In the comments to last month’s Gerrothorax story, Jerry wondered if the bite animation could be saved as a movie that could be used in conjunction with a PowerPoint file for presentation & instruction purposes.
I’ve updated the animation a little bit and exported it as several versions, all of which are available via the original post, or compiled below for your convenience. Feel free to download & use them as you see fit!
Word of warning - not all of these will work on every computer, but hopefully at least one of them will work for you. If there is a specific format that you would like to see, let me know and I’ll see what I can do about posting it.
Gerrothorax was a meter-long flat-headed temnospondyl amphibian that swam the waterways of Europe during the Middle to Late Triassic Period, between 230 and 200 million years ago. Known from several well-preserved fossils, illustrations of this animal are not too difficult to find. With their large eyes, feathery gills, stubby limbs, and fat, flattened bodies, reconstructions of Gerrothorax always seem to resemble Mesozoic cherubs, or Kewpie dolls from the Land of the Lost.
Any notions of idealized amphibian innocence end, however, once you look inside of its mouth. Like any good temnospondyl, the jaws of Gerrothorax bristled with dozens of sharp, fanglike teeth, and the roof of its mouth sported a handful of large palatal tusks. The bite of Gerrothorax would have, no doubt, made short work of even the slipperiest prey.
In 1919, D. M. S. Watson first proposed the idea that some flat-headed temnospondyls might have been able to perform a sort of “upside-down” bite—that is, they could open their mouths by raising their heads, instead of the more typical tetrapod practice of lowering the jaws to open wide. This ability would have clear advantages for an aquatic ambush predator. Lying in wait, half-buried or camouflaged at the bottom of a pond, one of these temnospondyls could flip up its skull to snap up or suck down a passing fish, without having to lift its lower jaw up from the sediment.
The above image (from Panchen 1959) shows how the flattened head and long retroarticular process of the jaw would allow Gerrothorax (on the left) to “open up” with minimal protrusion of the lower jaw. On the right, the high-skulled Batrachosuchus performs the same trick with significant jaw projection. Panchen suggested this might indicate different feeding habits for the two forms. Subsequent studies proposed that muscles anchoring on temnospondyls’ massive shoulder girdles would have played a much greater role in lifting the skull than the jaw muscles shown here.
Recently described fossils of Gerrothorax pulcherrimus from the Late Triassic of East Greenland have shed new light on the flip-top features of this charming little temnospondyl. A team led by Dr. Farish Jenkins, Jr., of Harvard University, has used these specimens to reconstruct the key skull-lifting joint between the condyles at the back of the skull and the atlas, or first neck vertebra. They found that, in Gerrothorax, this joint was uniquely shaped to allow significant flexing between the skull and the neck, allowing it to open its mouth about 50° without significantly opening its lower jaw. Additionally, they identified characters that would have served to lessen bending stresses on the spinal cord at this critical joint.
Jenkins et al. provide several photographs and illustrations of the new specimens, including fantastic drawings by L. L. Meszoly reconstructing the front of the skeleton with its mouth closed and opened wide. Using these as starting and end points, I put together a quick animation showing what the bite of Gerrothorax might look like. To see it yourself, click the “Bite!” button below (those reading via the RSS feed, click here to access this feature):
An animation of the bite of Gerrothorax, modified from diagrams by L. L. Meszoly published in Jenkins et al. 2008.
Of course, if this was indeed the way Gerrothorax caught its prey, then it certainly would perform much faster than I’ve shown here (the timing isn’t based on anything more scientific than my own aesthetic judgement, so take it with a grain of salt). What I do hope it shows somewhat accurately is the range of motion Jenkins et al. attribute to the joint between the back of the skull and the first vertebra; it appears to be quite an amazing adaptation.
Earlier workers have proposed skull-lifting for much longer-skulled temnospondyls (indeed, Watson first proposed it for the enormous Mastodonsaurus, and temnospondyls didn’t come much longer-skulled than that), but Jenkins and his team did not find substantial adaptations for skull-lifting outside the short-skulled plagiosaurid family to which Gerrothorax belongs.
