Microevolutionary patterns in some Jurassic bivalves of the Oxford Clay, England

Abstract

Over 10,000 bivalves were collected to investigate the microevolutionary patterns in the Peterborough Member of the Oxford Clay Formation. Two main bivalve lineages were studied Meleagrinella braamburiensis and Mesosaccella morrisi. M. braamburiensis was a thin-shelled suspension-feeder and M. morrisi was an infaunal deposit-feeder. These patterns were examined in the context of the new plus ca change model, which has extended the debate on microevolutionary patterns in the fossil record. The model proposes gradualism during narrowly fluctuating, relatively stable environments, and in contrast, net stasis in more unstable environments. The model also predicts for more gradualistic evolution on land in the tropics and in the deep sea, and for more stasis (and occasional punctuations) in shallow waters and temperate zones. Morphological patterns were detected from two distinctly different sets of environments within the Peterborough Member. The differences in environment were detected through lithological, palaeontological and geochemical evidence. It was found that a more stable period of environmental conditions occurred throughout the majority of the obductum Subzone of the Peterborough Member, whilst a period of more fluctuating environmental conditions (in terms of substrate, faunal composition, TOC, OI, carbonate carbon and total sulphur content) prevailed during the grossouvrei Subzone of the Peterborough Member. Bivalves were collected from three localities: Calvert landfill site, Buckinghamshire; Saxon and Orton brick pits, Peterborough. At Calvert and Saxon pits both the more stable environment and the more fluctuating environment, i.e. obductum and grossouvrei Subzones, were collected from. At Orton pit just the more stable environment, i.e. obductum Subzone, was collected from. Morphological patterns observed for the Length to Height ration for M. braamburiensis showed overall net stasis throughout the more stable environment at Calvert, Orton and Saxon pits, and throughout the more fluctuating environment at Calvert and Saxon pits. The Hinge Length to Anterior Length ratio for M. braamburiensis showed positive net morphological change throughout the more stable environment and net stasis throughout the more fluctuating environment at Calvert pit. Net stasis was observed throughout both the more stable and more fluctuating environments at Saxon pit, and was also observed throughout the more stable environment at Orton pit. Morphological patterns observed for the Length to Height ration for M. morrisi at Calvert and Saxon pits, showed the positive net morphological change throughout the more stable environment, with net stasis during the more fluctuation environment. The Hinge Length to Anterior Length ratio for M. morrisi showed positive net morphological change during the more stable environment, and net stasis during the more fluctuating environment at Calvert pit. Net stasis was observed throughout both the more stable and more fluctuating environments at Saxon pit. (M. morrisi was not studied at Orton pit). The morphological patterns observed, within the same species, obtained from the two separate geographically areas, are often distinctly different. Local microevolutionary changes or ecophenotypic variation are the two proposed alternatives for this difference. Overall, the morphological patterns observed from the two bivalve species within the Peterborough Member match, fairly closely, the plus ca change model's predictions for shallow marine invertebrates.