Yolk sac their ankles and major blood vessels. In some reptiles, one of the ankle bones is hooked, and provides extra leverage for one of the foot muscles (just as our projecting heel bone provides leverage for the Achilles tendon). This type of ankle is found in all lizards and chelo-nians (including modern turtles and tortoises), as well as in the "ruling reptile" lineage — the crocodiles, dinosaurs and their relatives.
The living representatives of all these groups also have an unusual arrangement of the major blood vessels near the heart. These twist around one another in a spiral fashion.
For all these reasons — skulls, ankles and blood vessels — paleontologists are confident that these groups of reptile are closely related to each other.
In the earliest reptiles, as in their amphibian ancestors, the skull was a box of bone, without any openings except for those of the eyes and nostrils. The muscles of the jaws were attached to the underside of the bony roof of the skull.
In most later reptiles, the weight of the skull was reduced by the development of areas in which the bone was replaced by a sheet of elastic, tendonlike material. This material decays, but the areas in which it was located appear in a .fossilized skull as holes (called temporal openings) between the bones. These openings not only serve to lighten the skull, but also provide additional attachment points to which the jaw muscles can attach, thereby increasing the bite-power of the jaws.
The presence or absence of these temporal openings in the skull form the basis for grouping reptiles into major groups or subclasses (above right).
The earliest reptiles (such as Hylon-omus and other protorothyridids, and mesosaurs) had no temporal openings, and their skulls are called anapsid. The chelonians (today represented by turtles, tortoises and terrapins) also lack openings in their skulls, and so they, too, are placed in the anapsids, although it is not certain from which group of early reptiles they evolved.
The ruling reptiles — including the thecodontians and their descendants,
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