Transitions: The Evolution of Life

Caimans,

Gavials,

Alligators,

and Crocodiles

have a long and complex evolutionary history. The Crocodylia have undergone at least three separate episodes of adaptive radiation during their history. They are all members of the order Crocodylia - which in turn is a member of the superorder Archosauria. Fellow members of the Archosauria include the Saurischia (dinosaurs such as Diplodocus), Ornithischia (bird hipped dinosaurs such as Stegosaurus), the flying Pterosauria and the Thecodontia. The origins of the Crocodylia lie over 200 million years ago in the Triassic period. The earliest know genus of Crocdylia are known as Sphenosuchia. One of them, Gracilisuchus of the upper Triassic, is pictured below.

Such as the above fossil (found in Argentina). There are several features of the skeleton that distinguish Crocodylia from other reptiles. In humans, for example, three bones come together in the pelvis to form a structure called the acetabulum. The acetabulum is a cup shaped area where the femur (or thigh bone) joins the trunk. As you can see in the picture of a human pelvis below.

In the Crocodylia, however, this is not the case. The pubis is almost completely, or is completely excluded from forming part of the acetabulum. A second feature of the skeleton that distinguishes Crocodylia from other reptiles occurs in the ankle (yes, Alligators and Crocodiles have ankles). In humans, the talus sits ontop of the calcaneus.

In the Crocodylia, however, they sit side by side (note the talus is also called the astragalus).

This arrangement allows the foot to be twisted forward and allows the leg to be more underneath the body (compared to lizards). The Gracilosuchus pictured above shares these features with the Crocodylia.
One of the interesting things about the Sphenosuchia is that they were relatively long limbed and land dwelling. The Terrestrisuchus below is a good example.

The next group of Crocodylia are the Protosuchia which appeared in the lower Jurassic. Like the Sphenosuchia, the Protosuchia have long limbs and were largely land dwelling. Although we begin to see some of the adaptations to water that characterize later Crocodylia - for example the nostil cavity is becoming separated from the mouth.

The Protosuchia were distributed throughout the world.
The Mesosuchia evolved in the lower and upper Jurassic and are excellent transitional fossils between the Protosuchians and the Eusuchians (modern crocodiles, alligators, caimans and gavials). The Mesosuchians were a highly diverse group containing over 70 genera. The Mesosuchia are divided into aquatic, semi-aquatic and terrestrial species. The Teleosauridae are good examples.

In the Teleosauridae the limbs have become shorter and the snouts have become larger. Another group is the Metriorhynchidae which have become highly adapted to water living. For example their forelimbs have become transformed into paddles.

land dwelling species include Baurusuchus.

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Semiaquatic forms were represented by the Goniopholidae and the Atoposauridae. It is from the Atoposauridae, and a species called Theriosuchus, that the modern Eusuchians are believed to have evolved.
Alligators first show up in the upper Cretaceous, Gavials in the Miocene (about 25 million years ago), Crocodiles in the Paleocene (about 60 million years ago) and Caimans in the Oligocene (about 30 million years ago).