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Mar 29

Are Crocodiles "Living Fossils"?

Posted on March 29, 2018 at 12:27 PM by Bonnie Man

Picture a crocodile in your mind. Did you imagine a scaly, toothy thing with short legs that lives in swamps and rivers? You’re not wrong – all modern members of the crocodile family (which includes crocodiles, alligators, caimans, and gharials) fit this description. For paleontologists, however, the word “crocodile” refers to a much more varied cast of characters. The modern crocs represent only one tiny branch of the crocodilian family tree that blossomed over deep time. 

Crocodiles first evolved around 225 million years ago, during the Triassic Period. This was around the same time as the first dinosaurs, and both crocs and dinosaurs descended from the same ancestral group. You might call crocodiles and dinosaurs distant cousins. Early crocs were very different from the swamp-dwellers of today. Called sphenosuchians (s-FEEN-o-SOOK-ee-ans), these crocodiles lived on land and were built for speed. They had long, thin legs and slender, flexible bodies – picture a cheetah with scales and a toothy, reptilian head. Ranging from a few inches to a few feet, sphenosuchians were agile hunters that pursued insects and other small, speedy prey. 

Over the course of the Mesozoic Era, crocodiles evolved to fill a variety of ecological niches. Metriorynchus of England was a marine crocodile, living most of its life in the open ocean. Metriorynchus traded bony armor for sleek, scaly skin, allowing it to pursue the large fish it fed upon. It even had a fin on its tail, like a shark. Meanwhile, multiple groups of crocodiles switched to a vegetarian diet. Revueltosaurus (pictured above) from Triassic Arizona and Simosuchus from Cretaceous Madagascar are examples of plant-eating crocs. Although only distantly related, both species developed round, bull dog-like heads, stout bodies, and leaf-shaped teeth.

All modern crocodilians belong to the eusuchian (you-SOOK-ee-an) family, which originated in the early Cretaceous Period. Most eusuchians are slow-moving aquatic ambush predators, specifically adapted to snag prey from the water’s edge. Eusuchian crocodile fossils are common at Dinosaur Park, especially their teeth and armor. We also find the remains of crocodile-like reptiles called champsosaurs. The swampy oxbow pond that became our fossil quarry was the perfect environment for newly-evolved eusuchians to practice their hunting technique. 

Crocodiles are often called “living fossils” – primitive creatures that have hardly changed in hundreds of millions of years. This is misleading, and the larger crocodile family tree is actually a stellar example of evolution in action. Crocodiles have evolved to live in a wide variety of environments – from deserts to the open ocean. They have adapted to eat a wide variety of food – insects, fish, large land animals, and even plants. Modern aquatic crocodiles are very different from their sphenosuchian ancestors, and are just one of many specialized types of crocodile. Just like birds are not representative of all dinosaurs, modern crocodiles and alligators are not representative of the myriad of shapes and sizes crocodilians have taken on over the course of deep time.

Public domain illustration of Revueltosaurus by Jeff Martz for Petrified Forest National Park.