The Flip-Up Skull of Gerrothorax
11:09 am
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.
- Primary Reference: Jenkins, F. A., Jr., Shubin, N. H., Gatesy, S. M., Warren, A. 2008. Gerrothorax pulcherrimus from the Upper Triassic Fleming Fjord Formation of East Greenland and a reassessment of head lifting in temnospondyl feeding. Journal of Vertebrate Paleontology 28(4): 935–950.
- Further Reading:
- Dracovenator: Picture of the Day: A Temnospondyl [Adam Yates's drawing of a Paracyclotosaurus lifting its skull]
- Panchen, A. L. 1959. A new armoured amphibian from the Upper Permian of East Africa. Philosophical Transactions of the Royal Society of London, Series B 242: 207–281.
- Society of Vertebrate Paleontology: Heads Up in the Triassic [press release]
- Watson, D. M. S., 1919. The structure, evolution and origin of the Amphibia. The “Orders” Rachitomi and Stereospondyli. Philosophical Transactions of the Royal Society of London, B 209: 1–73.
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:
- 550 x 400 px (348K, zipped)
- 1100 x 800 px (817K, zipped)
