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Bodyplan & Basic Anatomy

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.

crocodylian heart

A word on stance

Crocodylians are archosaurs. Archosaurs or the "ruling reptiles" as the name implies are a special group of reptiles that have certain characteristics that set them apart from other reptiles. These characteristics are:

That last one is what surprises most people, for most people don't realize that crocodiles have the ability to stand and walk just like a dog or cat. Their legs fit directly underneath the body...well almost. You see the front legs still stick out a bit, but the important set of legs (the back) are the ones that go directly underneath. Here see for yourself.

An _A.mississippiensis_ overhead shot

Pretty cool huh?

This is accomplished by two anatomical traits. One trait is the crocodylian hip. Their hips have been deeply recessed and the socket itself has a hole in the middle for the femur.This allows for the leg to fit under the body. The enhanced photo below shows the location of this hole.

This photo has been enhanced some to show off where the hole is located.

The other thing that allows for this erect stance is the ankle arrangement in crocodylians. It allows for the ankle to swivel 900 into a more in line postion with the leg.

For more on this check out the high walk in the locomotion part of the site.

A word on skulls

In the living species, most of the variation has gone into the skull. This really isn't that surprising since the skull is the most important part needed for survival in most vertebrates. Crocodylians have a secondary palate that fully separates their breathing tubes from the throat and therefore allows these animals to swallow and breathe at the same time, which is unlike us mammals who have to take breaths inbetween eating food. Another advantage of this secondary palate is that it helps reinforce the skull, which is quite helpful when holding a large thrashing animal in your jaws.

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).

Alligator skull cross section via CT scan
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