Deinosuchus was five times the size of the biggest crocodiles alive today and researchers have now discovered why - it lived far longer than its present-day cousins.
Unlike the dinosaurs, which became giants by putting on weight quickly, Deinosuchus grew slowly and became gigantic simply by continuing to get bigger while living to a relatively great age.
"How Deinosuchus attained sizes to rival its dinosaurian contemporaries, on which it undoubtedly preyed, has remained a mystery," say Gregory Erickson and Christopher Brochu, of Stanford University in California and the Field Museum of Natural History in Chicago, in the journal Nature.
The scientists studied the growth rings of Deinosuchus bones and found that the reptiles must have lived for at least 50 years, about 20 years more than living crocodiles, and considerably longer than the dinosaurs alive at the time. This suggests that the ancient crocodile grew slowly and steadily, like the modern-day cold-blooded reptiles cousins, rather than in spurts which is the pattern seen in today's warm-blooded animals.
Fossil specimens of Deinosuchus excavated in Montana and Texas indicate that it grew up to 32 feet long and had huge jaws which were capable of gripping animals as big as a rhinoceros.
"We're almost certain they fed occasionally on dinosaurs but there is also evidence that they ate large turtles because of puncture holes found in [turtle] shells which match the teeth of Deinosuchus," Dr Erickson said.
Dinosaurs' bones show that they went through a rapid period of growth in early life while Deinosuchus grew at a modest 0.3 metres a year and over several decades rather than the five or ten years of modern crocodiles.
Dr Erickson said that Deinosuchus would have been among the longest- lived animals of prehistoric times. "Each Deinosuchus must have seen several generations of dinosaurs come and go," he said.