FOSSIL INSECT BORINGS IN WOOD: AN UNDERUTILIZED ARCHIVE FOR ECOLOGICAL AND GEOCHEMICAL DATA

Fossil wood preserves a significant, but often overlooked, legacy of trace fossil evidence by insects that can provide useful data on community paleoecology and paleoenvironment. Although scant observations are recorded from the Paleozoic, insect wood borings become locally abundant in the Triassic period, and they are ubiquitous throughout the Cenozoic. Owing to the rare preservation of bark and the outermost phloem and cambial layers, many insect interactions and traces confined to these layers are absent in the fossil record, but insect borings often penetrate deeply into the woody tissues. Borings in the wood are filled either with partially-digested wood fragments (frass or pellets) or mineral spar that indicates an empty cavity at the time of preservation. The morphology of the boring, its orientation and association with vascular tissues, and the type of fill can be used to describe and infer behavioral aspects of the interaction. Descriptive ichnology in wood is a nascent field, but with ongoing surveys of petrified wood from the Late Triassic Chinle Formation in Utah and Arizona, several new morphologies indicate that wood utilization had diversified by this time. These surveys further indicate patchy spatial and temporal distribution of insect-bored wood, which may be linked to environmental factors. Beyond descriptive ichnology, the pellet and frass contained in the boring preserve organic matter in addition to permineralized wood fragments. Initial tests for organics by bleaching have shown positive results for borings in wood from the Black Forest deposits in Utah, raising the possibility of using geochemical markers to identify tracemakers or examining stable isotopes for paleoenvironmental investigations. Coupling the geochemical and descriptive ichnology of wood borings may provide a novel approach to examining the evolutionary paleoecology of wood utilization and plant defense mechanisms, and they may better define the environments in which these ancient interactions took place.