Are 'exceptionally' preserved skeletal fossils necessarily exceptional chemically and cytologically?
At the macroscopic scale, vertebrate fossils are considered exceptional when non-biomineralized (soft) tissues are preserved. Histologically, high quality is defined by trueness to original shape of a bone, preservation of fine details (e.g. canaliculi), and presence or absence of matrix material in void spaces. Some fossils are hypothesized to preserve cells and durable organelles. Traditionally, cytological details and biomolecular remains have been sought in exceptional fossils. Durable cytological features such as melanosomes do appear to follow feather preservation, but traditionally exceptional fossils are not necessarily exceptional on a microscopic scale. Here, we analyze a feathered dinosaur specimen from the Jehol Lagerstätte to assess claims of blood cell preservation and the state of potential biomolecular preservation. Beipiaosaurus inexpectus is a fairly complete specimen with preserved feathers. Though crushed, fine details in thin section are prevalent. Using Raman spectroscopy, Energy Dispersive X-ray Spectrometry, and Time-of-Flight Secondary Ion Mass Spectroscopy we found no evidence of exceptional molecular preservation. Instead, we found evidence that the vasculature, once hypothesized to contain preserved red blood cells, is filled with clay minerals, with the purported cells chemically indistinguishable from materials of other shapes infilling the vessels. Despite yielding exceptional fossils, the preservational environment of the Jehol biota does not necessarily preserve exceptional details cytologically or biomolecularly. Consequently, we conclude that a systematic approach to biomolecular and cytological preservation studies should rely on traits other than classic exceptional preservation.