view, the marine record of positive oxygen isotopes excursions can be read as a proxy for eustatic sea-level lowering. The second hypothesis relates first-order immigration episodes to high faunal turnover intervals and considers them to represent fundamental reorganizations of the continental ecosystem. In this view, the largest positive oxygen isotope excursions record major cooling episodes, which also radically alter terrestrial ecosystems. We consider both hypotheses as we compare the immigration data from the North American mammal record with oxygen isotope data from the marine realm.
The predicted correlation between first-order immigration episodes and rapid climatic cooling or broader land bridges, as represented by the oxygen isotope record, is corroborated in some cases but miscorrelated in others. Two clearly contradictory episodes are the Oligocene (35 to 30 Ma) and the Middle Miocene (16 to 6 Ma) when, despite major global cooling events, immigration rates were exceedingly low in North America. Possibly no land bridges were accessible in the Oligocene, but this explanation is less likely during the Middle Miocene when Beringian routes were probably accessible. It is more likely that these intervals carried few immigrants because they represent the two most stable chronofaunal phases in North American mammalian history. The Clarendonian chronofauna of the Middle Miocene, despite several episodes of global cooling and severe sea-level excursions, remained nearly closed to intercontinental immigrants. During such robust chronofaunal intervals the continental mammal fauna stood near its ecological capacity.
This analysis of the land mammal record indicates that climatic shifts represented by first-order changes in the isotopic record are necessary but not sufficient causes of first-order immigration episodes into the North American continent. Clearly the question of why the continental ecosystem was open to immigration during certain times of isotopic excursions and resilient to others has fundamental significance in understanding the stability of present and future continental ecosystems.
The rich record of Cenozoic land mammals provides not only a panoramic perspective on evolutionary processes, but also a valuable history of environmental change on the continents. As presently known, Cenozoic mammals number about 4500 genera, including 1051 living genera (M. McKenna, American Museum of Natural History, personal communication, 1992). Excluding bats, the number of extinct genera exceeds that of living genera by a factor of about four. Taxonomically and stratigraphically, land mammals are especially well documented in North America and Europe; on most other continents they are moderately well documented but with significant chronological gaps. Even on Antarctica, fossil mammals are now represented, albeit by only a few Paleogene genera from Seymour Island. Improved methods for sampling smaller forms (e.g., screenwashing and flotation) and continued success in discovering fossils in previously unsampled regions have greatly strengthened the fabric of the global mammalian succession.
Similarly, the chronostratigraphic framework for recording the Cenozoic history of land mammals has vastly improved in the past two decades. The succession of fossils, including evolutionary events, immigrations, and extinctions, provides the fundamental data set, but these biostratigraphic data are integrated via the stratigraphic context with other methodologies, most importantly with radiometric dates (principally K/Ar and more recently Ar/ Ar) and paleomagnetic chronology (Lindsay et al., 1984). In North America, the Cenozoic Era is divided into 19 land mammal ages, including 52 (mainly informal but widely recognized) subdivisions (Woodburne, 1987). Thus 66 m.y. of Cenozoic mammalian history in North America can be divided into units that average less than 1.3 m.y. In the late Cenozoic the precision of mammal faunal dates approaches 0.5 m.y., and in favorable sections, wherein paleomagnetic and radiometric data are integrated with biostratigraphic data, particular strata may be resolved chronologically to less than 0.1 m.y. Such sections are not uncommon in western North America (Woodburne, 1987).
As the history of mammalian evolution has become more precisely calibrated, it has become increasingly evident that its tempo is strongly syncopated. Nowhere is that more evident than in the record of land mammals that immigrated to North America. We show that about 70% of immigrant genera were concentrated during about 10% of the Tertiary Period. Of the seven first-order immigration episodes that we define, one may be artifactual, and the six "real" ones occur in closely yoked pairs. In this chapter we investigate the global and regional environmental significance of such major land mammal immigration episodes.