Taphonomic significance of Aristotle's 'Wheel Paradox': an example with the cerithid gastropod Cerithium atratum (Born, 1778) from Conceição Lagoon, Santa Catarina State, Southern Brazil

Abstract

In the book Mechanical Problems, attributed to Aristotle, the author relates the differential degrees of wearing and rounding of pebbles and shells moved by waves on the seashore to the variable radii and diameters of these particles. This phenomenon, which became known as Aristotle’s ‘Wheel Paradox,’ seems to explain the pattern of abrasion observed in shells of gastropods Cerithium atratum (Born) found in Conceição Lagoon, on the Island of Santa Catarina, Brazil. In situ observations show that the elongated turriform shells of C. atratum deposited at shallow (< 30 cm) depths on a sandy bottom with pavements of disarticulated valves of Anomalocardia flexuosa Gmelin are subject to bidirectional wave-generated currents under fairweather conditions. The 47 specimens analyzed exhibit a pattern of abrasion characterized by the loss of the shell wall, apparently proceeding from the wider last whorl toward the narrow apex, culminating in the exposure of the columella. Experiments designed to test whether this pattern could be related to the ‘Wheel Paradox’ principle show that under bidirectional flow, the shells describe circles on the vertical plane around the longitudinal axis (rolling) and a pendulum movement (revolution) on the horizontal plane centered on the apex. The whorls located farther from the apex have larger diameters and describe broader arc segments. Thus, they have higher tangential velocities and travel longer paths, which could explain why they are abraded earlier than the narrower whorls. Recognizing similar abrasion features in fossil shells can provide information on the local paleo-currents, hydrodynamics, and relative time-averaging of death assemblages and allow distinguishing between natural or anthropogenic modification in archaeological specimens. Although the effects vary between shells with different shapes, Aristotle’s ‘Wheel Paradox’ principle explains the general abrasion pattern among gastropods under certain hydrodynamic conditions. Keywords: Actualistic Taphonomy, Aktuopaläontologie, shell abrasion, shell orientation, biostratinomy, hydrodynamics