A reconsideration of the origins of the ‘typical’ Cretaceous inoceramid calcitic hinge plate in the light of new ultrastructural observations from some Jurassic ‘inoceramids’

: Marked differences occur between the hinge plates of Jurassic and Upper Cretaceous British ‘inoceramid’ species. Albian and Upper Cretaceous inoceramids had the poiriform calcite hinge plate with closely spaced resiliifers that is widely associated with this extinct bivalve family. The whole structure was dominated by compressible ligament. Jurassic species, in contrast, had an ultrastructurally less familiar aragonite hinge plate with widely spaced resiliifers and a ligament reminiscent of Isognomon with equally spaced elastic and compressional components. The wider taxonomic relationships of the Inoceramidae still remain uncertain, although the majority of the evidence links the family to the Pterioida. This study, however, suggests that the ancestral inoceramid did not necessarily have a calcitic hinge plate, but rather that a Jurassic ‘inoceramid’ species could have been developing such a structure via an aragonite precursor. Observations on British Kimmeridgian ‘inoceramid’ specimens indicate that calcite was being incorporated into the aragonite hinge plate by these times. Structures in the ligamentat of Jurassic species suggest that the fundamental change in mineralogy was initiated by valve architectural modification at the byssal notch, linked to inoceramid epifaunalism. Worldwide poor preservation of Lower Cretaceous inoceramids makes it very difficult to further prove this hypothesis via intermediate evolutionary stages leading to the fully poiriform calcite hinge plate observed in Albian and Upper Cretaceous species of Inoceramidae.