shorter snout and fewer teeth in the maxilla. Miodentosaurus differed with an even shorter, more robust, snout. Endennasaurus, while superficially similar to its cousins, has a narrow, toothless snout—it must have been limited to eating soft-bodied prey. Further, Endennasaurus appears to have been more strongly aquatic than its cousins, showing a stiffer trunk and thicker bones for ballast.


Miodentosaurus

Anshunsaurus
The remaining thalattosaurs comprised the Thalattosauroidea. These thalattosaurs were better adapted to a marine existence, with stiffer trunks and shorter legs, and started doing some very strange things with their snouts. Unlike the more generalized diets of askeptosauroids, thalattosauroids trended toward a diet of hard-shelled prey (durophagy).

I won’t discuss the fragmentary Agkistrognathus or Paralonectes except to note that, aside from the unpublished Alaskan taxon, they are the most northerly-known thalattosaurs. The remaining thalattosauroids are quite well known. Xinpusaurus was a Chinese genus that contained between one and four species, depending on which author you’re reading (more on this in a minute). Unlike the relatively straight, no-nonsense snouts of the askeptosauroids, Xinpusaurus had a snout featuring a considerable notch between the maxilla and premaxilla that was occupied by sizable triangular teeth. In some specimens of Xinpusaurus, the premaxillae extended forward into an exaggerated point like that of a swordfish. Close relative Concavispina had a similar skull, if a bit more robustly constructed. It also has weirdly notched neural spines (that it shares with Xinpusaurus xingyiensis). These animals had sharp teeth in the front of the jaws and rounded, crushing teeth in the back.

Endennasaurus
Concavispina; note how ridiculously long its tail is.
Xinpusaurus presents an interesting taxonomic entanglement, actually. There are several specimens known, divided into four species: X. suni (2000; initially described as a cymbospondylid ichthyosaur), X. bamaoliensis (2003), X. kohi, (2004), and X. xiangyiensis (2016). The skulls assigned to X. suni have a relatively short, blunt-tipped snout (see comparison images below) but all other species an have elongated, spear-like snout that extend far beyond the front of the dentary. Is the spear simply missing in X. suni specimens? Perhaps it has to do with ontogeny? How different are any of these species? Liu (2013) writes that the tip of the snout is  not completely preserved in many Xinpusaurus specimens.

Where the spear exists, Liu writes that "the snout is relatively longer in smaller specimens and varies in length in specimens. Therefore, the size of the snout cannot be used as a good diagnostic character." After discussing other potential points of differentiation between the species, Liu concludes that X. kohi and X. bamaoliensis are junior synonyms of X. suni.

Xinpusaurus suni
Maisch (2014) described the type of X. kohi after it had been more completely prepared. He concurred with Liu on the uselessness of the snout re: taxonomy. However, he notes that snout length was not used to differentiate X. kohi from X. suni in the former's description. For other reasons (among them Chen's careless 2003 description of X. bamaolinensis), Maisch supports sinking X. kohi into X. bamaolinensis, but a full description of the latter will be necessary before that proposal can be solidified. However, Maisch does not find support for synonymizing X. kohi and X. suni and considers both to be valid species of Xinpusaurus.

Xinpusaurus kohi; note the (broken) toothless spear-snout
Most recently, Li et al. (2016), in their description of X. xingyiensis, agreed with Maisch's interpretation and consider X. bamaolinensis as a "species inquirendae" for the time being. Interestingly, X. xingyiensis' spear-nose is equipped with at least six small teeth at the anterior end, a feature that distinguishes it from the edentulous spear of X. kohi and X. suni (although I might be wrong about that--Li et al. don't mention it as a differentiating character). I also note that in Li et al.'s phylogeny, X. kohi and X. bamaolinensis are sister taxa, which may support their synonymy.

Xinpusaurus xiangyiensis (note the teeth at the end of the spear-snout)
I still question whether the original specimens of X. suni have incomplete snouts. The ends of their snouts are rounded, not obviously broken-off (as is the case in the type of X. kohi). If it's not a good taxonomic indicator, per Liu (2013), could it be ontogenetic?

