Crayfish-like arthropod trackways from the fluvial Wapiti Formation (Upper Cretaceous), Alberta, Canada

Abstract and figures

This study documents and analyzes arthropod trackways from the Upper Cretaceous Wapiti Formation, located in west-central Alberta, Canada. In general, the ichnotaxonomy of arthropod trackways is problematic due to inconsistent use of diagnostic criteria in previous studies. Thus, the trackways from the Wapiti Formation are described following recently defined criteria to facilitate future ichnotaxonomic reevaluations. Judging primarily from the number and morphology of the imprints and the symmetry of the track series, the trackways are identified as Octopodichnus cf. O. raymondi Sadler, 1993. Other relevant track attributes include heteropody and great imprint depth. The mudstone slabs bearing the trackways also contain other well-preserved invertebrate trace fossils along with abundant silicified plant material. The silicified plant fragments, taken together with bentonite overlying the trace-bearing surface, suggest volcanic ash played a role in preserving the trackways. The associated traces include burrows, fecal mounds, and surficial marks and contribute to a diverse invertebrate trace-fossil assemblage. The Wapiti Formation trackways are attributed to crayfish or similar decapods and were emplaced in a succession interpreted to have been deposited in a swampy floodplain environment. By contrast, previously described examples of Octopodichnus have typically been attributed to arachnids in aeolian/desert environments. This paper expands the ichnological record of crayfish-like arthropods from the Late Cretaceous and contributes to the underexplored invertebrate fossil record of the Wapiti Formation and, more broadly, Cretaceous strata in the Western Canada Sedimentary Basin.