Finally, Jenkins et al. note that head-lifting is known from some modern amphibians, and they draw an interesting comparison to Leurognathus marmoratus, the Shovel-nosed salamader. L. marmoratus lift their head not only to feed, but to burrow as well. Perhaps the flip-up skull of Gerrothorax was not an adaptation for capturing its prey, but instead (or additionally) was a key part of some richer behavioral repertoire we have yet to discern.
Update 1/5: I’ve tweaked the animation a little bit so that the lower jaw moves with the rest of the skull instead of simply rotating open. Downloadable Quicktime movies of the bite are now available in two chomptastic sizes:
Update 1/9: Some readers have had some trouble with the Quicktime movies on their machines, and recommended a couple of additional formats that will embed more cleanly into a PowerPoint presentation (right-click to “Save As…” to your desktop):
I am induced to lay before the Geological Society the annexed representations of parts of the skeleton of an enormous fossil animal, found at Stonesfield near Woodstock, about twelve miles to the N. W. of Oxford ; in the hope that, imperfect as are the present materials, their communication to the public may induce those who possess other parts of the same reptile, to transmit to the Society such further information as may lead to a more complete elucidation of its osteology.
The good Reverend William Buckland published these words in 1824, introducing an article entitled, “Notice on the Megalosaurus or great Fossil Lizard of Stonesfield.”* Eighteen years later, when Sir Richard Owen became the first person to utter the word “dinosaur,” Buckland’s Megalosaurus was a charter member of the group. Buckland’s 1824 notice is recognized as the first scientific description of a dinosaur.** A PDF of this article was recently made available for download (along with a sampling of other notable 19th century papers) from the Geological Society of London.
* This notice illustrated a scant 7 pages of text with 5 beautiful lithographic plates of the fossils, reproduced at 25%, 50%, and full scale. I am inclined to believe that this very nearly approaches an ideal ratio of text to figures in a fossil description.
**Although historians are aware of at least one notable early attempt to describe probable Megalosaurus remains.
Name Means: Bustingorry’s Scorpion Hunter (Manuel Bustingorry owned the land where the fossil was found, and the describers report an “abundance of living scorpions moving around the excavation.”)
Relations: Abelisaurid theropod
Holotype: MMCH-PV 48, an almost complete skeleton
Location: Neuquén Province, Argentina
Age: Late Cretaceous (~93,000,000 years old)
Info:Skorpiovenator belongs to the Abelisauridae, a distinctive family of large carnivorous dinosaurs that prowled the Gondwanan supercontinent during the latter half of the Cretaceous period, from about 95 to 65 milion years ago. During this time, Gondwana was beginning to break up into several more familiar landmasses—South America, Africa, Madagascar, India, Australia, Antarctica—and palontologists have suggested that the relationships and distribution of abelisaurids might help determine the order in which Gondwana split apart.
In Canale et al.’s phylogeny, Skorpiovenator is most closely related to other South American abelisaurids, including Ekrixinatosaurus, Ilokelesia, Carnotaurus, and Aucasaurus. They share a suite of features that suggest that they were turning their skulls into shock-absorbers: hyperossified ornamentation atop their heads, struts of bone projecting into or sealing off parts of the orbit, and shortened muzzles. This last feature inspired a name for this abelisaur subgroup—the Brachyrostra, or “short snouts.”
If this interpretation is correct—that South America had its own endemic radiation of short-snouted abelisaurs for the last 30 million years of the Cretaceous, this might suggest that South America was isolated from other Gondwanan landmasses with non-brachyrostran abelisaurs (places like Africa and Madagascar). However, other abelisaur phylogenies present different conclusions, and, as Canale and his coauthors point out, there are certainly other groups of Cretaceous Gondwanan animals that have their own stories to tell.
