Hybodontiform sharks from Middle Triassic Chang 7 Member of the Ordos Basin, Shaanxi, North China: palaeobiological and palaeoecological significances

Abstract

The lacustrine ecosystem of the early Ladinian of Chang 7 Member of Yanchang Formation in the Ordos Basin (Shaanxi, North China) was proposed as the earliest known Mesozoic-type, trophically multileveled lacustrine ecosystem after the end-Permian mass extinction (EPME). However, the speculated top predator shark was a mere conjecture from coiled coprolites. Herein, thirty-one shark teeth from the organic-rich mudstones of the Chang 7 Member at the Bawangzhuang section, Tongchuan City, Shaanxi Province, North China are described in detail. Two taxa of hybodontiformes, Hybodus? youngi and Hybodus sp., are identified. Based on new material and a re-examination of original material, the previously described Hybodus youngi is substantially revised by adding several newly recognized anatomical features that include flared lateral cusplets, orthodont crown with a pulp cavity surrounded by numerous dentine tubules and a monognathic heterodonty. The results indicate that at least two different hybodont sharks, associated with Saurichthys of ca. 1 m in total length, occupied the higher trophic levels of the Chang 7 Member’s lacustrine ecosystem. The re-establishment of large predator niches encompassing diverse large predators with multiple dietary habits further supports the full recovery of the lacustrine ecosystem structure 10 Myr after the EPME.