First occurrence of hybodontid teeth in the Luoping Biota (Middle Triassic, Anisian) and recovery of the marine ecosystem after the end-Permian mass extinction

Abstract

Hybodont sharks were one of the most successful chondrichthyan lineages of all time, first occurring in the Devonian and becoming extinct in the Late Cretaceous. The end-Permian mass extinction had a limited effect on hybodont sharks, but there are fewer records of hybodonts in the Triassic than in the Jurassic and Cretaceous in China. The Middle Triassic Luoping Biota (Pelsonian, Anisian) is famous as an example of complete re-establishment of a shallow marine food web after the end-Permian mass extinction. Actinopterygian fishes are abundant and diverse in this assemblage, with 31 taxa described, including Halecomorphi and Ginglymodii. However, we still know very little about chondrichthyan fishes. Here we describe three taxa of hybodontid sharks based on teeth for the first time. One of them is ascribed to a new taxon of Lonchidiidae, Luopingselache striata gen. et sp. nov. The other two belong to Acrodus. The dentition of Luopingselache striata gen. et sp. nov. is preserved in situ and shows pronounced monognathic heterodonty. This represents the first detailed account of hybodont dentition with information of tooth replacement rate from the Mesozoic of China. We show that the tooth replacement rate is very rapid, namely 2.6 days/row on average. This rapid dental replacement rate might be an adaptation to replace broken teeth and insure the maintenance of an adequate dentition. Further, the newly discovered heterodont Luopingselache and the durophagous Acrodus sp. A and B not only made the food web more complex, but also emphasized the recovery of the marine ecosystem in the Middle Triassic after the end-Permian mass extinction.