ICHNOFOSSILS OF ANCIENT SOIL BIOTA SHOW CHANGES IN ABUNCANCE, STRATIGRAPHIC DISTRIBUTION, AND SIZE DURING A RAPID GLOBAL-WARMING EVENT: PALEOCENE-EOCENE THERMAL MAXIMUM INTERVAL, PALEOGENE WILLWOOD FORMATION, BIGHORN BASIN, WYOMING

Soil organisms, as recorded by ichnofossils in Willwood Formation, Bighorn Basin, Wyoming, show changes in abundance, distribution, and size during a global warming event known as the Paleocene-Eocene Thermal Maximum (PETM). The PETM coincides with a foraminifera mass extinction and a turnover of North American mammal assemblages. Paleobotanic, paleopedologic, and oxygen isotope studies indicate high temperatures during the PETM and sharp declines in precipitation compared with late Paleocene estimates. Willwood paleosols and fluvial deposits contain a diverse assemblage of ichnofossils, including Naktodemasis bowni, Camborygma litonomos, Edaphichnium lumbricatum, cf. Skolithos isp., cf. Planolites isp., cf. Steinichnus isp., and cocoon traces. Ichnofossils attributable to insects and oligochaetes increase in abundance within the PETM interval, probably in response to improved soil drainage. Freshwater crayfish burrows and molluscan body fossils, abundant below and above the PETM interval, are nearly absent during the PETM likely due to drier floodplain conditions and dramatically lower water tables. Burrow diameters of the most abundant ichnofossils are 33–47% smaller within the PETM interval, suggesting that the tracemakers were smaller bodied. Reduced size may have been an adaptive response to high temperatures, lower soil moisture, or reduced nutritional values in high CO₂ vegetation.

These results are important because increases in greenhouse gases and surface temperatures since the beginning of the industrial revolution are similar in magnitude to climate change experienced during the PETM. Our research points toward a potentially powerful biomonitoring system using extant organisms, particularly insects, because we predict that they likely have responded or will respond to anthropogenic global warming in a fashion similar to tracemaking soil fauna during the PETM. Museum insect collections, some dating back to pre-industrial times, could be reexamined and compared with living specimens. Looking forward, ongoing and future insect monitoring programs collecting morphometric data from the field may be able to detect body size changes over the coming decades and quantify potential climate warming effects.