THE BEGINNING OF THE AGE OF MAMMALS
Authors: SIMPSON GEORGE GAYLORD 
Published: January 2008

The mammals of the Paleocene, first epoch of the Tertiary, the Age of Mammals, are essential for the elucidation of numerous zoological, biological, and geological problems. Among these problems are determination of the affinities of mammals in general, of their ancestral and primitive structures and of the course of their evolution, as well as problems of the origin and nature of adaptations and habits, and more special and, in one sense, practical problems of stratigraphy and some other branches of geology.

The first known Paleocene mammal was described in 1841, but intensive work began with Lemoine's first publication in 1878. Since that time work has continued at ever accelerated pace, by Cope, Osborn, Wortman, Matthew, Granger, Sinclair, Douglass, Gidley, Schlosser, Teilhard, Jepsen, Russell, Patterson, Simpson, and others. A nearly complete sequence of Paleocene mammalian faunas is now known from North America, and more limited but also important faunas are known from Europe, Asia, and South America.

These faunas include multituberculates, marsupials and placental mammals, classified in seventeen orders, the general characters of which in the Paleocene are eviewed. From these mammals it is possible to infer with high probability the ancestral characters of placental mammals in general, the evidence for a primitive “tritubercular” or trigonal‐tuberculosectorial primitive molar type being particularly conclusive and important.

The Cretaceous‐Paleocene transition in North America is marked by the disappearance of dinosaurs and the appearance of several orders of mammals apparently as immigrants from some unknown region. The Paleocene sequence on the same continent, which still has two breaks not represented by known faunas, is marked not only by great evolutionary advance but also by progressive enriching of the faunas, chiefly by the appearance of new and generally more progressive mammalian groups as immigrants. The Paleocene‐Eocene line is drawn at the culmination of this faunal change. Although in detail the change is by intergradation and gradual transition, from a broader point of view it marks a very radical difference in mammalian faunal type, the Paleocene forms eventually disappearing and the Eocene forms being the forerunners of the later Tertiary and Recent faunas. The same faunal change eventually occurred in South America, but at a much later date, around the end of the Tertiary.

In the Upper Paleocene Asia, Europe, North America, and South America all show considerable local differentiation but give evidence of the derivation of their faunas from a common source. Those of North America and Europe are fairly similar, although not identical, and that of South America is most distinctive, evidence of longer separation from the other continents.

In a general summary of known mammalian faunal history the few known Triassic mammals have no clear significance. The Jurassic mammals of Europe and North America are of distinctive type, with four primitive orders. From two of these developed the multituberculates, marsupials and insectivores of the Upper Cretaceous. Further differentiation of these three, but particularly of the general placental, carnivore‐insectivore stock produced the typical Paleocene faunal type. Finally, progressive evolution and diversification of the several Paleocene placental mammal stocks gave rise to the Eocene faunal type which still exists to‐day.

Summary of Mammalian Faunal History

The oldest known mammals, from the Rhaeto‐Lias in Europe and Africa, do not include the ancestors of the later mammals and have little bearing on mammalian faunal succession (see Simpson, 1928b). The Middle Jurassic fauna of England and the Upper Jurassic faunas of England, the United States, and East Africa (one specimen) are of a distinctive faunal type and suggest that this sort of mammalian fauna had then spread over a large part of the world. They include multituberculates, triconodonts, symmetrodonts, and pantotheres (Simpson, 1928c, 1929c). The multituberculates reappear, in more advanced and varied form, in the Cretaceous and Lower Tertiary. The triconodonts and symmetrodonts do not reappear and probably became extinct during the early Cretaceous. The known pantotheres seem to represent a Jurassic radiation from the common marsupial‐placental stock.

The known Upper Cretaceous faunas also are of a distinctive faunal type, but one quite different from that of the Jurassic. They consist of multituberculates (Asia and North America), marsupials (North America), and very primitive placentals of rather undifferentiated insectivore‐carnivore type, classified as Insectivora (Asia and North America) (Gregory & Simpson, 1926; Simpson, 1928d). The latter apparently represent primary dichotomous differentiation of the general pantothere stock, with a secondary local radiation within each group.

The known Paleocene faunas of North America, Europe, Asia, and South America probably all had a common source and represent the radiation of a fauna derived from one of the known Cretaceous types but much more highly differentiated. Among the multituberculates and marsupials this differentiation was of relatively minor grade, in taxonomic terms of family or at most subordinal rank, while the more progressive and adaptive placentals show the beginnings of a more profound splitting, ultimately of ordinal rank,1 and are more numerous and varied. In North America, at least, this new faunal type appears as an invasion from some unknown evolutionary center.

In Europe, North America, and Asia, a new type of fauna began to appear during the Paleocene, the change culminating at the end of the epoch and becoming entirely complete during the Eocene. The new fauna is less markedly different from the old than in the previous changes noted, and consists of the appearance of new or “modernized” groups clearly derived from an already partly differentiated fauna of Paleocene type. The new forms appear to be immigrants where found, and came from some unidentified area where the earliest Paleocene fauna was well developed and where its rapid and diversified evolution was permitted and stimulated. Only placental mammals were involved, the few surviving multituberculates and marsupials clearly being stragglers from the known Paleocene.

There has not been any other major spread of mammals or great change in faunal type. With positive changes resulting from long evolution and from repeated intermigration and negative changes resulting from extinction, the mammals now peopling at least the Holarctic continents are essentially those that appeared there in the Eocene invasion. In South America this change was long delayed, and what is essentially the incursion of the Holarctic Eocene fauna into the previous habitat of the Paleocene fauna took place at the end of the Tertiary and not toward its beginning as in Holarctica. In Australia this change never took place (aside from the agency of man). The early faunal history of Africa is unknown and still beyond logical conjecture.