MAMMALIAN RESPONSES TO CLIMATE CHANGE AT THE PALEOCENE-EOCENE BOUNDARY: TRANSIENT DWARFING AND COSMOPOLITAN DISPERSAL

The Paleocene-Eocene transition in North American land-mammal faunas is well documented on the south side of Polecat Bench and in surrounding badlands of northwestern Wyoming. Here a rich fossil record is known from a stratigraphic section with an established geomagnetic polarity time scale and medium- and high-resolution carbon and oxygen isotope records. The Paleocene-Eocene carbon isotope excursion (CIE) and temperature maximum (PETM) associated with greenhouse warming are both represented, starting at about 55 Ma and lasting approximately 86 k.y., during the transition from Paleocene to Eocene epochs and from Clarkforkian to Wasatchian land-mammal ages. The most salient changes in mammals at the Paleocene-Eocene boundary are: (1) transient dwarfing of earliest Wasatchian (Wa-0) mammals during the CIE/PETM interval; and (2) cosmopolitan dispersal and more-lasting changes in faunal composition.

Body size dwarfing in Wa-0 mammals cannot be explained as a response to temperature alone, and elevated atmospheric CO₂ probably played an important role. Body size is influenced by food quality. Experiments with plants grown in CO₂-enriched atmospheres (double present levels) yield enhanced photosynthesis, with more carbon and less nitrogen in plant tissues. The extra carbon enhances production of secondary compounds, inhibiting digestibility. CO₂ enrichment reduces rubisco enzyme in foliage, decreasing leaf protein and lowering nutritional value for herbivores, which causes the herbivores to grow more slowly. Slower growth, coupled with any of the usual seasonal temperature, rainfall, or day-length effects controlling reproductive cycles, would lead to reproduction at smaller body size. Such an explanation involves temporary and reversible environmental selection leading to a transient evolutionary dwarfing response.

Faunal changes lagged onset of the CIE by some 13 and 22 k.y., consistent with the idea of a stepped response to climate change. In Wyoming, endemic Meniscotherium appeared in the first step of response some 13 k.y. after initiation of the CIE. Cosmopolitan Artiodactyla, Perissodactyla, and Primates appeared in the second step, some 9 k.y. later, at about the time they appeared on all three northern continents. Climate change can have profound and lasting effects on mammalian faunas.