New dinosaur swim tracks from the Middle Jurassic Zhanghe Formation in Yunnan, China

Abstract

ABSTRACT Recently, dinosaur tracks were discovered in the vicinity of Zhemujiu village in Yimen County, Yunnan Province, representing the first dinosaur footprints found in the purplish-red sandy mudstone of Lower Jurassic Zhanghe Formation. This marks a significant discovery as evidence of dinosaurs from this formation has previously only been reported from body fossils. Nearly 100 tracks were observed at the site and two morphotypes were identified: Characichnos-like morphotype A, and Wintonopus-like morphotype B. Some dinosaurs scraped the substrate with the distal portions of their digits (morphotype A), while others punted by pushing either the digital plantar surface against the sediment or plunging their digits vertically into the sediment (morphotype B). These track morphotypes suggest small-bodied dinosaurs swam (fully buoyant) and punted (partially buoyant) aqueous palaeo-environments (respectively). The orientation of these Zhemujiu tracks indicates that some dinosaurs were moving along the same axis as the flow of the water while others were moving perpendicular to it, yet despite the high abundance and shared orientations of tracks, we find no evidence of gregarious behaviour. This Zhemujiu tracksite discovery expands our understanding of dinosaur swimming behaviour in the Mesozoic era, as well as the occurrence of these track morphologies in China.