The history of reptiles, from their first appearance during the Carboniferous to the present, is traced on this chart. (Each white area represents a major order plotted according to when it first began to flower and how long it lasted.) In addition to the major groups, many short-lived offshoots developed. For reasons of space only two of them, represented by the marine forms Geosaurus and Tylosaurus, have been included here. Solid bars on the chart indicate lines of descent which have been fairly well established by the fossil record. Broken bars are used where the fossil evidence is sketchy.
A striking aspect of reptile history is how, from the primitive cotylosaurs (here represented by Seymouria), these creatures radiated to occupy an enormous variety of niches on land, in the water, and in the air. One group of cotylosaurian descendants that played a profound role in the development of reptiles was the thecodonts, primitive archosaurs. Not only did they give rise to the Ornithischia and Saurischia (popularly called dinosaurs), but also the Pterosauria (flying reptiles) and the Crocodilia. Thecodonts were even related to the ancestral birds. The mammals evolved from another group, the therapsids, shown at lower left.
Another curious fact of reptilian evolution revealed by this chart is the relative suddenness with which order after order disappeared toward the end of the Cretaceous, described as “the time of the great.”
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