Evolution of Alligators and Crocodiles

It's hard to imagine, but modern crocodylians (alligators, crocodiles and gavials) are very different from their ancestors. The ancestors of crocodylians were not aquatic ambush predators as their descendants are today. Fossil crocodylians were mostly land animals. Their legs did not sprawl like those of modern alligators and crocodiles, but instead they stood with their legs tucked under their body, like mammals and dinosaurs. They were much faster and more manueverable on land. Most were small cat and dog sized animals, but some were as large as bears. And even more surprisingly, some did not even eat other animals, but ate plants instead!

Crocodylians, dinosaurs, birds and pterosaurs are all archosaurs. Archosaurs are reptiles which have teeth that attach to the jaw in a socket, rather than sitting on top of the jaws or along the side of them as they do in lizards. They also have two unique openings in the skull, one in the snout called the antorbital fenestra which was probably filled with a sinus, and the other is in the lower jaw and called the mandibular fenestra which was probably an attachment for muscles. Archosaurs also share another unique feature which is the presence of a special knob on their thigh bones for extra muscle attachment. This special knob let them stand up with their legs tucked under their body, and let them move much more quickly than other reptiles of their time.

Crocodylians and their more distant cousins are part of a group called "Suchia". Suchians split away from other archosaurian reptiles in the Triassic Period. The earliest known suchians were the aquatic phytosaurs and the plant eating armored aetosaurs. More closely related to crocodylians were larger predators like rauisuchians which had large boxy skulls similar to theropods (the meat eating dinosaurs from which birds evolved) and the bizarre poposaurs which were just too strange to sum up easily.

The very first crocodylians were small, cat and dog-sized predators called sphenosuchians. These had a special joint in their skull which would have let their skull deform slightly to withstand the forces of biting and struggling prey. One sphenosuchian from China, called Phyllodontosuchus, ate plants. It is one of the earliest known plant-eating crocodylians.

Around the same time that sphenosuchians evolved, another group of crocodylians also appear, the protosuchians. Protosuchians probably are not a group with a single common ancestor but may in fact be multiple groups of similar looking fossil crocodylians. My favourite protosuchian is the plant-eating Edentosuchus, known from the Early Jurassic of Arizona and the Early Cretaceous of China. Edentosuchus was a very small rabbit sized animal which probably burrowed to hide from predatory dinosaurs such as Coelophysis and Dilophosaurus.

The group to which modern crocodylians evolved also probably came about in the Early Jurassic, the first known being the aquatic Calsoyasuchus. These are neosuchian crocodylians, and the first aquatic crocodylians. Not all neosuchians were aquatic, for example, some forms like Iharkutosuchus were stocky land dwelling plant eaters, and others like Pristichampsus were bear sized meat eaters which probably hunted primitive horses and other early hoofed mammals. Thalattosuchians were probably the first marine crocodylians. They show evidence of salt glands (necessary for living in salt water to shed excess salt). They had paddle like limbs and finned tails. Pholidosaurs and dyrosaurids are close relatives of thalattosuchians and greatly resembled modern crocodylians in their appearance and probably in their life style as well. The giant bus-sized Sarcosuchus (or "SuperCroc") was a pholidosaurid from the Cretaceous of Niger in mid Western Africa. Stomatosuchids were also probably aquatic (possibly even marine) neosuchians. The best known remains of Stomatosuchus were destroyed in WWII but it has been suggested that it was a large filter feeding crocodylian because it lacked teeth in its lower jaws and the skull superficially resembled the rough shape of a duck (long, broad and U-shaped). Sadly, despite numerous expeditions across Saharan Africa, no one has recovered new fossils of Stomatosuchus or any related species, so for now, we really don't know what they looked like or how they fed.

Another group of crocodylians are the notosuchians. Notosuchians are very interesting and different. Some of the meat eating notosuchians, such as Sebecus, were the only other crocodylians aside from neosuchians to survive extinction at the end of the Mesozoic. Notosuchians such as the sphagesaurids and notosuchids have oddly mammal-like teeth and show special adaptations in their skulls which might have led them chew their food like we and other mammals do.