Local extinction and ecological re-entry of early Eocene mammals

Abstract

The use of high-resolution stratigraphical control in studying the species sequencing patterns of early Eocene mammals has recently been demonstrated by Gingerich et al.^1–5. These studies have documented the nature of the tempo and mode of evolution and have stimulated the debate between adherents of phyletic gradualism and punctuated equilibria⁶. All studies have been largely verticalist (evolutionary) in outlook, equating change within a single basin of deposition with evolution. However, when I applied these techniques to the early Eocene condylarth, Phenacodus, I found a strong lateral (biogeographical and ecological) component influencing the sequencing of species in a single basin. I report here that Phenacodus species are relatively static in size and morphology throughout the local section studied, and two of these show statistically significant discontinuities in temporal range. The clumped re-entry of these species after local disappearance along with the introduction of other new taxa points to ecological control of vertical events within a local section.