New papers:
Jun Wang, Mark A. Norell, Rui Pei, Yong Ye & Su-Chin Chang (2019)
Surprisingly young age for the mamenchisaurid sauropods in South China.
Cretaceous Research (advance online publication)
doi:
https://doi.org/10.1016/j.cretres.2019.07.006 https://www.sciencedirect.com/science/article/pii/S0195667119300333Mamenchisauridae, a sauropod group named after the largest known proto-Asian sauropod Mamenchisaurus, is generally believed to range from the Early to the Late Jurassic in age. Fossil remains of Mamenchisaurus are primarily found in the Upper Shaximiao Formation and less frequently in the overlying Suining Formation in the Sichuan Basin. An accurate absolute age for Mamenchisaurus is critical for understanding the evolution and radiation of the mamenchisaurids. However, reliable age determinations for the Mamenchisaurus-bearing formations are lacking. In this study, we provide robust detrital zircon U/Pb ages (mean ages of 114.4Â1.1 Ma) for rocks from the classic fossil outcrops of the Suining Formation in the Sichuan Basin. Our data indicate an extremely long chronological gap between M. anyuensis (114.4Â1.1 Ma) and the earlier Mamenchisaurus (Late Jurassic). This indicates mamenchisaurids may have lived at least 30 million years longer than previously thought.
Maximum depositional age of Rhoetosaurus brownei is constrained to 162.6âÂâ1.1âMa.
Maximum depositional age of Siderops kehli is constrained to 176.6âÂâ2.0âMa.
Detrital zircons are useful for refining the age of fossils in museum collections.
Abstract
Australia's Jurassic vertebrate fossil record remains extremely sparse with only two dinosaur taxa and two temnospondyl amphibians identified to date. Of these, the spectacular and extremely well-preserved giant amphibian, Siderops kehli, and the only known pre-Cretaceous sauropod in Australia, Rhoetosaurus brownei, are perhaps the most important. The age of both specimens, and the stratigraphic context of Rhoetosaurus brownei, are weakly constrained and imprecisely defined, limiting our understanding of their evolutionary relationships within a broader Gondwanan context. To clarify and contextualise the evolutionary relationships and ages of these two iconic Jurassic taxa, we used UPb detrital zircon geochronology to date the sandstone matrix from around the bones of the historic museum specimens. The robust maximum depositional age for Siderops was calculated at 176.6âMaâÂâ2âMa, indicating that it is no older than late Toarcian, which refines existing biostratigraphic estimates. The Rhoetosaurus maximum depositional age determined is 162.6âÂâ1.1âMa, no older than early Oxfordian, demonstrating that the fossils are younger than expected, and definitely recovered from the Walloon Coal Measures.
Keywords