WHAT MAKES FOR AN EXCEPTIONAL FOSSIL? THE PRESERVATION OF THE THERIZINOSAUR BEIPIAOSAURUS INEXPECTUS FROM THE YIXIAN FORMATION OF CHINA

A vertebrate fossil is often considered excellently preserved when most elements of the skeleton are present, articulated, and recognizable. Rare features such as preserved skin and feathers can deme a fossil exceptional. The next step is to determine whether these fossils are as exceptional on a chemical level as they are visually. By these standards, both specimens of Beipiaosaurus inexpectus from the Early Cretaceous Yixian Formation in northern China, with articulated elements and preserved feathers, are exceptional. It has been suggested that the holotype specimen is also exceptional in its cytological and molecular remains – that red blood cells are preserved in the dinosaur’s blood vessels and visible in thin sections of its humerus.

In this study, we have examined the original thin sections from B. inexpectus’ humerus and made additional sections from ribs and petrified wood from the same formation for chemical analysis. We have gathered additional data through field visits to the collection locality, examination of both specimens, Raman spectroscopy (for chemical and mineralogical characterization), and ToF-SIMS (for elemental and molecular characterization) to provide a more detailed story of the taphonomy of the holotype. Raman spectroscopy has also been employed to test the alternative hypothesis that the purported blood cells are pyrite framboids or their oxidation products. High levels of organics in the blood vessels obscure the small peaks of Raman spectra, but we have found that we can reliably detect major minerals (such as apatite in bone as well as quartz and calcite in petrified wood) and peaks of carbonaceous material in the blood vessels. Upon examination of the specimens and the Yixian Formation strata, the abundance of iron and manganese minerals deposited on and around the fossils indicates active mineral precipitation throughout the taphonomic history of the specimens. Other researchers have also reported the occurrence of pyrite in fossiliferous strata of the Yixian Formation. The chemical story of this visually stunning specimen indicates that the usual definition of exceptional preservation may not point clearly to exceptional chemical preservation.