SCIENCE

Pakasuchus: The Croc That Ate Like a Mammal

Science Correspondent

August 6, 2010

Modern crocodylians—from alligators to gharials—can't chew their food. Their jaws are adapted for snapping shut quickly and powerfully on prey, but once these archosaurs have captured their meal, they must either swallow it whole or tear off a smaller piece and bolt it down. Given that these extant species are often cast as "living fossils" which have gone "unchanged for millions of years," it might seem like a safe assumption that crocs have always favored high bite forces over masticatory ability, but a new discovery from southern Africa shows that this isn't the case.

Known from a nearly-complete skull and skeleton discovered in the 105-million-year-old strata of Tanzania, Pakasuchus kapilimai did not look very much like its modern cousins. A crocodyliform—or a member of a diverse group containing living crocodylians and their close, extinct relatives—Pakasuchus had long legs, a gracile skeleton, and a short, broad skull instead of an elongated crocodile smile. What's in its mouth, however, is what makes it so remarkable. Whereas living crocodylians have interlocking rows of conical teeth, Pakasuchus had different kinds of teeth in its jaw, some of which looked like molars and would have made contact with each other while processing food.

As described by scientists Patrick O'Connor, Joseph Sertich, Nancy Stevens, and colleagues in the latest issue of the journal Nature, the tooth-to-tooth contact in Pakasuchus is extremely unusual. Other fossil crocodyliformes with mammal-like teeth have been found before, but Pakasuchus has a mammal-like arrangement of molar-like teeth that could come into direct contact for chewing. Based upon the articulation of the lower jaw with the skull and the correspondence of its teeth, it appears that Pakasuchus would have been able to bring its teeth together when moving its lower jaw upward and forward, crushing its meal between its robust rear teeth.

But what was Pakasuchus eating? The Cretaceous crocodyliform did not have the body form of an aquatic ambush predator, but instead this croc—about as big as a medium-sized dog—appears to have been well-suited to running about on land. Insects, small mammals and other quick-moving small prey would have undoubtedly been on the menu. If so, that might explain why Pakasuchus was poorly-armored by crocodyliform standards. In Pakasuchus, the majority of the tough bony plates—called osteoderms—that cover the bodies of many of many crocs are primarily found in its tail. It did not have the tough, full-body protection that living crocodylians have. As hypothesized by the authors behind the new description, Pakasuchus would have been the sportscar of crocodyliforms—a light, gracile form in which adaptation traded off armor for speed.

Unfortunately for us, there is nothing like Pakasuchus alive today. Even though it and its close relatives flourished in the southern landmass of Gondwana during the Early Cretaceous, but by about 99 million years ago most of them were gone. Interestingly, however, during their heyday Pakasuchus and its close crocodyliform relatives occupied ecological roles similar to those of mammals elsewhere in the world. Even though it is often said that dinosaurs kept mammals down during the Mesozoic, perhaps some competition from mammal-like crocs also influenced the pattern of mammalian evolution in the southern continents, meaning that the extinction of the mammal-like crocodyliforms may have opened evolutionary opportunities for their furry little neighbors.

For more on this discovery, see Surprising Science and Ed Yong's post at Not Exactly Rocket Science.

References:

O’Connor, P., Sertich, J., Stevens, N., Roberts, E., Gottfried, M., Hieronymus, T., Jinnah, Z., Ridgely, R., Ngasala, S., & Temba, J. (2010). The evolution of mammal-like crocodyliforms in the Cretaceous Period of Gondwana Nature, 466 (7307), 748-751 DOI: 10.1038/nature09061