A statistical examination of the change in body size of mammalian communities across the Eocene-Oligocene Boundary

The body mass change of mammals across the Eocene-Oligocene Boundary (EOB) and the potential relationship with global climate change was studied. Global climate went through a period of dramatic cooling and drying during the Eocene-Oligocene Transition. The effects on the environments of North America were dramatic, causing a shift from abundant, dense rainforest canopy in the Eocene to open savannah environment in the Oligocene. Mammal faunas from the White River Group of the continental interior of North America were investigated. Statistical surveys of large mammal faunas (> 1 kg) from the Douglas, Wyoming, and surrounding area and small mammal fauna (< 1 kg) from northwestern Nebraska were performed to assess three hypotheses regarding body mass change: (1) climate change has no effect on body mass, (2) climate cooling increases mammalian body mass, and (3) climate cooling decreases mammalian body mass. Mass of large mammals was estimated from the length of skulls and the width of occipital condyles. Mass of small mammals was estimated based on the size of the first lower molar. The Eocene fauna studied here contains some of the largest and smallest mammals of all time; body mass estimates in this study ranged from as small as 4.5 g in the insectivore genus Oligoryctes up to titanotheres massing 1,589 kg. Large mammals were found to decrease in body mass across the EOB, whereas body mass of small mammals increased. This phenomenon of extreme body masses approaching the middle of the range of body masses is likened to the Island Effect, where large mammals decrease in body size and small mammals increase in body size to accommodate a decrease in available habitat and resources. The change in body mass was attributed to environmental degradation brought on by climate change. Lower body mass evolved in large, herbivorous mammals as they adapted to diminished food resources. Small mammals likewise responded to climate change by adapting to the prevalence of open environments, and granivorous diets evolved as tropical vegetation waned.