The Nutcracker Croc of Cretaceous Texas

Bite marks (arrows) on a piece of turtle shell (top left) and dinosaur bone (bottom left) made by a large crocodyliform from the Arlington Archosaur Site. Right: The AAS crocodyliform species trying to crack the shell of a turtle (by Jude Swales, Seattle, Washington). Images courtesy Chris Noto.

The Arlington Archosaur Site doesn’t fit the traditional vision of a remote fossil deposit situated in rocky, windswept badlands. Rich in turtle, crocodile, fish, and dinosaur remains, the assemblage is within walking distance of a Starbucks in the Arlington, Texas, metro area. But those fossils tell of a very different time — a snapshot of life roughly 95 million years ago, when Texas was on the coast of a warm, shallow seaway that cleft North America in two.

According to University of Wisconsin-Parkside paleontologist Chris Noto, one of the researchers who has been carefully picking over the site, this place “was probably very similar in overall appearance to the Florida Everglades.” But the Cretaceous fauna was significantly different. While turtles and crocodyliforms were familiar players, dinosaurs, lungfish, sharks, stingrays, and other animals called this prehistoric swamp home. All endured the harsh fluctuations between the wet and dry seasons. “Preservered in the sediment is the story of a seasonal climate,” Noto said, “one which was punctuated by intense storms that may have felled the large trees” and may have sparked wildfires. “Life there was certainly not easy.”

Turtles may have had an especially difficult time. As explained by Noto and collaborators Derek Main and Stephanie Drumheller in the latest issue of PALAIOS, the Cretaceous turtles shared their habitat with a large, powerful crocodyliform capable of crushing through the defenses of the shelled reptiles. And the aquatic ambush predator didn’t just specialize on turtles — the crocodyliform was formidable enough to dismember dinosaurs, too.

As yet, the crocodyliform does not have a name. A formal description of the animal is still in the works. But Noto suggests that this predator would have reached approximately 20 feet in length. “I think in many ways it resembles salt water crocodiles,” he said, although the prehistoric species differed from modern ones in some significant ways. In addition to a “robust A-shaped skull” set with “a pair of large, prominent teeth (pseudocanines) in the upper and lower jaws,” the croc’s bony armor — known as scutes — “were large and rectangular, as opposed to the smaller, rounded scutes seen on living forms,” Noto explained.

This sharp-toothed archosaur left behind a telltale pattern of damage on turtle and dinosaur bones. Out of a sample of two hundred dinosaur bones and 29 turtle shell pieces, Noto and colleagues found tooth marks attributable to the crocodyliform on two dinosaur limb bones and 17 pieces of turtle shell. The pits, punctures, and scores along these bones are permanent signs of prehistoric interactions and might provide paleontologists with clues about how the ancient croc fed.

Turtles seemed to be the crocodyliform’s specialty. About 60 percent of the turtle shell fragments found so far have tooth marks on them. And those bite marks indicate that the croc was a Cretaceous nutcracker. “The marks suggest that the croc flipped turtles on their sides in its mouth using inertial motions of the jaws and head,” Noto said, “then crushed the shell about its long axis.” What is missing from the turtle shells is an additional clue. “We are completely lacking fragments from the central portions of the shell”, Noto explained — the crocodyliform’s bite obliterated this part of the turtle shell. “We believe that this is the first documented evidence in the fossil record of this specific ‘nutcracking’ behavior of turtle shells by crocodyliforms,” Noto said.

Exactly what happened to the dinosaurs isn’t as clear. Two limb bones — one from a subadult ornithopod dinosaur, and the other from a juvenile ornithopod dinosaur — show tooth marks. This indicates that the crocodyliform consumed dinosaurs, but, Noto said, “Whether it actively killed them or only scavenged their remains is uncertain.” Still, the crocodyliform was certainly large enough to tackle juvenile dinosaurs. “Many crocs go after juveniles because they make easier targets than adults, and these remains lead me to think something similar may be going on here,” Noto said, and the paper imagines that “Crocodyliforms may have killed unwary dinosaurs that journeyed too close to the water’s edge and scavenged their carcasses when available, possibly even dragging them to the water from further inland.” The nature of the evidence obscures the details of whether the crocs hunted dinosaurs or simply tore off chunks of already dead animals. However they grabbed dinosaur dinners, though, the crocodyliforms unwittingly helped paleontologists. By biting and caching bones, the crocs created a macabre record of life and death 95 million years ago.

References:

NOTO, C.R., MAIN, D.J., DRUMHELLER, S.K. (2012). FEEDING TRACES AND PALEOBIOLOGY OF A CRETACEOUS (CENOMANIAN) CROCODYLIFORM: EXAMPLE FROM THE WOODBINE FORMATION OF TEXAS PALAIOS, 27 : 10.2110/palo.2011.p11-052r