MAMMAL BODY SIZE CHANGE IN THE ZONE OF THE EARLY EOCENE HYPERTHERMALS ETM2 AND H2

The early Eocene is relevant for testing the recent hypothesis that decreasing body size is the “third universal response” to warming. Increasing temperature during the early Eocene culminated in the Early Eocene Climatic Optimum (EECO), the hottest sustained period of the Cenozoic. Superimposed on this long-term trend were several short-term ‘hyperthermals’, including the Paleocene-Eocene Thermal Maximum (PETM) at the base of the Eocene, and ETM2 and H2, smaller events ~2 my later in the lead-up to the EECO. A superb mammal fossil record from the Fifteenmile Creek (FC) section of the Willwood FM in the south-central part of the Bighorn Basin, WY, spans these events and provides the opportunity to test the universality of body size response to rapid warming. The PETM has been comprehensively studied, but changes in body size at ETM2 and H2 are not well understood.

Two carbon isotope excursions (CIEs) attributed to ETM2 and H2 have been described in the northern Bighorn Basin in proximity to geomagnetic and biostratigraphic events. More than 32000 mammal fossils from >100 lineages are known from a 220-meter thick stretch of the FC section that brackets and includes these levels. Previous analysis identified two brief episodes of change, faunal events B-1 and B-2, hypothesized to be related to ETM2 and H2 based on a similar relationship to the geomagnetic and biostratigraphic events near their CIEs. More than 7500 complete lower first molars from the FC sample are used to calculate natural log-transformed occlusal surface areas as a proxy for body size.

Mean molar area for the whole FC sample is 10-20% smaller (statistically significant) during faunal events B-1 and B-2 than before and after the events. Similarly, nearly half of all mammal lineages decreased in size at the PETM through the immigration of small species and the temporary dwarfing of lineages, probably via metabolic effects. However, at faunal events B-1 and B-2, there are no documented immigrants and new species appearing through morphological innovation are both larger and smaller than their close relatives. Dwarfing is apparent in only a few lineages. Instead, the overall size decreases at faunal events B-1 and B-2 are driven by shifts in (standardized, proportional) relative abundance, which favor the smaller species in eight of the 10 most common families.