The Recognition of Ophiomorpha irregulaire on the Basis of Pellet Morphology: Restudy of Material from the Type Locality

Abstract

Serial grinding of a specimen of Ophiomorpha irregulaire collected from Coal Creek Canyon in the Book Cliffs of Utah, USA—the type locality of the ichnospecies—allows us to assess the validity of pellet morphology as an ichnotaxobase for species-level identification. The importance of O. irregulaire stems from its abundance in petroliferous shallow marine strata of post-Paleozoic age. Our three-dimensional reconstruction of the burrow wall of O. irregulaire demonstrates for the first time that it is composed of sand-cored pellets. The typically spiky nature of the pellets in cross-section is inferred to result from differential compaction of the sand core and pelletal lining. Rupturing of the thin outer clay coating of the sand pellets is considered to produce the distinctively attenuated, spiky outer surface to the burrow wall of this ichnospecies. This study demonstrates the utility of pellet morphology in species-level classification of Ophiomorpha and lends support to O. irregulaire being a cosmopolitan ichnospecies present in post-Paleozoic strata. It also demonstrates the need to ensure that ichnotaxonomic identifications are based on three-dimensional knowledge of the trace fossil in question.