New evidence for avian and small non-avian theropod ichnotaxa from the Lower Cretaceous of Canada: Implications for theropod ichnodiversity

Abstract

A diverse continental ichnofauna from the Lower Cretaceous (Aptian) Gething Formation occurs at the Ninesting Creek site in northeastern British Columbia. Vertebrate tracks including Wupus and Saurexallopus are inferred to represent medium-sized (footprint length ∼10 cm) tridactyl avian and larger (footprint length ∼15–17 cm), gracile, tetradactyl non-avian theropod trackmakers respectively. Shape, length-to-width and divarication ratios, suggest two morphotypes that are distinct from the poorly known, and infrequently-cited, Gething Formation ichnogenera Columbosauripus and Irenichnites (footprint lengths ∼12.5 and ∼15.0 cm respectively). This suggests a diverse theropod ichnofauna representing size classes which are quite distinct from the better-known large theropod ichnogenus Irenesauripus with footprint lengths in the range of ∼38.0–53.5 cm. Thus, current evidence suggests that the ichnodiversity of smaller avian and non-avian theropod tracks (footprint length ∼<15.0 cm) outnumbers large ‘megatheropod’ morphotypes by about 7:1. The implications of this distribution are consistent with recently proposed palaeoecological models. The assemblage also includes probable pterosaur tracks and distinctive arcuate compound Archaeonassa-Lockeia traces, attributed to freshwater unionid bivalves. The trace fossils comprise a composite Mermia-Scoyenia Ichnofacies assemblage. Palaeoenvironmentally the site represents the low energy margin of an open lake basin or floodplain pond, or a fluvial point bar.