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Revision of the British Lower Jurassic Arcoidea (Bivalvia): palaeobiological, palaeoecological and evolutionary implications

R. I. Knight,N. Morris

Published 2020 in Historical Biology

ABSTRACT

ABSTRACT The majority of species of the Arcoidea identified from the British Lower Jurassic are taxa within the Grammatodontinae. Although this study has further partitioned the Grammatodontinae fossils into the genera Grammatodon, Cosmetodon and Catella based on distinct defining characters, it is clear that differentiation at a species level becomes problematic as adults exhibit characters that grade between two distinct end states. Additionally, there is variability in characters between the juvenile and adult shells within species and the two valves of individuals. These factors have led to an over-splitting of British Lower Jurassic arcoid species, with implications in the study of marine environmental change and extinction events. After revision of the taxonomy of Lower Jurassic Arcoidea, it is clear that there was a gradual change in arcoid form through this epoch, moving from sulcate Cosmetodon to sulcate-free Grammatodon s.s. Arcoid species that survived the Toarcian extinction event in the UK were non-sulcate. This study concurs with previous research suggesting the Parallelodontidae, via the Grammatodontinae, led to the Cucullaeidae. Indeed, hinge dentition and ligament area gradational end states of some British Late Jurassic Grammatodon appear very Cucullaea-like. Dynamic environmental conditions through the Early Jurassic appear to have helped drive this evolutionary change.

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