Activities - A Skeletal Map

This is the skeleton of Sarcosuchus. About 50% of the skeleton was discovered. This drawing represents a fully grown adult--nearly 40 feet long. We put an average-sized human (5' 10") near the skull for scale to show the enormous size of this ancient croc.

Illustration by Paul Sereno and Carol Abraczinskas

The drawing you see here is a "composite" based on several fossil skeletons, none of which were 100% complete. The best partial skeletons the 2000 Niger Expedition team discovered belonged to two young adults. The one of these was particularly important because it preserved the armor plates in order along its back. This find was the key to understanding how the plates changed shape along the body.

A skeletal map like this requires a lot of detective work. You need to "size adjust" different skeletons so that you can make a single composite skeleton that has all of the bones that you discovered in the right proportions. You need to enlarge the bones of juveniles or young adults to match the bones of the adults.

B. Neck vertebra
Did you ever notice that, unlike a lot of mammals, a croc's neck is not narrower than its head? Look at drawing A. Sarcosuchus had a very strong neck - but one that did not allow a lot of bending. The neck of living crocodilians is composed of eight vertebrae and, judging from the fossil bones we found, we think that Sarcosuchus had the same number. The neck vertebrae have a relatively stout centrum (body) that is spool-shaped. From this we can tell that the neck was relatively stiff and projected straight forward to the head.

C. Back vertebra
The back of a croc is very different than ours. It is very stiff. The vertebae are tightly joined and connected by strong ligaments to the overlying armor plates. The dorsal vertebrae (back vertebrae) have a large centrum (body), long transverse processes (tp) that attach to the ribs, and a short thick neural spine (ns) on top that supported the armor plates.

D. Neck armor plate
The armor plates in the neck are asymmetrical--they have an angled outer edge because each successive plate is getting broader toward the back. The osteoderms (armor plates) of Sarcosuchus overlapped each other like roofing tiles. You can see a smooth surface (om) on the front edge of each plate that was overlapped by the plate in front.

E. Back armor plate
These beautiful rectangular plates are the largest of all the body armor. In life these plates were covered with skin. All of the little depressions were filled with blood and used by Sarcosuchus to adjust its body temperature, just as in living crocs. Ever see a croc basking in the sun? As the back heats up, the hot blood is circulated to other parts of the body. So the plates did more than protect the animal from attack.

F. Tail armor plate
The tail is one of the most powerful parts of a croc and is used much more than the limbs while swimming. The tail armor has square plates that get smaller toward the middle of the tail. There are no armor plates on the last half of the tail. In Sarcosuchus, as in living crocs, the front half of the tail has a flat upper surface between two rows of spines.
The last half of the tail, in contrast, is very narrow with a single median row of spines.

G. Pelvic girdle
The pelvic girdle is composed of three bones--the ilium (il), ischium (is) and pubis (pu). These bones surround the hip socket (acet), where the hind limb attaches. Sarcosuchus had a very heavy body, and the limbs were sprawled out to each side of the pelvic girdle. It would have dragged itself over the ground, although it was probably capable of lunging with incredible speed. Sarcosuchus would have been more at home in the water, where it could swim with grace.

• Number of bones (not counting skull and armor) = about 250
  
• Number of armor plates along the neck, back and tail = about 70
  (2 rows of 35 plates)
  
• Number of vertebrae in the spinal column = about 65
  
• Longest bone in skeleton = femur