• Growth rate, extinction and survival amongst late Cenozoic bivalves of the North Atlantic

      Johnson, Andrew L. A.; Harper, Elizabeth M.; Clarke, Abigail; Featherstone, Aaron C.; Heywood, Daniel J.; Richardson, Kathryn E.; Spink, Jack O.; Thornton, Luke A.H.; University of Derby (Taylor & Francis, 2019-09-12)
      Late Cenozoic bivalve extinction in the North Atlantic and adjacent areas has been attributed to environmental change (declines in temperature and primary production). Within scallops and oysters—bivalve groups with a high growth rate—certain taxa which grew exceptionally fast became extinct, while others which grew slower survived. The taxa which grew exceptionally fast would have obtained protection from predators thereby, so their extinction may have been due to the detrimental effect of environmental change on growth rate and ability to avoid predation, rather than environmental change per se. We investigated some glycymeridid and carditid bivalves—groups with a much lower growth rate than scallops and oysters—to see whether extinct forms from the late Cenozoic of the North Atlantic grew faster than extant forms, and hence whether their extinction may also have been mediated by increased mortality due to predation. Growth rate was determined from the cumulative width of annual increments in the hinge area; measurements were scaled up to overall shell size for the purposes of comparison with data from living species. Growth of the extinct glycymeridid Glycymeris subovata was at about the same rate as the slowest-growing living glycymeridid and much slower than in late Cenozoic samples of extant G. americana, in which growth was at about the same rate as the fastest-growing living glycymeridid. Growth of extinct G. obovata was also slower than G. americana, and that of the extinct carditid Cardites squamulosa ampla similarly slow (evidently slower than in the one living carditid species for which data are available). These findings indicate that within bivalve groups whose growth is much slower than scallops and oysters, extinction or survival of taxa through the late Cenozoic was not influenced by whether they were relatively fast or slow growers. By implication, environmental change acted directly to cause extinctions in slow-growing groups, rather than by increasing susceptibility to predation.
    • Marine climate and hydrography of the Coralline Crag (early Pliocene, UK): isotopic evidence from 16 benthic invertebrate taxa.

      Vignols, Rebecca M.; Valentine, Annemarie M.; Finlayson, Alana G.; Harper, Elizabeth M.; Schöne, Bernd R.; Leng, Melanie J.; Sloane, Hilary J.; Johnson, Andrew L. A.; University of Derby; University of Cambridge; et al. (Elsevier, 2018-05-24)
      The taxonomic composition of the biota of the Coralline Crag Formation (early Pliocene, eastern England) provides conflicting evidence of seawater temperature during deposition, some taxa indicating cool temperate conditions by analogy with modern representatives or relatives, others warm temperate to subtropical/tropical conditions. Previous isotopic (δ18O) evidence of seasonal seafloor temperatures from serial ontogenetic sampling of bivalve mollusk shells indicated cool temperate winter (< 10 °C) and/or summer (< 20 °C) conditions but was limited to nine profiles from two species, one ranging into and one occurring exclusively in cool temperate settings at present. We supplement these results with six further profiles from the species concerned and supply seven more from three other taxa (two supposedly indicative of warm waters) to provide an expanded and more balanced database. We also supply isotopic temperature estimates from 81 spot and whole-shell samples from these five taxa and 11 others, encompassing ‘warm’, ‘cool’ and ‘eurythermal’ forms by analogy with modern representatives or relatives. Preservation tests show no shell alteration. Subject to reasonable assumptions about water δ18O, the shell δ18O data either strongly indicate or are at least consistent with cool temperate seafloor conditions. The subtropical/tropical conditions suggested by the presence of the bryozoan Metrarabdotos did not exist. Microgrowth-increment and δ13C evidence indicate summer water-column stratification during deposition of the Ramsholt Member, unlike in the adjacent southern North Sea at present (well mixed due to shallow depth and strong tidal currents). Summer maximum surface temperature was probably about 5 °C above seafloor temperature and thus often slightly higher than now (17–19 °C rather than 16–17 °C), but only sometimes in the warm temperate range. Winter minimum surface temperature was below 10 °C and possibly the same as at present (6–7 °C). An expanded surface temperature range compared to now may reflect withdrawal of oceanic heat supply in conjunction with higher global temperature.