MAMMALIAN IMMIGRATION AND CHANGE IN FOREST STRUCTURE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

Global warming of 5-10 °C occurred during the Paleocene-Eocene Thermal Maximum (PETM), beginning ~56 Ma and lasting ~175 kyr. Representatives of Euprimates, Artiodactyla, and Perissodactyla first appeared in North America during the PETM, presumably in response to warming that opened high-latitude dispersal routes between Asia, North America, and Europe. We present fossil and stable isotope records from a stratigraphically well-resolved composite section in the Cabin Fork area of the southern Bighorn Basin, Wyoming. The carbon isotope excursion (CIE) associated with the PETM spans 34m of strata and is well-defined in this area, based on δ¹³C values in mammalian tooth enamel. The first PETM immigrants to arrive were cursorial perissodactyls and artiodactyls, which first appear near the onset of the CIE. In contrast, the arboreal euprimate Teilhardina first appears ~8-10m stratigraphically higher in the CIE, implying a later arrival. We use δ¹³C values from mammalian tooth enamel to test the hypothesis that a closed forest canopy developed in response to changing climate, facilitating the dispersal of arboreal euprimates. The presence of a closed canopy can be recognized by mammals with exceptionally negative δ¹³C values, relative to taxa feeding in open gaps in the forest, due to the consumption of ¹³C-depleted leaves in the understory. Because canopy development is precipitation dependent (~ ≥1700 mm for closed canopy), we also calculate a humidity proxy by comparing δ¹⁸O values from the pantodont Coryphodon (aridity insensitive) with those from the equid Sifrhippus (aridity sensitive). Results indicate a lack of negative δ¹³C outliers in the fauna that would indicate the presence of a closed canopy during the CIE. Our humidity proxy suggests a pattern of dry/wet/dry cycles in the PETM with cursorial immigrants first appearing in the lower dry interval at the beginning of the CIE, and arboreal Teilhardina appearing in the succeeding wet interval. These results are consistent with the idea that drier conditions early in the PETM resulted in open forests favoring the dispersal of cursorial mammals, whereas later wetter conditions resulted in denser vegetation, but probably not a closed canopy, favoring the dispersal of Teilhardina.