Slip-resistant silver-feet: shell form and mode of life in lower Pleistocene Argyropeza from Fiji

Abstract

Asymmetric sculpture in the form of tubercles or ridges on the shells of large (mean shell length ~60 mm) turritelliform gastropods has previously been shown to enhance efficiency of the shell as a penetration anchor in burrowing through coarse sediment. The small (<8 mm) turritelliform shells of all species of Argyropeza (Family Cerithiidae) have both of these ratchetlike sculptural features. Four lines of evidence from Lower Pleistocene A. divina and A. schepmaniana from Fiji support the interpretation that they help resist back-slippage: cross-orientation, frictional asymmetry, allometric shallowing, and allometric densing of sculptural elements. In addition, excellent state of preservation including protoconch sculpture, near coplanarity of aperture and ventral side of shell, and susceptibility to drilling predation by naticids support the conclusion that these gastropods burrowed in the sediment that formed the siltstone in which they are now preserved. A direct relationship is proposed between height of ratchet sculpture on turritelliform gastropod shells of approximate length range 5–100 mm and the grain size of sediments they inhabit ranging from silts to coarse sands.