A Review of two Middle Jurassic Theropod Tracksites Discovered in the 1980s from Sichuan Basin

Abstract

Tracks from two sites in the Middle Jurassic Xintiangou Formation in the Wuma Village area, Wuhuang Township, Zizhong have been known and intermittently studied and excavated since the 1980s. The track-bearing surfaces were exposed by a combination of natural weathering and deliberate excavation by residents in a rural agricultural area. The surfaces were used as “threshing floors” for the processing of agricultural crops in an area subject to weathering under a humid sub-tropical climatic regime. Despite the negative effects of weathering on the quality of track preservation, the sites are historically significant in Chinese ichnology as the type areas for many controversially named theropod ichnotaxa. Subsequent researchers challenged the ichnotaxonomy as provincial and over-split, suggesting that many of the tracks, belong to well-known Lower Jurassic ichnogenera. The present study reviews these two sites, providing new information, and confirming that the tracks belong to the ichnogenera Grallator, Eubrontes and Kayentpus which are typical of the globally widespread Lower Jurassic tetrapod biochron. This suggests the Middle Jurassic ichnofauna in Sichuan is like Lower Jurassic ichnofaunas elsewhere. Previous efforts to transfer the ichnospecies to globally, better-known ichnogenera were important in reducing ichnogenus diversity, but did not reduce ichnosepcies diversity. Herein the ichnotaxa are reviewed and it is shown that the ichnospecies names have no utility for comparative study or in assessing assemblage diversity, or biochron composition. It is therefore proposed that the multiple ichnospecies names proposed based on tracks from these two localities can mostly be accommodated under the labels Grallator isp. indet., and Eubrontes isp. indet.