Bite marks reveal behavior of dinosaur-eating croc
Research by Dr. Christopher Noto and a team of paleontologists published this week in the international journal Palaios describes recently discovered fossils from the Cretaceous Period (145-65 million years ago) of Texas that show evidence of attack by a new species of giant crocodyliform (croc-relative). Bite marks on fossil bones provide a rare glimpse of predatory behavior that indicate this animal was a top predator that regularly consumed turtles and even ate dinosaurs. Dr. Noto, an assistant professor at the University of Wisconsin-Parkside, Derek Main, a doctoral student at the University of Texas at Arlington, and Stephanie Drumheller, a doctoral student at the University of Iowa authored the paper.
For most extinct species, scientists can never directly observe such predatory behavior. Paleontologists must resort to other, indirect indicators. Bite marks on fossil bone are a great way to figure out who ate who and how.
The fossils come from the Arlington Archosaur Site (AAS), a recently discovered fossil locality in the Dallas-Fort Worth metroplex. The AAS preserves the remains of a 100 million year old coastal delta that includes a diverse assemblage of animals. "The AAS is a special fossil site for many reasons: its unique urban location, the unique fossils found there, and what they can tell us about Cretaceous coastal ecosystems," said Main, who led the team that discovered the fossils. When studying the fossils in detail, researchers discovered dozens of turtle and dinosaur bones had bite marks that they were able to match to the crocodyliform, whose remains are found at the same site.
"We can study bite marks made by modern crocodilians (alligators, crocodiles, and gharials) in order to make predictions about what kinds of feeding traces extinct croc-relatives might have left behind in the fossil record," said Drumheller, who helped conduct the research. "The bite marks identified on the AAS fossils are very similar to those made by living crocs." This allowed the researchers to reconstruct the most likely predatory behavior of the crocodyliform.
What they found is that the AAS crocodyliform fed in much the same way as its living counterparts. "The two dinosaur bones have bite marks on the femur (upper leg bone) that are located close to the hip joint," said Dr. Noto. "Living crocodilians often remove the limbs of large prey in this way with a 'death roll' making the prey easier to consume." The dinosaur remains come from hadrosaurs, or duck-billed dinosaurs, which were major herbivores of the Cretaceous. The bones represent younger individuals, which also happen to be favorite targets for living crocodilians.
The turtle fossils paint an even more dramatic picture. Tooth marks on turtle shell pieces occur mainly around the shell edges and the central parts of shells are missing. "These places happen to be major weak points in the shell, and living crocodiles and alligators have been observed crushing turtle shells in a manner that creates a very similar breakage pattern," said Noto. "The marks suggest that this animal flipped turtles on their sides in its mouth, then crushed their shell to access the fleshy insides. It's almost as good as having a 100 million year old photograph of our animal in action." This is the first documented evidence in the fossil record of the specific "nutcracking" behavior of turtle shells by crocodilians.
"The AAS could be among the few Cretaceous ecosystems where crocs were the top predators rather than dinosaurs," said Main. Scientists generally thought theropod dinosaurs like Velociraptor or Tyrannosaurus rex were the top predators of the Cretaceous. This discovery adds to the growing picture that, like today, the croc-relatives of the Cretaceous Period were the dominant predators around swamps and rivers. "Though not nearly as large as other giant crocs of the time, our 20 foot crocodyliform was still a very important predator in its community and shows that large croc-relatives did consume dinosaurs," said Noto.
In the illustration, left, example of bite marks (arrows) on a piece of turtle shell (top left) and dinosaur bone (bottom left) made by a giant croc-relative from the Arlington Archosaur Site near Dallas, Texas; right, reconstruction of AAS crocodyliform trying to break open the shell of a turtle caught in its jaws (by Jude Swales, Seattle, Washington)
For more information about the Arlington Archosaur Site go to: http://www.arlingtonarchosaursite.com