Multiple geniculated types in a single inoceramid (Bivalvia) species: “Inoceramus” nebrascensis

ABSTRACT Inoceramids have been studied extensively over the last 200 years and, along with ammonites, have become an important tool for Late Cretaceous biostratigraphy. Geniculation, the abrupt change in convexity/inflation of the valve disc, occurs in many inoceramids and has been recognized for decades. But despite extensive inoceramid research, it remains a relatively under-documented phenomenon. To understand the phylogenetic relationships within this cosmopolitan bivalve family, it is necessary to recognize the range of intraspecies variation. “ Inoceramus ” nebrascensis is known to geniculate, but its morphological variability has not been effectively documented. This study identifies five morphotypes of “I.” nebrascensis from a single locality of the Late Campanian Pierre Shale in South Dakota. The most common form is non-geniculated (Type I). The four geniculated forms include: marked positive geniculation (Type II); slight positive geniculation followed promptly by marked negative geniculation (Type III); marked positive geniculation generating a conspicuous neck, followed by marked negative geniculation (Type IV); extreme positive geniculation with a “butterflied” configuration of the pre-adult shell (Type V). This study also argues for a three-stage shell development for this species (juvenile, intermediate, and adult) rather than the two stages described in previous studies.