Anatomically linked occurrence of concretionary preservation of mosasaurs from the Pierre Shale of South Dakota

Fossils of mosasaurs, late Cretaceous marine reptiles, are frequently found partially preserved in concretions (i.e., hard mineral masses), but little is known about this mode of fossilization. Concretionary fossils are difficult to extract, so are often ignored in the field and laboratory. Accounts suggest that skulls, the most taxonomically diagnostic element, are most often preserved in concretions. The presence of concretions was documented in four anatomical regions (head, torso, tail, limbs) in 48 mosasaur fossils from the Pierre Shale of South Dakota. Specimens were compiled from a restricted lithostratigraphic interval to minimize the effects of lithologic variation on concretion formation. In this sample, the skull and torso were preserved more often in concretions than were limbs and tails. The occurrence of concretion formation was found to vary depending on anatomical region. The results suggest that concretion growth begins in the core of the organism with a higher volume of organic matter, and successively precipitates towards the periphery. The skull and torso contain many taxonomically diagnostic features; thus concretionary specimens should not be overlooked. Additionally, increased likelihood of preservation of specific anatomic regions was dependent on concretion formation, suggesting that our view of the fossil record is affected by concretions.