Christine M. Janis, Professor
MAMMALIAN PALEBIOLOGY (esp. UNGULATES)
Ph.D., Harvard University, 1979

e-mail: Christine_Janis@brown.edu

The Eocene-Oligocene boundary has long been considered an important stratigraphic marker in mammalian evolution, the point of major extinctions in the Northern Hemisphere, known in Europe as the Grand Coupure. The Eocene is also a time of important climatic changes. The boundary between early and middle Eocene was the time of the maximum mean annual temperatures in the Northern Hemisphere, and the mammalian extinctions of the late Eocene have been considered to be correlated with climatic deterioration and corresponding effects on the vegetation.

Stratigraphic redating has shown that the major extinctions in North America occurred earlier than had previously been assumed, primarily in the late middle Eocene (within the Duchesnean Land Mammal age), although a further wave of extinctions occurred at the end of the Eocene. The patterns of extinction have been assumed to mainly affect archaic mammals, surviving from the Paleocene, with the more modern groups appearing in the Eocene being more immune to the late Eocene events.

A more detailed examination of patterns of diversification and extinction in North American ungulates and ungulate-like mammals reveals a complex pattern correlated with climatic changes. In this paper, the taxa are divided into groups according to dental morphologies, representing mammals with an omnivorous diet (bunodont cheek teeth), with a frugivorous-folivorous diet (semi-lophed cheek teeth), and with a fibrous, folivorous det (fully lophed cheek teeth). The general pattern of ungulate diversification differs from that of mammals in general. Although, like other mammals, ungulate generic diversity increases in the early Eocene, the ungulate diversity increases to a maximum at the end of the middle Eocene, but the general curve of mammalian diversity decreases in the middle Eocene. There is a slight decline in ungulate diversity in the late Eocene, regaining early Eocene levels by the end of the Oligocene.

This overall pattern masks three separate evolutionaty patterns in ungulates of different dietary types. Bunodont ungulates were small and abundant in the Paleocene. Their numbers declined in the middle and late Eocene, but the total diversity of bunodont taxa was boosted in the Oligocene by the emergence of large, specialized modern forms (suoid artiodactyls). Today suoids (pigs and peccaries) represent the only bunodont ungulate taxa, and are not very taxonomically diverse. They are found in faunas in both tropical and temperate habitats. Semi-lophed ungulates were fairly abundant in the late Paleocene and early Eocene, but underwent dramatic diversification in the late middle Eocene, almost doubling their generic diversity by the end of the period. They experienced some decline in the late Eocene, and underwent some profound extinctions at the end of the Eocene and in the Oligocene. Today only a few genera semi-lophed ungulates exist, found only in tropical habitats. Fully lophed ungulates did not appear until the latest Paleocene, and were of only moderate diversity, and of fairly small size, through the early and early middle Eocene. In the late middle eocene they, like the semi-lophed ungulates, underwent a great diversification and also expended their range of body sizes into larger forms. Unlike the semi-lophed ungulates, the fully lophed ones maintained their elevated taxonomic diversity through the late Eocene and into the Oligocene. Today fully lophed ungulates are the predominant ungulates world-wide, found in tropical, temperate, and even artic habitats. The parrerns of radiation and extinction among North American ungulates can be linked to the patterns of climatic change rather than to the geological boundaries per se. The cooling climate of the later Eocene appears to have promoted the diversification of the more folivorous semi-lophed and fully lophed taxa, but the more profound cooling at the end of the Eocene left only fully lophed taxa and specialized, modern bunodont taxa as the main components of the Oligocene ungulate fauna, similar to the conditions of the present day.

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*presented at the 5. Intern. Congr. Of Vertebrate Morphology, Briston, July 12-17, 1997.

Zoology 100 (1997/98), 203-220 „ by Gustav Fischer Verlag

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