THE BEGINNINGS OF PRIMATES

When and from what mammalian group did primates originate? This question cannot yet be answered as accurately as students of Cretaceous and early Tertiary mammals would like. Although the evidence is still fragmentary and meager, however, the beginnings of primate radiation and their origin can be relatively well recognized. The insectivore-primate transition and the taxonomic boundary have been referred to, mentioned, and occasionally briefly and vaguely discussed in the literature. In discussing the insectivore-primate boundary, one is restricted mostly to known changes and inferred adaptations of dentitions, and to the sparse evidence offered from the pertinent cranial anatomy. Many teeth, dentitions, and a few skulls of Cretaceous and early Tertiary insectivores, condylarths, and primates are the only facts that serve as the basis of the present evaluation. The only inferred evidence that is used relates to dietary habits of some of the fossil forms. This inference is largely based on the vaguely consistent dietary habits of recent primates, that is, on sound behavioral and physiological facts. It was clearly recognized by Simpson (1959:268) that the most primitive known prosimians ". had acquired a rather vague but real grade distinction from the primitive insectivores. " McKenna (1963), in spite of the title of his fine study of Paleocene and Eocene apatemyids, did not specifically discuss the primate-insectivore boundary. The same author in his 1966 paper on the origin of the primates, however, gave a very penetrating, accurate, and relatively detailed examination of some characters (with emphasis on the origin of the bulla and changes in the carotid circulation) of practically all known groups of mammals that have immediate bearing on the nonprimateprimate boundary. An attempt will be made in the following discussion to delineate the earliest known primates from their taxonomically and presumably adaptively different insectivore ancestors. The very first major grade of primate evolution, which probably incorporates all known Cretaceous, Paleocene, some Eocene, and several recent groups, will be referred to as the prosimian grade, briefly sketched by Simpson (1959:268). Before the question of the nature of the insectivore-primate transition is pursued further, several of the pertinent early insectivore families will be briefly examined. In addition, some recognizable important differences and similarities of certain aspects of the dentition and its occlusion will be assessed. It is disputable which of the known Cretaceous or early Tertiary eutherians is most primitive. Although it is not doubted among specialists that the Insectivora and the primitive families of Deltatheridia of the Cretaceous and Paleocene are the two most primitive orders of eutherian mammals, some of the genera must be examined for a more detailed analysis. It appears most probable that a long, roughly transversely running preparacrista and postmetacrista are primitive in the Metatheria and Eutheria. In the recently published lower Cretaceous therian Pappotherium (Slaughter, 1965) and in the Lance marsupials these features are obvious. These long and necessarily transverse preparacrista and postmetacrista are closely correlated with a very efficient transverse shear. Correlated with the noted feature of the upper molars is a rather transverse trigonid, relatively more so than