THE PROTEROSUCHIA AND THE EARLY EVOLUTION OF THE ARCHOSAURS; AN ESSAY ABOUT THE ORIGIN OF A MAJOR TAXON OSVALDO A. REIG^ CONTENTS Abstract 229 [ntroduction 230 Acknowledgments 231 Foundations 231 rhe extension of the Proterosnchia-concept ._.. 234 rhe intension of the Proterosuchia-concept ... 236 Statement and analysis of the proterosuchian character-states 237 Evolutionary and taxonomic significance of the proterosuchian character-states 245 rhe origin of the Proterosuchia 247 ideological and evolutionary features within the Proterosuchia 259 'roterosuchian descendants 262 Classification and evolutionary significance of the euparkeriids 263 Relationships within the Pseudosuchia 266 The origin of the Crocodilia 270 Saurischian ancestry 274 The case of the phytosaurs and other archo-saurian groups 280 'Ummary of the major events in early archo-saurian evolution 281 evolutionary and taxonomic conclusions 282 iesumen 284 bibliography 287 xldendum 290 vBSTRACT After comments on several methodo-,,)gical and theoretical questions connected ith the classification and the origin of ^Institute de Zoologia Tropical, Facultad de iencias, Universidad Central de Venezuela, Apar-ido 59058, Caracas, Venezuela. major taxa, various hypotheses on archo-saurian origins are discussed. A compar-ative survey of the characters of the early archosaurs, the proterosuchian thecodonts, shows that they are probably derived from the ophiacodont-varanopsid group of pely-cosaurian synapsids. As the synapsids are known to have separated very early from the captorhinomorphs, and as the mil-leretids and younginids, which are cap-torhinomorph derivatives, are considered closely related to the origin of modern lepidosaurian orders, it is concluded that the two groups of diapsid reptiles, lepi-dosaurians and archosaurs, have quite dif-ferent origins. A survey is also made of the present state of knowledge of the origin of the various archosaurian groups. The conclusion is that the final estab-lishment of archosaurian orders as the dominant reptiles of the Jurassic and Cretaceous was the outcome of a gradual process, one which had an exploratory phase during the Middle and Upper Triassic. During this phase, various archo-saurian lines of evolution developed, com-peting among themselves and with the therapsids in the exploitation of two basic food resources: green plants and animals. In the Upper Permian, the roles of plant-eaters and carnivores were mainly plaved by synapsids; from the uppermost Triassic to the end of the Cretaceous, they were mainly played by archosaurs. The Bull. Mus. Comp. Zoo)., 139(5), April, 1970 ongm 229
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