The Early Cenozoic Mammalian Radiation

The fossil record documents an extensive and rapid— often described as "explosive"—adaptive radiation of mammals during the first third of the Cenozoic, characterized by a dramatic increase in diversity of therian mammals soon after the mass extinctions at the end of the Cretaceous (e.g., McKenna and Bell, 1997; Alroy, 1999; Novacek, 1999; Archibald and Deutschman, 2001). Nearly all of the modern mammal orders, as well as many extinct orders, first appear in the fossil record during this interval (Rose and Archibald, 2005). This era was the "Beginning of the Age of Mammals" alluded to by Simpson (1937c, 1948, 1967).

The adaptive radiation was particularly intense soon after the final extinction of nonavian dinosaurs at the K/T boundary. In the famous Hell Creek section of Montana, for instance, Archibald (1983) found that diversity increased from an average of about 20 mammal species immediately following the K/T boundary to 33 species within the first half-million years, 47 after 1 million years, and 70 after 2-3 million years. For the same intervals, the number of genera rose from about 14 to 30, then 36, and finally 52. Although some of these numbers could be inflated as a result of reworking (discovered subsequent to Archibald's analysis), the overall pattern was upheld in a more recent study by Clemens (2002), who reported that 70% of early Puercan mammals of Montana were alien species new to the northern Western Interior of North America. Similarly, Lille-graven and Eberle (1999) observed a significant mammalian radiation, particularly involving condylarths, at the beginning of the Cenozoic (after the disappearance of nonavian dinosaurs) in the Hanna Basin of southern Wyoming. Only nine mammal species, including just two eutherians, were present in uppermost Cretaceous strata. By contrast, 35 species (75% of them eutherians), almost all presumed immigrants, were recorded from the earliest Paleocene. They further reported that "major experimentations in dental morphology and increasing ranges of body sizes had developed within 400,000 years of the [K/T] boundary" (Lille-graven and Eberle, 1999: 691).

Based on ranges provided by McKenna and Bell (1997), 52 families of mammals are known worldwide from the early Paleocene, but only eight of them continued from the Late Cretaceous—more than 80% were new (Fig. 1.1). Only five therian families are known to have crossed the K/T boundary, two of which are present in late Paleocene or Eocene sediments but have not yet been found in the early Paleo-cene. On a more local level, Lofgren (1995) reported that the survival rate of mammalian species across the K/T boundary in the Hell Creek area of Montana was only about 10%.

Fig. 1.1. Family diversity of mammals from the Cretaceous to the present. Bars indicate the number of families recorded from each interval; the shaded portion denotes the number of those families also present in the immediately preceding interval. Key: Cret., Cretaceous; E, early; L, late; M, middle; Olig., Oligocene; Pal., Paleocene; Plei., Pleistocene; Plio., Pliocene, R., Recent. (Compiled from McKenna and Bell, 1997, with minor modifications.)

Fig. 1.1. Family diversity of mammals from the Cretaceous to the present. Bars indicate the number of families recorded from each interval; the shaded portion denotes the number of those families also present in the immediately preceding interval. Key: Cret., Cretaceous; E, early; L, late; M, middle; Olig., Oligocene; Pal., Paleocene; Plei., Pleistocene; Plio., Pliocene, R., Recent. (Compiled from McKenna and Bell, 1997, with minor modifications.)

Thus there appears to have been a sharp decline in mammalian diversity at the end of the Cretaceous, followed by a fairly rapid rise in diversity soon after the K/T boundary

Although the geographic source of many of the newcomers is uncertain, it is important to note that many early Paleocene metatherians and eutherians can plausibly be derived either from other early Paleocene forms or from known Late Cretaceous therian families (including some that did not cross the boundary). For these mammals, it is not necessary to postulate long periods of unrecorded evolution. But it is questionable whether all the diversity that emerged in the Paleocene can be traced to the small number of lineages that we know crossed the K/T boundary Could the alien species of the northern Western Interior represent clades that were evolving in areas that have not been sampled? And if so, could these clades have existed for a substantial period during the Mesozoic? The answers to these questions are unknown. However, as shown in Fig. 1.1, the fossil record documents that family-level diversity continued to increase through the middle Eocene, then declined somewhat into the early Oligocene, after which it rose again to an all-time high in the middle Miocene (a standing diversity of 162 families). Notably, up to the middle Eocene, the number of new families equaled or exceeded the number that continued from the previous interval.

The present volume is an attempt to summarize current knowledge of the record of this extensive Paleocene-Eocene radiation and the roles of mammals in the world of the Early Cenozoic, which are essential for understanding the structure and composition of present-day ecosystems. This volume focuses on the fossil evidence of these early mammals and what their anatomy indicates about interrelationships, evolution, and ways of life. First it is necessary, however, to touch on several issues that affect the interpretation of that record. These include the timing of the radiation, how phylogenetic relationships are established, the interrelationships and classification of mammals, and the chronologic framework of the Early Cenozoic.

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