The origin of mammals, often treated as a discrete but obscure event that took place sometime between the Late Triassic and Early Jurassic, can also be viewed as the product of many transitions - from the early synapsid radiations during the Pennsylvanian and Early Permian through the emergence of placentals and marsupials in the Cretaceous (see Hopson, 1969). A fundamental dichotomy appeared early in the evolution of amniotes; sauropsids (represented today by living reptiles and birds) constituted one lineage, synapsids (which includes the mammal-like reptiles- pelycosaurs and therapsids- and their mammalian descendants) the other. Thus, mammalian ancestry may be traced to pelycosaurs that first appear in the fossil record as part of the earliest known reptilian fauna (Carroll, 1982). And mammalian bony and dental structures continued to undergo substantial modification throughout the Mesozoic, long after the appearance of forms technically classified as Mammalia. Given these phylogenetic changes, the suggestion that there occurred a point when mammals “originated” seems simplistic. Yet the major evolutionary stages may still be evaluated with the purpose of identifying the inception of features and functions basic to the radiation of modern mammals. Accordingly, this survey summarizes our current understanding of the evolution of mammals with full acknowledgment that an account of mammalian origins has neither a definitive beginning nor a climactic end.
