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!
Origins of crocodylians and changes through time
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.
Features evolved by crocodilians for life in the water
9 months ago, someone asked why crocodiles and alligators are suited for life in the water. Well, that's a pretty easy though longish thing to explain.
- Secondary palate which allows the mouth to be open without water flowing into the throat as well as a flap that covers the throat
- Raised nostrils, eyes and ears which allow it to keep its senses above the water and be able to see above the water line without revealing the entire body
- Advanced touch system on the snout that can tell when objects are moving in the water, especially when objects break the surface
- Ability to secrete excess salt through salt glands in the snout for saltwater crocodiles (Crocodylus porosus)
- Very thick skin and armor which keeps excess salinity from being absorbed into the skin (this lets many crocodilians swim up coast lines and between islands and move in and out of very brackish areas for a short time)
- Body armor on the belly has special structures which can detect too high of salinity and lets them stay out of water that is too brackish
- Strong tail to hip muscles that allow them to propel strongly through the water
- Ear flaps that keep water out of the ears when going under water
- Webbed feet for paddling in shower water
- Strong conical teeth to snag prey in the water that move quickly, like fish
I can't think of any others, but I hope this helps!