New observations on some Mollusca from the Jurassic of Dorset.
John Whicher
Abstract:
The fossils of the Inferior Oolite formation (Middle Jurassic) of Dorset were the subject of detailed study at the end of the last century, but while the ammonites have received modern analysis, little has been done with the gastropods and bivalves. The rocks were deposited in the Wessex Basin, that was undergoing active faulting during the period of deposition. This resulted in the formation of hollows and fissures in the sea floor that filled with sediment and protected it from removal by erosion. The development of high resolution biostratigraphy in the last 20 years has allowed the faunas of these localised deposits to be integrated into a unified framework. This has made possible accurate stratigraphical collecting, something that the Victorian workers were unable to do.
We now believe that ammonites were sexually dimorphic, evolved rapidly and underwent migratory influxes into Dorset. These ideas have both simplified and complicated their taxonomy. Unfortunately the patchiness of the fossil record means that we still do not have a clear understanding of their evolutionary relationships.
Gastropods are abundant at some horizons in the Inferior Oolite but are relatively scarce otherwise. Pleurotomariidae are present as three major lineages typified by the genera Obornella, Pyrgotrochus and Pleurotomaria. Innumerable species have been described, often from limited material, which are not taxonomically useful. Careful collecting is beginning to make sense of the chaos. Some show considerable variation in morphology at a single horizon (notionally isochronous) whereas others do not. Evolutionary trends are very slow when compared to ammonites, however over the Aalenian-Bajocian, a time of about 5 million years, it is possible to see consistent evolutionary change.
Of all the bivalves of the Jurassic one of the commonest is the Limid Ctenostreon. Some 15 species have been described of which 5 occur in the Inferior Oolite. We have observed that the morphological variation at one horizon is often as great or greater than the difference between described species. Over the 5 million years of our study we doubt that it is possible to distinguish any consistent shift in morphology..
I will review recent findings from digs that we have conducted which highlight the difficulties surrounding the species concept in palaeontology.