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From Reptiles: Conqueors of the Land
Radiation of the reptiles
The reptiles evolved from the early tetrapods some time in the Late Carboniferous (Pennsylvanian) period. The earliest-known reptile is Hylonomus (see p.62), preserved in the Late Carboniferous rocks of Nova Scotia. These rocks are about 300 million years old -- 60 million years after the early tetrapods started the invasion of the land.
From Hylonomus, many different types of reptile evolved. Like the early amphibians, all of the early reptiles seem to have been confined to the ancient continent of Euramerica. Most of these different lineage of reptile can be distinguished by the pattern of openings in the skull.
In addition to skull structure, evidence for the interrelationships of reptiles can be seen in the structure of 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 chelonians (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 spital 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 of the later reptiles, the weight of the animal's skull was reduced by the development of areas in which the bone was replaced by a sheet of lighter, elastic, tendonlike material. This material decays and does not fossilize, 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, but recently doubts have been cast on their validity. The earliest reptiles (such as Hylonomus and other protorothyridids) had no temporal openings, and their skulls are called anapsid.
The ruling reptiles -- including the dinosaurs, pterosaurs, and crocodiles -- have diapsid skulls, with two pairs of openings behind the eyes on each side. The primitive lizards, the sphenodonts (which today are represented by the sole surviving member, the tuatara), also have a diapsid skull. Later lizards have made the skull even lighter and more flexible, by losing the bar of bone below the lower opening on each side. Snakes have taken this evolutionary tendency even farther, by dispensing with the bar of bone between the upper and lower openings.
Other groups of reptiles are not so easily defined. The extinct marine reptiles, the nothosaurs and plesiosaurs, for example, developed a skull pattern (sometimes called euryapsid) similar to that of most lizards. These groups would also seem to have evolved from diapsid ancestors. Two other groups of extinct marine reptiles, the ichthyosaurs and placodonts, also have a euryapsid type of skull. But these animals are very different from one another and also from the plestosaurs. Each reptile group may have evolved from diapsids, but there is no evidence to detail their precise lines of ancestry. A similar uncertainty surrounds the origin of the diapsid herbivorous rhynchosaurs, which are also extinct.
Copyright © 1998 by Marshall Editions Limited.
Copyright © 1999 by Marshall Editions Developments Limited.