A Greater Alligator

Weighing in at more than 5000 kilograms and equipped with Tyrannosaurus rex-sized teeth, Deinosuchus was not the sort of crocodile you would try to make a handbag out of. In today's issue of Nature, paleontologists show how Deinosuchus could grow so big: Unlike dinosaurs from the same era and modern-day crocodiles, the Cretaceous monster apparently kept on growing throughout its lifetime.

In its heyday, some 85 million years ago, Deinosuchus, measuring well over 10 meters from head to tail, was the terror of North American estuaries. It must have fed on big game and large fish, says Gregory Erickson, a paleontologist at the University of California, Berkeley. "We suspect it ate dinosaurs--it certainly had the size to do it." Together with geologist Christopher Brochu, Erickson wondered if Deinosuchus, like dinosaurs, experienced an intense growth spurt early in life, or whether there was some other secret to its size. To find out, Erickson and Brochu sliced up Deinosuchus fossils and counted and measured the concentric rings in the bones, which, in modern crocodiles, are deposited yearly. From the rings' sizes, they estimated the animal's size at every age.

Modern crocodiles gain about 25 centimeters per year during the first 10 years of their lives, after which their growth slows down. But the curves for Deinosuchus looked different: Rather than leveling off at age 10, the creature maintained a juvenile growth rate throughout most of its 50-year life. "Imagine if our teenagers kept going for another 10 years--they'd get gigantic," says Erickson.

David Schwimmer, a paleontologist who studies Deinosuchus fossils at Columbus State University in Georgia, says the results are "very interesting." But he points out that the climate of the Cretaceous is still under debate, and he is skeptical about the assumption that the growth rings represent years. "They could reflect nutritive cycles or seasonal droughts," he says, in which case they could occur less or more frequently than today, making Deinosuchus's growth curve look a lot different.