Crocodile Facts: A Definition
Crocodile Facts: We all have a picture of what a crocodile looks like, and for most of us it looks like this beast to the right.
But there are many different types of crocodiles that all look a bit different. There are also creatures from the past that look remarkably like crocodiles. So how do we define what a crocodile is, if it's not just based on its external appearance?
It's Not Just About Looks
While a description of the appearance of living crocs serves us well enough in the present, things are not as clear cut when it comes to looking at fossils. Generally, there is no fossilization of soft tissue when a creature dies. The soft tissues are the blood, skin, muscles, organs and so on. Every now and then scientists are lucky enough to find a fossil with an impression of the skin in the rock.
So when looking at fossil remains we cannot go on lifestyle, skin, organs, behavior or any of the characteristics we use in daily life to identify crocodiles. If we did, we would more than likely conclude that the two animals in the following picture are also crocodiles, especially the one on the top.
The creature on top is a phytosaur. Phytosaurs were a group of reptiles only distantly related to the ancestors of crocodiles. They became extinct over 200 million years ago.
The creature on the bottom is Platyoposaurus, an amphibian ambush predator from 255 to 250 million years ago. Another example of a crocodile-like amphibian is Eryops.So if we can't use appearance or soft tissue structure, that leaves us only with the bones. And it is the bones that we use to identify what group of animals a fossil belongs to. The most useful bones are the skull.
Crocodile Facts: The Skull
Let's start by looking at the skulls of a phytosaur (left) and a modern crocodile (right).
Clearly they have evolved a similar shape in order to live a certain lifestyle. Mind you, even the Alligator Gar has developed this jaw, and it's not even a reptile!But anything more than a glance reveals that these skulls are very different. The crocodile has nostrils on the end of its nose, while the phytosaur has nostrils on the top of its head. The bones of the head and the holes in them are also arranged differently.
Crocodile Facts: Bones And Holes
In fact, each main group of animals can be tracked through the fossil record from the arrangement of the bones and holes in the skull. While this is a complex science, the definition of what makes a crocodile can be condensed down to this:
* They all possess a diapsid skull, meaning that the outer layer of bone has two openings in it behind the eye socket. This also applies to the vast majority of modern reptiles though.
* The skulls have what is called cranial kinesis. This means that parts of the skull can move; this is usually associated with being able to swallow larger prey than would otherwise be allowed. It may also allow for the release of stresses in the skull.
* The nostrils are in the tip of the snout. They are on the top of the snout in modern forms, but this was not necessary for the early land dwelling forms and developed later. In order to achieve this, crocodiles use different skull bones to make their snout than did the phytosaurs or amphibians of the same shape. This identifies them from creatures such as phytosaurs when only fossil bones remain.
Crocodile Facts: The Archosaur Connection
Fossil evidence points to crocodiles evolving from a group of reptiles calledarchosaurs, some 250 million years ago. Other descendants from the Archosaur stock are birds. All dinosaurs were also descended from Archosaurs, but these no longer grace the earth.
Like birds, crocodiles have a four chambered heart. This has been suggested as evidence that they were once warm blooded, and that they re-evolved cold-bloodedness to better suit their environment. They also, like birds, have a cerebral cortex. It's really small compared to mammals, but it's there none the less. While they may look dopey, crocodiles are capable of a great deal more thought and planning than other reptiles. I have experienced their cunning myself, having been on the receiving end of a couple of well planned
crocodile attacks.
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