Crocodylians have characteristics that set them apart from all other reptiles. Even though they are one of the first reptilian forms, their internal anatomy is much more birdlike than other reptiles.
In the crocodylians there is really one basic body plan. That of a long body with four short stout legs (although they really aren't that short) a long laterally compressed tail and a skull with a relatively long snout. Throughout their over 200 million year history there have been many many variations on this body plan. There were dozens of terrestrial and fully aquatic species of crocodyliformes along with at least one bipedal form and maybe even an arboreal one at that. Still in times of stress the group always reverted to this as their back up.
The hearts have it
Crocodylians have a four chambered heart. This heart, which is like that of a bird, allows complete separation of oxygenated and deoxygenated blood, which in most cases is more efficient that the reptilian
three chambered heart. Yet unlike the four chambered hearts of birds and mammals, crocodylians have a special valve that allows their hearts to revert the their more archaic state of three chambers. This allows the deoxygenated (don't listen to the word there is still some oxygen left in it) blood to mix with oxygenated blood and thus allows crocodylians to stay submerged underwater for longer periods of time.
For more on this check out the high walk in the locomotion part of the site.
The eyes, ears, and nostrils are all set up at the top of the skull. This allows the crocodylians to submerge most of their bodies underwater while just keeping the smallest part of their bodies visible. Truly a brilliant setup.Although modern day crocs & gators lack their antorbital fenestras, their ancestors did have them. In many cases crocodylians solidified much of their skulls, making them strong enough to handle strong prey animals.
But crocodylians didn't become totally thick headed. They still have areas of their skulls that lighten it. Some of these passages are located at the top rear of the skull and called pneumatic (air) passages. This is another feature that the crocodylians share in common with their birdy relatives. While these openings allow for some lightening of the skull it is also believed that they are a form of cooling system for the brain (which is also very large and birdy in appearance).