Breaking: Mosasaurs Are Awesome

This is Platecarpus tympaniticus, and yes, it has a caudal fluke. Confused? I recommend clicking this link for the answers to all your questions.

Weekend Sketches

After over a month of not really producing any new art, I finally forced myself to come up with some material for a book I'd like to write that I'll talk about later. I'd talk about it now, but I have two podcasts to listen to before bed, one of which I'm on. Anyway, The Missus and I spent the weekend in an isolated cabin in Soldotna, which gave me plenty of quiet time to sketch. I took Richard Ellis' excellent Sea Dragons with me and let inspiration take its course...



This is, as handily scrawled above, Utatsusaurus, one of the basalmost known ichthyosaurs. The general outline is basically lifted straight from Ellis' excellent pen-and-ink representation, though I opened the mouth, switched up the colors (I think it looks rather like a penguin here) and added the "back" flippers. For being so basal, I think Utatsusaurus is very much an ichthyosaur. Are there more basal ichthyosaurs known now? I know that Grippia and Cymbospondylus are supposed to be pretty low on the ichthyosaur family tree, but I've never been all that educated on the phylogeny of the group.



Here is a rather cartoony Nothosaurus. I know, the proportions are all wrong, but I like how intent he looks. I think that nothosaurs and pachypleurosaurs are interesting in that they're essentially pre-plesiosaur sauropterygians, but all three groups co-existed at one point, so they were all doing different things. And they all mix and match body parts, too. Definately some modular evolution going on in that group. It's also interesting that while placodonts are sauropterygians, they look NOTHING like plesiosaurs and their allies.

Actually, if one of you in Readerland is an expert on plesiosaurs and pliosaurs, I've got a quick question: at one point, I know that the two groups were considered separate, but I remember reading somewhere that, in fact, some former pliosaurs are actually plesiosaurs, and some former plesiosaurs are actually pliosaurs, and that the separation between "plesiosaur" and "pliosaur" is actually very superficial and does not adequately reflect the phylogeny of either group. Is that true?

A Carpaceless Turtle

This week's Nature reports on a new ridiculously primitive turtle, Odontochelys semitestacea, described by Li, Wu, Rieppel, Wang, & Zhao (above). It comes hot off the heels of another basal turtle, Chinlechelys tenertesta, and the two offer competing ideas about how the turtle shell originally formed. First, let's talk about Chinlechelys, because I never got around to posting about it. It's an incredibly fragmentary turtle, but interesting in that one of the fragments shows that the carpace (upper shell) is made of osteoderms which fuse to expanded thoracic ribs. In Chinlechelys, that fusion is incomplete. In modern turtles, the dorsal osteoderms fuse onto the expanded thoracic ribs in the embryo. The reason Chinlechelys is important is because it demonstrates two things: (1) The carpace was originally made up of rows of armor plating; and (2) the thoracic ribs expanded independantly of armor fusion. Which event happened first is hard to say, but Chinlechelys shows that the two are separate occurrances.

The picture above, from Chinlechelys' description, shows a hypothetical sequence of events leading to the modern turtle shell. The two animals on the left are speculative. The third one from the left is Proganochelys, and the far right is Chinlechelys.

Well, leave it to evolution to screw up our understanding of the world. Enter Odontochelys, a turtle more primitive than either Chinlechelys or Proganochelys by a few degrees. It has a fully-developed plastron (lower shell) but no carpace. Did you hear that? It doesn't have any carpace armor! However, it does posess expanded thoracic ribs, so it was a wide-bodied animal with. It was also marine and, better yet, had lots of teeth in all regions of the mouth.

So that throws the Chinlechelys' carpace hypothesis out the window. Odontochelys presents the idea that ancestral turtles were shallow marine animals that developed a plastron to protect their bellies from predators, and only evolved the carpace later, but not initially from osteoderms. The skeletal structure (expanded thoracic ribs) was already in place, and probably evolved originally to join with the plastron. The ribs probably expanded for structural support. So while the armored carpace hypothesis isn't completely wrong, Odontochelys shows that more works needs to be done. The authors suggest that dermal osteoderms may have never been present, and that the full carpace evolved through "intramembraneous ossification within the carapacial disk." Or, more simply, the spaces between the expanded ribs, filled with dermal or cartiligionous tissue, may have simply ossified through evolution. No dermal armor necessary!

