ALBERT

Description: In their “alternative interpretation” Marshall and Hecht raise two interesting questions: I)Is “Taxocene Analysis” a permissible approach to community evolution, and if so under what conditions?II)What really happened to the ten South American ungulate genera that vanished during the Early Pleistocene?As these are fundamentally different questions, I shall consider each separately.

Description: The American interchange of land mammals reached its acme during the late Blancan and early Irvingtonian in North America and during the Chapadmalalan and Uquian in South America. It lasted about two million years and included taxa adapted to diverse habitats. It was preceded in the early Hemphillian in North America and the Huayquerian in South America by the interchange of a few heralding genera. The MacArthur-Wilson faunal equilibrium hypothesis correctly predicts a marked increase in originations, number of genera, and turnover rate for the South American fauna during the peak of the interchange. Subsequent further increases were not so predicted but closely resemble patterns also observed in late Pleistocene land mammals of Europe and North America. The continued increase in South American land mammal genera after the interchange had largely ceased resulted principally from autochthonous evolution of northern immigrant stocks. A marked decrease in South American ungulate genera (from thirteen to three) coincided with the appearance of fourteen northern ungulate genera and therefore appears to be a replacement phenomenon. The area/diversity relationship predicts no important change in generic diversity if a maximum of only nine percent of North America is occupied by the interamerican mingled fauna. At the family level, however, diversity is seriously overestimated due to the nomenclatural artifact of increased relative diversity by filtering.

Description: When the isthmian land bridge triggered the Great American Interchange, a large majority of land-mammal families crossed reciprocally between North and South America at about 2.5 Ma (i.e., Late Pliocene). Initially land-mammal dynamics proceeded as predicted by equilibrium theory, with roughly equal reciprocal mingling on both continents. Also as predicted, the impact of the interchange faded in North America after about 1 m.y. In South America, contrary to such predictions, the interchange became decidedly unbalanced: during the Pleistocene, groups of North American origin continued to diversify at exponential rates. Whereas only about 10% of North American genera are derived from southern immigrants, more than half of the modern mammalian fauna of South America, measured at the generic level, stems from northern immigrants. In addition, extinctions more severely decimated interchange taxa in North America, where six families were lost, than in South America, where only two immigrant families became extinct.This paper presents a two-phase ecogeographic model to explain the asymmetrical results of the land-mammal interchange. During the humid interglacial phase, the tropics were dominated by rain forests, and the principal biotic movement was from Amazonia to Central America and southern Mexico. During the more arid glacial phase, savanna habitats extended broadly right through tropical latitudes. Because the source area in the temperate north was six times as large as that in the south, immigrants from the north outnumbered those from the south. One prediction of this hypothesis is that immigrants from the north generally should reach higher latitudes in South America than the opposing contingent of land-mammal taxa in North America. Another prediction is that successful interchange families from the north should experience much of their phylogenetic diversification in low latitudes of North America before the interchange. Insofar as these predictions can be tested, they appear to be upheld.