Variation in sauropod trackway pattern from the Tuchengzi Formation (Jurassic-Cretaceous boundary) in western Liaoning, China

Abstract

Sauropod tracks, particularly those attributable to the ichnogenus Brontopodus are becoming increasingly well known in the Jurassic and Cretaceous of China. Globally there are indications that wide gauge forms, attributable to titanosauriform sauropods increase in the Cretaceous. However, differentiating wide and narrow gauge, broadly categorized as large manus (low heteropody) Brontopodus or small manus (high heteropody) Parabrontopodus, respectively is complicated by the occurrence of intermediate forms with medium gauge and intermediate heteropody, and the unknown role played by size variation. We here describe two morphotypes from the Jurassic-Cretaceous boundary sequences of the Tuchengzi Formation of western Liaoning that are medium sized and mainly distinguished by striking variations in manus rotation angles. Currently Brontopodus birdi from the Lower Cretaceous of North America is typified by wide gauge and low manus rotation (~10°), whereas Brontopodus pentadactylus from the Lower Cretaceous of Korea is characterized by high outward rotation (~90°). We consider such large differences in footprint rotation could be useful in distinguishing morphotypes and diagnostic gaits in sauropod ichnology as they have proven to be in the case of some other dinosaurs. The trackways from the Tuchengzi Formation and their peculiar patterns are a useful basis and starting point for future discussions on the ichnotaxonomy of sauropod trackways.