Thalattosaurus skull restoration (Nicholls 1999)
The rest of the thalattosauroids are informally called “claraziids” after namesake weirdo Clarazia. These animals differed mainly in respect to how downwardly-deflected their snouts are. Thalattosaurus and Clarazia were the more “normal” side of the spectrum, with robust dentaries and reduced dentition. The teeth at the front of their snout were small and sharp. In Thalattosaurus, the “teeth” at the front of the snout were not teeth at all, but hook-like extensions of the premaxillary bones. A distinct diastema separated the premaxillary and maxillary teeth. The maxillary teeth were rounded, as were the teeth in the vomers. They must have been plucking shellfish with their hooked snouts and crushing them with their back teeth. 

Things get significantly stranger with Hescheleria and Nectosaurus: their snouts appeared to be deflected strongly downward, like the original interpretation of Atopodentatus, but without the hare-lip. The dentary symphysis was also equipped with a large bony “tooth,” the purpose of which is unknown. Hescheleria had similar dentition to Thalattosaurus and Clarazia. In Nectosaurus, however, the maxillary teeth were more numerous and pointy. Despite sharing a downturned snout, Hescheleria and Nectosaurus must have been eating different things.

Hescheleria skull reconstruction, from Rieppel et al. 2005.
Note the downturned snout and giant dentary "tooth."
The group has a rather interesting taxonomic history (as with most extinct animals). John C. Merriam named the Thalattosauria in 1905, basing the family on two California taxa, Thalattosaurus and Nectosaurus. Clarazia and Hescheleria, both from Monte San Giorgio in Switzerland, were described by Bernhard Peyer as strange marine reptiles of "uncertain affinity" in 1936.

In fact, thalattosaurs have been of uncertain affinity throughout most of their history. While many authors considered them to be diapsids (Benton, 1985; Evans, 1984, 1988; Rieppel, 1987, 1998) nobody was quite sure of their relationships to any other diapsid group. Kuhn-Schnyder (1988) disputed the diapsid idea, mainly because thalattosaurs don’t have upper temporal fenestrae.

In 1999, Elizabeth Nicholls re-described Thalattosaurus and Nectosaurus and assigned them to Diapsida, saying only that “Thalattosaurs are clearly diapsids as defined by most recent workers,” although no specific characters are given. She also examined some other thalattosaurs and reclassified as needed (Thalattosaurus shastensis, for example, was moved into Nectosaurus; Thalattosaurus perrini was sunk as a nomen dubium, etc.). Thalattosaurs secondarily ditched their upper temporal fenestrae by greatly reducing the size of the squamosals and postorbitals. The result is that the parietal is the only significant bone remaining. 

As Rieppel (1987) writes, “Posteriorly, the parietal skull table forms prominent posterolateral (supratemporal) processes, resulting in a deep posterior emargination of the skull roof.”

There’s been a lot of hot descriptive thalattosaur action since 1999, and most authors have issued phylogenetic hypotheses of thalattosaurs as a group but very few have discussed thalattosaurs in the broader context of diapsids. In their redescription of Anshunsaurus huangguoshuensis, Liu & Rieppel (2005) included Petrolacosaurus, Youngina, and Prolacerta as outgroup taxa. As Müller (2005) notes in his description of Askeptosaurus italicus:

“…a problem of any hypothesis on the…origin of thalattosaurs is that there is currently no consensus on the identity of their sister group. In recent investigations on diapsid phylogeny (Müller 2003, 2004), there was some indication that ichthyopterygians might be the closest relatives of thalattosaurs.”

If you’re curious, that indication amounts to two features shared between thalattosaurs and ichthyopterygians: (1) an elongated, sharp snout formed almost exclusively by the premaxilla; and (2) tall, slender vertebrae whose rib articulation facets are often ventrolaterally positioned.

In 2014, Chen et al. & Motani et al. found support for Müller’s idea that thalattosaurs and ichthyopterygians form a clade, but it seemed weakly supported—thalattosaurs moved around depending whether strictly marine adaptations were included in the dataset or not. It is somewhat alarming that, a hundred and ten years after the group was named, the best we’ve got is “I dunno, maybe they’re related to ichthyosaurs?” And it’s not like thalattosaurs are only known from scrappy remains—almost every taxon has a virtually complete, well-preserved skeleton. However, as a friend of mine reminded me, what we really need is a large, monograph-type work that summarizes and fully describes currently-known thalattosaurs. Perhaps something like Nicholls (1999) but on a larger scale. This might also elucidate the Xinpusaurus situation.

