Article: Functional studies on the Cretaceous oyster Arctostrea

The three Cretaceous species-groups Arctostrea colubrina (Lamarjsk), A. ungulata (Schlotheim), and A. diluviana (Linnaeus) are described. Particular attention is paid to the detailed morphology of A. colubrina, from which its life history is reconstructed. It is inferred that the unusual characters of the genus (especially the arcuate shape, the zigzag commissure and the funnel spines) relate to the size and importance of the gills as primary food-collecting organs. The subspecies A. colubrina ricordeana (d'Orbigny) possesses, in addition to these special adaptations, a set of long spines secreted sub-parallel to the plane of the commissure on the lower valve; these are interpreted as a specific response to the hazards of inhabiting a substrate of soft ooze. Behavioural and structural adaptations used by Recent oysters for combating conditions of high turbidity are discussed, and it is suggested that similar methods were utilized by many extinct species. Funnel spines occur in other bivalves, including Pinna and Etheria; it is likely that they also are connected with inhalent current streams. Some taxonomic implications of the functional interpretation of Arctostrea are presented in an appendix.