This is an unbelievably exciting and important find, and what's more, the authors did a phylogenetic analysis of the animal and found it group with sauropterygians! That would make turtles diapsids, a contention I've never really understood. It's still exciting, though!

UPDATE: In the "News & Views" section of the issue, Reisz & Head suggest that Odontochelys descended from carpaced ancestors, but lost the dermal ossifications secondarily after moving into the sea. Many modern turtles do this, like leatherbacks and soft-shelled turtles. I could really go either way here.

Predatory Mesozoic Marine Reptiles

During the Mesozoic period (and even stretching back to the Permian), reptiles coontinuously invaded the seas. One of the first marine reptiles was Mesosaurus, an Early Permian anapsid with needle-like dentition. During the Triassic period, an enormous range of marine forms developed, primarily belonging to two distinct groups--the Ichthyopterygia and Sauropterygia. But even as these more basal forms filled the oceans, still more reptiles poured into the sea. During the Jurassic period, crocodilian archosaurs invaded nearshore environments, and during the Cretaceous period, varanid lizards got in on the action, producing some of the largest predatory marine animals to ever exist. In this post, I will examine three very distinct groups of Mesozoic marine reptiles, all of which produced species well suited for large-game hunting. Far from being strictly picsivorous, these animals were the ones to look out for while diving (or snorkling) in the Mesozoic seas.

Askeptosaurus italicus, a long-snouted thalattosaur from the Late Triassic

The Sauropterygia developed into a menagerie of forms, including pistosaurs, plesiosaurs, and placodonts, but one of the more understated groups are the thalattosaurs. These near-shore lizard-like creatures grew fairly long (two meters or so) and probably lived a life similar to that of the modern marine iguana (Amblyrhynchus cristatus). Anguilliform swimmers, the thalattosaurs had long, flexible bodies and short but stocky limbs. They are divided roughly into two main groups: Clazariids and askeptosaurids. The former have bizarre downturned snouts and vomers, and their diet is problematic because of that. Askeptosaurids, however, were predators through and through. Askeptosaurus italicus and Anshunsaurus huangguoshuensis are the best-known species within the group, and while they certainly would have snacked on fish, the large, recurved teeth lining the jaws suggest a more varied diet.

Additionally, the skull of Askeptosaurus has plenty of room for jaw muscles. Note the large recess behind the orbit, where jugal simply fails to connect to the back of the skull. The bump on the lower jaw also suggests a muscle attachment point, and unlike other sauropterygians, Askeptosaurus may have possessed a sizeable adductor mandibulae muscle, allowing the jaws to snap shut strongly and quickly. The recurved teeth may imply a method of feeding enjoyed by mosasaurs, varanids, dromaeosaurs, and allosaurs--that is, grabbing a chunk of from a victim and running, leaving the prey to bleed to death and/or go into shock. But what would Askeptosaurus be eating? Ichthyosaurs and other basal sauropterygians are a possibility. By the Late Triassic, enormous ichthyosaurs (Shonisaurus) were already swimming the seas.

Pelagosaurus typus, a large Jurassic teleosaurine thalattosuchid

During the Jurassic, crocodilians briefly took to the sea. Like thalattosaurs, thalattosuchids can be divided into two groups: the gharial-like teleosaurs (above), and the far more menacing metriorhynchids (below). Although not terribly closely related to actual gharials, the teleosaurs evolved long, slender jaws with needle-like teeth. Their bodies are dorsoventrally compressed, and the limbs are more laterally placed on the body than gharials, with surprisingly short forelimbs. Teleosaurs kept their dorsal armor, but augmented it with significant ventral armor. They were mid-sized crocodilians, about the size of the average American alligator. Their dentition reveals teleosaurs to be specialized fish-eaters.

