A dinosaur skeleton also reveals several features unique to dinosaurs.
These are found in no other animals, and because of this paleontologists can use them to identify a new fossil as a dinosaur.
For example, dinosaurs had jaw muscles that extended out onto the top of the skull, through the highest opening behind the eye.
The humerus (first bone in the arm) had a long, strong ridge for the attachment of a large muscle.
There is also a distinct process at the knee joint, for the attachment of another muscle.
Other features are not unique to dinosaurs—they can be found in some other animals—but do serve to help distinguish dinosaurs from most other reptiles.
The limbs of dinosaurs were held upright, like those of birds and mammals.
This is clear from the fact that the femur (thigh bone) was straight and had a distinct head, which stuck out from the shaft and directly into the hip socket.
The ankle was a hinge joint, only allowing back-and-forth motion. Dinosaurs also walked on their toes (this is called digitigrade posture), with the ankle held well off the ground.
In addition, the dinosaur hip socket was open, and did not have any bony wall along the inside. The neck tended to be long and curved. In the hand, the longest finger was the second, whereas in the foot the longest toe was the third.
Edmontosaurus mounted skeleton
Thus each particular feature of a dinosaur tells us something about the evolutionary history of the animal.
Most of what you see in a dinosaur comes from some point in its ancestry, either close or distant. Other features help to distinguish dinosaurs from all other animals, and still others can identify each type of dinosaur
