Belichnus new ichnogenus, a ballistic trace on mollusc shells from the Holocene of the Benguela region, South Africa

Abstract

Small holes of irregular outline and mainly 0.2 to 1.5 mm in diameter are present in the exteriors of molluscan shells obtained from the latest Quaternary transgressive sequence on the Orange Shelf, Benguela region, off southwestern Africa. In shell interiors, the shell surrounding the holes is broken out to form flaring, conical spalls. The holes are clearly not borings, but are punctures caused by a small, pointed object having been forcefully driven into the shell. The holes in bivalves occur preferentially in small, thin-shelled tellinids and are nonrandomly placed in the central portions of valves. It is proposed that the holes are praedichnia produced by the rapid, ballistic-like raptorial strikes of spearing stomatopods, the pointed dactyl tips having penetrated the shells. Associated evidence of shell smashing, consisting of large holes in valves and gastropod whorls, supports the suggestion of stomatopod activity. The only stomatopod that inhabits the Benguela ecosystem, the spearer Pterygosquilla armata capensis Manning, 1969, is the probable tracemaker. Although shell nearly 1 mm thick has been pierced, the apparent selection of thin-shelled tellinids (0.14 to 0.34 mm thick) may reflect the requirement of shelling smashing in order to access the tissues. Nevertheless, the spearing of shelled prey is atypical for stomatopods and therefore confirmatory zoological observations are required, particularly to clarify the context of shell spearing. The ichnogenus Belichnus is proposed to accommodate these unique traces of pointed impacts, with ichnospecies B. monos (single punctures) and B. dusos (combined punctures) to accommodate the main morphological varieties observed.