Land Mammal Faunas of North America Rise and Fall During the Early Eocene Climatic Optimum

Climatic warming at the beginning of the Early Eocene Climatic Optimum (EECO) resulted in major increases in plant diversity and habitat complexity reflective of temporally unique, moist, paratropical conditions from about 53–50 Ma in the Western Interior of North America. In the early part of the EECO, mammalian faunal diversity increased at both local and continental scales in conjunction with a major increase in tropicality resulting from mean annual temperatures reaching 23 ̊C and mean annual precipitation approaching 150 cm/yr. A strong episode of taxonomic origination (high number of first appearances) in the latest Wasatchian and earliest Bridgerian Land Mammal Ages apparently was in response to these greatly diversified floral and habitat associations along with increasing temperature and precipitation. This is in contrast to a similar increase in first appearances at the beginning of the Wasatchian (Paleocene-Eocene Thermal Maximum, or PETM) that can be traced instead to climate-induced transcontinental immigration. In the later part of the EECO, from Br-1b–Br-3, climatic deterioration resulted in a major loss of faunal diversity at both continental and local levels, apparently mirroring climatic deterioration. Relative abundance shifted from diverse, evenly distributed communities to much less diverse, skewed distributions dominated by the condylarth Hyopsodus. Evolutionary innovation through the 53–50 Ma interval included a modest overall increase in body size and increased efficiency in carnivory and folivory as reflected by within-lineage patterns of evolution. Rather than being “optimum,” the EECO engendered the greatest episode of mammalian faunal turnover of the first 15 million years of the Cenozoic era, with both first and last appearances at their highest levels. Both the PETM and EECO faunas were climatically shaped.