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The Pelyeosauria, or Theromorpha, of the late Paleozoic, reptiles particularly characteristic of the Permo-Carboniferous redbeds of the American Southwest, form a group of major importance in the history of land vertebrates. As first recognized by Cope, they represent an early stage in the beginning of mammal-like forms. They are also the dominant types in the oldest adequately known reptilian fauna and possess many archaic features which illustrate the structure of the primitive reptilian stock.

Most of the earlier work on pelycosaurs was done by Cope. He realized the significance of the material, but his brief and often inadequate descriptions and meager illustrations failed to give the group the scientific prominence it deserved. A decade of work on the group by Case culminated in the publication in 1907 of his Revision of the Pelyeosauria of North America. Case figured most of the important material then known, straightened out many taxonomic snarls, and gave the first account of the general structure of a number of the more adequately represented types. Through the influence of this work, the pelycosaurs were at length generally accorded a place of importance among fossil vertebrates.

This earlier period of the study of the pelycosaurs occupied essentially 3 decades. Since Case’s 1907 work increases in our knowledge have been slow. The paucity of material of even the most common types necessitated the leaving of lacunae in Case’s descriptions, few of which have since been filled. Many new data have been obtained, but little has . . .

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