Tetrapoda is diagnosed by the appearance of limbs with the distinctive arrangement of bones shown in Figure 4.4. So now let's take a closer look at Tetrapoda and, because we're interested in dinosaurs, we'll try to understand the part that's generally best preserved: the skeleton.

The tetrapod skeleton made easy

Figure 4.5 shows a typical tetrapod skeleton - in this case a prosauropod dinosaur - blown apart. Not surprisingly (because they're monophyletic), tetrapods are all built in the same way: a vertebral column is sandwiched by paired forelimbs and paired hindlimbs. The limbs are attached to the vertebral column by groups of bones called girdles. At the front end is the head, composed of a skull and mandible, or lower jaw. At the back end is the tail. It's that simple!

Vertebral column. The vertebral column is composed of distinct, repeated structures (the vertebrae), which consist of a lower spool (the centrum), above which, in a groove, lies the spinal cord (Figure 4.5). Planted on the centrum and straddling the spinal cord is the neural arch. Various processes, that is parts of bone that are commonly ridge-, knob-, or blade-shaped,

Figure 4.5. Exploded view of a tetrapod skeleton exemplified by the saurischian dinosaur Plateosaurus.

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(finger bones)

Caudal (tail) vertebrae

Caudal (tail) vertebrae

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