Surprisingly young age for the Mamenchisaurid Sauropods in South China

Abstract

The giant herbivorous sauropod dinosaurs that characterized by long-neck and gigantic size, dominated many terrestrial ecosystems on the planet from Jurassic to Cretaceous periods. Mamenchisauridae, a basal sauropod dinosaur family known from 4 genera and 16 species, is only discovered from western China, especially in the Sichuan basin. It evolved in geographical isolation from other sauropods, when Proto-Asia was a separate landmass. It is known as a special sauropod with the longest neck among all the dinosaur species, with huge size that have reached more than 35 meters in length and possibly weighted tens of tons.Mamenchisaurus is the youngest and the most phylogenetically derived genus among four genera of the Mamenchisaurodae, as well as the most giant dinosaur in Asia and one of the most giant dinosaurs in the world. Since all the fossils of Mamenchisaurs were excavated from Middle to Late Jurassic strata, it was traditionally assigned to be Middle-Late Jurassic in age in previous studies based on biostratigraphic correlations with other contemporary fossil assemblages. Determining the absolute age of the dinosaur-bearing formations in the Sichuan basin is critical to understanding the evolution and radiation of basal sauropods such as Mamenchisaurus in this geographically isolated system and throughout the world. However, radioisotopic age constraints for these formations are lacking since igneous events were rarely recorded within the Sichuan basin. We herein reported new zircon U-Pb age of 115±1.5 Ma for a tuffaceous sample collected from the Mamenchisaurus-bearing Suining Formation from the central Sichuan basin. These data serve to define the age of the Suining Formation, and to provide the youngest constraints yet reported for the radiation of the basal sauropods.