Bison evolution and zoogeography in North America during the Pleistocene.

Abstract

The fossil record and information about contemporary forms provide evidence that the evolutionary pattern of bison cannot be interpreted as either a unidirectional decrease in horn size or as a series of successive invasions to the New World from the Old. Rather, some species have persisted and remained relatively unchanged for long periods of time, while elsewhere other contemporaneous species were changing quite rapidly. Although the trends in the evolution of bison horn size have been remarkably regular, major reversals have taken place. Bison arose in Eurasia and have had a much longer history there than is North America. In spite of this longer history in the Old World, bison have undergone greater evolutionary changes in North America. This can be explained by a different mode and intensity of competition in the New World. The major points presented are the following: (1) The giant-horned B. latifrons was a New World product. (2) B. priscus (= B. crassicornus) appeared early as a holarctic northern species and remained in that niche until the late Wisconsin (Wurm). (3) Most of the other bison species in the late Pleistocene were derived indirectly or directly from this widespread northern species. (4) Middle and Late Pleistocene bison can be place into four species: B. priscus, which can be dated at least as far back as early mid-Pleistocene; B. latifrons, which extends back at least to late Illonoian (Riss) time (it is possible that B. latifrons gave rise to B. antiquus; if so the species B. alleni should be maintained); B. antiquus, which originated during the early to middle part of the Wisconsin (Wurm) glaciation; and B. bison, which was a late Wisconsin product. (5) B. latifrons became extinct, at least over most of its range, in pre-Wisconsin time. B. priscus and B. antiquus became extinct in the late Wisconsin, and B. bison still exists in relict populations. (6) Two or more species of bison have not occurred sympatrically for extended periods of time. (7) Neither the "orthogenetic" nor the "wave" theory adequately accounts for the evolution of bison in North America; rather, the fossils can only be explained by a combination of invasions from Siberia and evolutionary changes that occurred in the new environment.