Crocodiles are often called living fossils. Their bodies have not changed much for a very long time. Their genes also change very slowly.

But in the age of dinosaurs, crocodiles had many different relatives. Some swam in the sea. Some ran on land with long legs. Some even ate plants.

Scientists learned this by studying the shapes of their fossil teeth. So crocodiles have a more colorful history than we think.

Crocodiles are often described as living fossils, and there is some truth in this. In 2014, an international team read the genomes of three crocodilian species and found that they change very slowly at every level. The animals' lifestyle of waiting in the water to ambush prey has also stayed similar for a long time.

However, fossils tell a different story. During the age of dinosaurs, crocodile relatives included sea-swimming forms, long-legged land runners, and even plant-eaters. In 2019, Keegan Melstrom and Randall Irmis measured the complexity of fossil teeth and found that teeth suited to eating plants had evolved at least three separate times. These relatives were far more varied than today's crocodiles.

Crocodiles are frequently labeled living fossils, a description that is only half accurate. On one hand, a 2014 study comparing the genomes of the American alligator, the saltwater crocodile, and the gharial found very slow change at every level examined, from DNA substitutions to chromosome arrangement; comparisons with birds suggested that it was the bird lineage that had sped up.

On the other hand, the fossil record reveals an extraordinary diversity among crocodile relatives during the Mesozoic. There were marine forms with flipper-like limbs, long-legged terrestrial runners, and, most surprisingly, herbivores. In 2019, Keegan Melstrom and Randall Irmis quantified the complexity of fossil teeth and concluded that plant-eating dentition evolved independently at least three times, sometimes rivaling that of herbivorous mammals. A 2021 analysis of body size by Maximilian Stockdale and Michael Benton found that evolution proceeded in bursts separated by long stable periods, with faster change during warmer climates. These findings carry some uncertainty, though. Diets were inferred from tooth shape by comparison with living animals rather than observed directly, and the body-size study says little about how other traits evolved.