(Also, as Darren Naish has noted, we should be careful of assuming that marine adaptations are strictly convergent--they may well indicate common ancestry.)

Another thing that’s maddeningly lacking in descriptions of thalattosaurs is speculations about their behavior and ecology. In his initial description of the group, Merriam (1905) did provide some of that. After determining their marine habits, he writes that “the larger and more specialized species comprised in the genus Thalattosaurus were strictly natatory. They may have visited the shore but, like the Plesiosaurs, were better fitted for swimming than for crawling.” He goes on:

“The character of the paddles, the form of the skull, and the presence of slender prehensile teeth in the terminal portions of the jaws would indicate that they fed in part upon some swiftly moving prey which was caught by a quick snap of the jaws, deglutition being assisted by the curved teeth of the pterygoid. The heavy vomerine and posterior mandibular teeth may have been used for crushing the light shells of ammonites, which existed in vast numbers in the same seas.”

Rieppel (1987) considered the functional significance of the strange skull of Hescheleria, and in particular the surprising “symphysial protuberance” of the lower jaw:

Hescheleria might have searched for prey in nearshore areas, as may indeed be indicated by the structure of the fore-limb…suited for terrestrial habits, and the “symphysial protuberance” may have had some function in this context. However, in view of the incomplete knowledge of the animal as a whole, and taking the absence of any extant model into account, there is no sound basis or the construction of a scenario which would explain the jaw apparatus of Hescheleria to a satisfactory degree.”

In her redescription of Thalattosaurus and Nectosaurus, Nicholls (1999) wrote that:

“With its powerful crushing teeth, Thalattosaurus probably fed on shellfish, especially cephalopods, which were abundant in the associated invertebrate fauna…The blunt, conical teeth at the anterior end of the dentary would be well suited for grasping the rubbery flesh of cephalopods….[it] probably spent much of its time along the shoreline, swimming out to sea to feed. The very powerful claws at first seem incongruous in a marine reptile. However, the marine iguana…also has powerfully developed, recurved claws which help it cling to rocks in heavy seas.”

Müller et al. (2005) tried to make sense of the toothless Endennasaurus at the end of their description:

“…Endennasaurus clearly occupied a specialized niche, relying on soft-shelled invertebrates, fish fry or small crustaceans that did not require teeth for either prey capture, holding or crushing...Endennasaurus was primarily adapted to an aquatic lifestyle, although the strongly ossified limbs imply it could have also moved on land (at least for reproduction).”

Bizarrely, Rieppel, Müller and Liu (2005) never discuss the behavioral or dietary possibilities associated with the various thalattosaur snout types that they describe in such detail.

Thalattosaurus by Ken Kirkland, from Hilton 2003,
still the best thalattosaur illustration out there.
Müller (2005) delves a bit into the depositional environments and specializations of the two branches of the Thalattosauria (long story short, thalattosauroids were more strongly adapted to life at sea). In their full description of Concavispina (2013), Liu et al. briefly address its swimming prowess compared to Xinpusaurus (“poorer swimming ability”) as well as their dietary preferences: “Xinpusaurus probably had the power to crush shells, while Concavispina could have eaten softer food such as fish and jellyfish.”

The rarity with which thalattosaurs’ lifestyles are discussed is very surprising. How much time did they spend on land vs. in the water? What were askeptosauroids eating vs. Xinpusaurus vs. claraziids? How different, really, are the skeletons of askeptosauroids and thalattosauroids? If they are different, what does that say about their ecologies? How did Hescheleria and Nectosaurus even get food in their mouths? Do any thalattosaurs show evidence of decompression sickness, or “the bends,” as reported in other marine reptile groups (Rothschild 1987, Rothschild & Martin 1987, Rothschild & Storrs 2003, and Rothschild et al. 2012)? What’s that spear-snout on Xinpusaurus for?

Future authors might focus more on the ecological implications of the odd thalattosaur skulls and skeletons, rather than spending all their efforts on the alpha taxonomy of the group, which remains surprisingly stable.

Special thanks to Nick Gardner for edits and suggestions.

2 comments:

  1. Hi!
    I study thalattosaurs and I must say this is quite the review.
    The best I can say is...wait and see many of your questions have answers in the work.
    Appreciate that thalattosaur love!
    Eric Metz

    ReplyDelete