Fossils indicate ... wow, what a croc!

Newly discovered fossils of an ancient cousin of modern crocodiles suggest that adults of the species may have been dinosaur-munching behemoths that grew to the length of a school bus and weighed as much as 8 metric tons.

Paleontologists first found remains of Sarcosuchus imperator--which translates as emperor of the flesh-eating crocodiles--in the Tenere Desert of Niger in the 1960s. The initial description of the species was based on only a few bones and a partial skull, says Paul C. Sereno, a vertebrate paleontologist at the University of Chicago. Those fragments enabled the scientists to determine that the animal was related to today's crocodiles but didn't provide many clues about its lifestyle or ultimate size.

A year ago, Sereno and his colleagues unearthed in Niger the 110-million-year-old remnants of five super-crocs. The sediments bearing the fossils were laid down in an ancient river more than 160 kilometers from the nearest coastline. Remains included skulls and partial skeletons of juveniles, as well as the complete, 1.5-meter-long skull of an adult. By comparing that length with the dimensions of modern-day relatives, the researchers estimate in a forthcoming issue of SCIENCE that Sarcosuchus could have grown to lengths of 12 m. Only three or four species of living crocodilians can attain even half that size as adults.

Sarcosuchus' snout is broad and extends about 75 percent the length of its skull. The animal's teeth are stout, smooth, and rounded and are therefore suited to grabbing prey and crushing bones, says Sereno. Unlike those of fish-eating crocodiles, Sarcosuchus' upper and lower teeth didn't intermesh when the jaw was closed, which suggests that the animal had a diet of larger prey.

The researchers estimate that one of the juveniles they discovered last year was about 80 percent the size of an adult. Microscopic analysis of some of that juvenile's scutes--the large bony plates embedded in the skin that provide crocs a sort of armor--showed nearly 40 growth rings, which are much like a tree's annual growth rings. This finding suggests that the giant crocs may have taken 50 or 60 years to grow to full size.

The overlapping scutes stretched without a seam from just behind the head to about halfway down the tail, says Wann Langston, a vertebrate paleontologist at the University of Texas at Austin. This stiff suit of armor, as well as the shape of Sarcosuchus' vertebrae, would have restricted the animal's flexibility and speed. "This animal probably wasn't an active fisherman," notes Langston.

As in many crocodiles, Sarcosuchus' eyes and nostrils are located atop the skull. This configuration would have permitted the animal to hide almost totally submerged and supports the idea that it was an ambush predator, says Sereno. Along with an occasional large fish, Sarcosuchus could have dined on turtles and other riverside prey.

"Sarcosuchus could easily have taken a 20-foot-long dinosaur," Sereno suggests.