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To Raśl Esperante & all other Paleonetters, There has been much debate about whether the famous Yixian Fm. fossil assemblage is datable to the Late Jurassic or Early Cretaceous. According to Joshua Smith in 1998, the biostratigraphic data is split between a Late Jurassic & an Early Cretaceous age. But, Ar-Ar dates indicate an Early Cretaceous age. Smith suggests the Sihetun Quarry assemblage is likely Early Cretaceous, not Tithonian, as earlier suggested. According to Spencer Lucas in 1999, the Yixian Fm. has psittacosaurs - good Early Cretaceous index fossils. According to Wang Xiao-Lin et al. (1999 - Palaeoworld vol. 11: 310-327), tuff beds at the Sihetun Quarry have 124.6 my dates & suggest that this is clearly indicative of an Early Cretaceous age. According to Lo Ching-Hua et al. (1999 - Palaeoworld vol. 11: 328-340), volcanic beds in the Sihetun Quarry interval of the Yixian Fm. have Ar-Ar dates ranging from 136.2 to 155.4 (mean 145.3) my, which they see as consistent with an earlier published K-Ar date of 147.3 my, meaning the biota is Late Jurassic in age. Who's right? Not sure... Best, James St. John stjohn.2@osu.edu _______________________________ James St. John Founders Hall 156A 1179 University Drive Ohio State University at Newark Newark, Ohio 43055 USA ____________________________________
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