Dakosaurus andiniensis, a giant metriorhynchid thalattosuchian

Among the most fearsome creatures in the Jurassic sea, however, must have been Dakosaurus andiniensis, a giant South American marine predator. D. andiniensis is very much unlike its sister species, D. maximus, so much so that I wonder why it didn't recieve a separate genus. While most metriorhychids were fish and cephalopod feeders, D. andiniensis must have been able to eat whatever it wanted. At six meters in length, the beast could have fed on fish, cephalods, ichthyosaurs, plesiosaurs, and sharks. Dakosaurus andiniensis' skull is short and massive, quite unlike its metriorhychid cousins. The large frontal (prefrontal?) bone projects laterally outward as sort of a palpebral immitation, giving Dakosaurus a sort of "eyebrow" bone. The jaws are huge, with plenty of room for powerful jaw muscles. Additionally, the teeth are ridiculously long and large, and reminds me of Ceratosaurus' oversized dentition. Dakosaurus' teeth are serrated, and the teeth of the premaxilla are noticably shorter than those of the maxilla. I am reminded somewhat of Tyrannosaurus rex' teeth, all of which are huge, but the teeth of the premaxilla are smaller and more tightly packed than those of the maxilla. Given the obvious strength of the teeth, skull, and musculature, one wonders if Dakosaurus andiniensis was able to slice through flesh and crush bone.

Metriorhynchid thalattosuchians had a unique body plan. The body is devoid of armor, and the end of the tail is downturned, indicating the presence of a vertical tail fluke. Metriorhynchids may have also had a short dorsal fluke. What's really odd, though, is that the forelimbs became incredibly short and stocky and would have resembled muscular "flappers" rather than true flippers in the mode of, say, mosasaurs or ichthyosaurs.The hindlimbs, however, were long and lean, with elongated metatarsals. They, too, are completely unlike the flippers of other marine reptiles. Metriorhychids probably swam in a manner similar to modern crocodilians, with the limbs folded against the body, only to be used for maneuvering.

Clidastes prophyton, a small Creataceous mosasaur

With the extinction of most ichythosaurs and plesiosaurs at the close of the Jurassic, varanid lizards took advantage of this ecological vacuum and invaded the Cretaceous waters, eventually becoming the mosasaurs, giant predatory monsters who ruled the seas until the end of the Mesozoic era. These beasts grew from between two and fifteen meters, and probably ate whatever they felt like, including other mosasaurs. The creatures are unique in a number of ways, but especially in terms of skull anatomy. The mandible is double-hinged, allowing the dentary to slide back and forth and outward, allowing the mosasaur to engulf large prey items and swallow them whole. Aiding in this effort was a double-row of sharply recurved palate teeth, which, along with the motions of the jaw and tongue, would have "pulled" the prey deeper into the animal's gullet. Clidastes prophyton was a fairly typical, if small, mosasaur from Kansas. Like Askeptosaurus, Clidastes has a recess behind the orbits, suggesting very powerful jaw muscles. Also like thalattosaurs, mosasaurs were anguilliform swimmers, but unlike thalattosaurs, they were fully marine and would have been unable to come ashore.

Relative sizes of Askeptosaurus (bottom), Clidastes (middle), and Dakosaurus (top)

With the close of the Cretaceous period, only one branch of marine reptile survived: the turtles. They became fairly diverse during the ensuing Cenozoic, only to face gradual extinction during the current Holocene with the rise of humanity. The predatory roles once dominated by thalattosaurs, thalattosuchids, and mosasaurs are now in the hands (fins?) of sharks and whales.

Postscript: Thalattosaurs might not be sauropterygians. They are usually considered close to that group via good deal of hedging, but after reading a paper on sauropterygian skull morphology provided by Neil, I believe that even if they're not sauropterygians proper, thalattosaurs must be darn close, and a lot closer than they would be to ichthyopterygians. Also, as you'll see in the comments, Will mentions that a few ichythosaurs and plesiosaurs did push through the J/K boundary, so they weren't entirely absent. Finally, Dakosaurus is one of the few inherently evil tetrapods on the planet. It would not hesitate to eat a baby. Gleefully.