ICHNOFOSSILS OF THE PALEOGENE WILLWOOD FORMATION AND THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM): RESPONSE OF AN ANCIENT SOIL ECOSYSTEM TO TRANSIENT GLOBAL WARMING

Abstract

This research combines paleopedological, paleontological, neoichnological, and ichnological methods to understand better the paleoenvironmental, paleoecological, and paleoclimatic significance of the trace fossils of soil-dwelling organisms preserved in paleosols. Such ichnofossils are well-preserved and abundant in the continental deposits and are important in situ indicators of paleohydrologic regime, degree of pedogenesis, paleoecology, and paleoclimatic setting during the time of their formation. Naktodemasis bowni, a new ichnogenus and ichnospecies erected for adhesive meniscate burrows common in paleosols, were likely produced by burrowing insects based on their morphology and ichnopedologic associations. These burrows do not represent subaqueous, sediment-ingesting organisms as has been previously suggested. Neoichnologic experiments examining the traces and burrowing behaviors of cicada nymphs (Hemiptera: Cicadidae) demonstrate that some burrowing hemipterans produce distinct backfilled burrows in modern soils identical to N. bowni. Such backfilled burrows indicate periods of subaerial exposure associated with pedogenic modification and can be used to differentiate alluvial paleoenvironments from marine and lacustrine paleoenvironments. Freshwater crayfish burrows in paleosols of the Willwood Formation, Bighorn Basin, Wyoming, illustrate the important role that burrowing organisms played in initiating and promoting pedogenic development. Crayfish burrows and other ichnofossils in the Willwood Formation show changes in their distribution, increased abundance and diversity, and reduced size through a transient period of global warming known as the Paleocene-Eocene Thermal Maximum (PETM). Significant changes in the distribution and abundance likely indicate that soil biotic communities responded to improvements in drainage conditions coincident with the global warming. Burrow diameters of the most abundant ichnofossils are 30-46% smaller within the PETM interval, suggesting that the tracemakers were smaller bodied. Smaller tracemaker body sizes may have been an adaptive response to higher temperatures, lower soil moisture, or reduced nutritional values in high CO2 vegetation. Similar body size changes in extant insect populations may be used as a new biomonitoring tool to gauge the impact of modern anthropogenic-increases in greenhouse gases and surface temperatures. This research demonstrates for the first time that ancient burrowing organisms in the continental realm were not buffered by soil environments and responded to climate perturbations in ways that are detectable in their ichnofossil record.