• Anti-predation strategy, growth rate and extinction amongst Pliocene scallops of the US eastern seaboard

      Johnson, Andrew L. A.; Valentine, Annemarie; Leng, Melanie J.; Sloane, Hilary J.; Schoene, Bernd; Surge, Donna; University of Derby; University of Loughborough; British Geological Survey; University of Mainz; et al. (2017-06-29)
      Placopecten, Chesapecten and Carolinapecten are scallop genera occurring in the Pliocene of the US eastern seaboard. The first (extant) is a smooth, streamlined form, adept at escaping predators by swimming (‘flight’ strategy). The other two (extinct) are plicate forms. Plication facilitates a ‘resistance’ strategy towards predators which is benefited by large size and high shell thickness - maximally so if these states are achieved early in life. Oxygen isotope profiles show that in early ontogeny, Pliocene Placopecten grew at the same moderate rate as modern Placopecten. By contrast, Chesapecten grew as fast as the fastest-growing modern scallop and developed an unusually thick shell, while Carolinapecten grew substantially faster still, this probably enabled by high primary productivity. Extinction of these genera, and survival of Placopecten, can be attributed to a decline in productivity which prevented a maximally effective ‘resistance’ strategy towards predators but had no deleterious impact on a ‘flight’ strategy.
    • Anti-predation strategy, growth rate and extinction amongst Pliocene scallops of the US eastern seaboard

      Johnson, Andrew L. A.; Valentine, Annemarie; Leng, Melanie J.; Sloane, Hilary J.; Schoene, Bernd; Surge, Donna; University of Derby; University of Loughborough; British Geological Survey; University of Mainz; et al. (2017-07-07)
      Placopecten, Chesapecten and Carolinapecten are scallop (pectinid bivalve) genera occurring in the Pliocene of the US eastern seaboard. The first, present in the area today, is a smooth, streamlined form, adept at escaping predators by swimming (‘flight’ strategy). The other two, which are extinct, are plicate (‘ribbed’) forms. Plication facilitates a ‘resistance’ strategy towards predators which is benefited by large size and high shell thickness - maximally so if these states are achieved early in life. Oxygen isotope (δ18O) profiles show that early ontogenetic extensional growth in Pliocene Placopecten was at the same moderate rate as in modern Placopecten. By contrast, in Chesapecten it was as fast as in the fastest-growing modern scallop (c. 80 mm/annum), and accompanied by development of an unusually thick shell, while in Carolinapecten it was substantially faster still (<140 mm/annum). Rapid growth in Chesapecten and Carolinapecten may have been enabled by high primary productivity, which is indicated by the abundance, diversity and large size of co-occurring vertebrates. The extinction of Chesapecten and Carolinapecten, and the survival of Placopecten, can be attributed to a decline in primary productivity which prevented a maximally effective ‘resistance’ strategy towards predators but had no deleterious impact on a ‘flight’ strategy.
    • Life history, environment and extinction of the scallop Carolinapecten eboreus (Conrad) in the Plio-Pleistocene of the U.S. eastern seaboard.

      Johnson, Andrew L. A.; Valentine, Annemarie M.; Leng, Melanie J.; Schöne, Bernd R.; Sloane, Hilary J.; University of Derby; Nottingham Trent University; British Geological Survey; University of Mainz (SEPM (Society for Sedimentary Geology), 2019-02-01)
      Plio-Pleistocene mass extinction of marine bivalves on the U.S. eastern seaboard has been attributed to declines in temperature and primary production. We investigate the relationship of growth rate in the scallop Carolinapecten eboreus to variation in these parameters to determine which contributed to its extinction. We use ontogenetic profiles of shell d18O to estimate growth rate and seasonal temperature, microgrowth-increment data to validate d18O-based figures for growth rate, and shell d13C to supplement assemblage evidence of production. Postlarval growth started in the spring/summer in individuals from the Middle Atlantic Coastal Plain but in the autumn/winter in some from the Gulf Coastal Plain. Growth rate typically declined with age and was usually higher in summer than winter. Many individuals died in winter but the largest forms typically died in spring, possibly on spawning for the first time. No individuals lived longer than two years and some grew exceedingly fast overall, up to 60% more rapidly than any other scallop species (, 145.7 mm in a year). Faster growth was generally achieved by secreting more rather than larger microgrowth increments. Some very fast-growing individuals lived in settings of high production and low temperature. No individuals grew slowly under high production whereas most if not all grew slowly under ‘average’ production and low temperature. In that the rapid growth evidently enabled by high production would have afforded protection from predators, Plio-Pleistocene decline in production was probably contributory to the extinction of C. eboreus. However, the negative impact of low temperature on growth under ‘average’ production suggests that temperature decline played some part.