Rapid morphological change in Miocene marsupials and rodents associated with a volcanic catastrophe in Argentina

Large fossil samples collected from a continuous series of stratigraphic levels are ideal for studying the tempo and mode of phyletic evolution. Study of two such coeval mammalian lineages (caenolestoid marsupials and octodontoid rodents) from Miocene rocks of southern Argentina revealed marked, simultaneous changes in molar dimensions in both lineages. The changes occur across a 1.65-1.80 m thick tuff unit that covers a widespread bonebed. Geologic and taphonomic data indicate that the tuff unit marks the rapid (geologically instantaneous) burial of an ancient land surface by pyroclastic products of a single major volcanic eruption. This suggests that the morphological changes observed in the fossil mammal samples were associated with catastrophic reduction in population size or the local or regional extermination of small mammal populations. Samples of both Lineages showed changes in dental dimensions that are not simply the continuation of long-term morphological trends established before the volcanic event. Rapid ''jumps'' in morphology are seen in the opposite direction to longterm trends. The observed changes are consistent with responses to new selection pressures imposed by a strongly altered environment, and with the genetic and phenotypic phenomena associated with rapid and severe population reductions (''bottlenecks''). This is one example of the effect that geologically instantaneous environmental events can have upon evolving lineages, both immediately following the event and for geologically significant periods of time thereafter. Such morphological ''punctuations'' may easily occur without lineage splitting (cladogenesis). This suggests that traditional punctuated equilibrium theory may place too much emphasis on processes associated with speciation in the generation of macroevolutionary patterns.