Non-predatory borings in Phanerotrema (Gastropoda), Early Silurian, Anticosti Island, Québec, Canada

Borings found in Early Silurian Phanerotrema sp. gastropod shells from the Goéland Member, Jupiter Formation (Anticosti Island) were analyzed to determine if the borings are evidence of predation. The analysis included a comparison of the borings to published criteria for predatory borings, scrutiny of the functional morphology and biostratinomy of the gastropod shell, and contextual information about borings produced as dwelling structures in skeletal substrates from the Anticosti sequence. Four of nineteen Phanerotrema contain borings having a roughly circular cross-section and a length that is slightly conical. Although the borings meet several predatory criteria including taxon-specificity and site-specificity of the borings, the occurrence of multiple borings within the same shell and the variable angle of penetration are atypical of most predatory borings. The functional morphology and stratinomy of the shell further supports a non-predatory origin. When the shell is oriented in life position, several borings penetrate anterior to where the snail would be if retracted into the shell as an anti-predatory action and two of the borings are shown to cut into the sediment infilling the shell. The borings therefore are interpreted as Trypanites dwelling cavities. The distribution of Trypanites in coralline substrates from Anticosti provides an explanation for the apparent taxon- and site-specificity of the borings observed in Phanerotrema. Absolute height of a substrate is demonstrated to positively correlate with Trypanites boring frequency, since high relief substrates are more likely to remain above the sediment–water interface than low relief substrates. Trypanites boring frequencies show that substrates from the Goéland Member are 1.6 times more likely to be bored by Trypanites as compared to background levels observed for all Anticosti members. This factor would facilitate the probability of borings in the large shells of Phanerotrema, which likely had longer exposure times to the water column and settling larvae of bioeroders. The peripheral band was the preferred target as it was the highest point on the recumbent conch, and the rugosity of the site also may have favoured settling and initiation of boring larvae. Borings in shells are evident throughout the fossil record, and are commonly used as evidence of predation. Viewed individually, some of the borings in this case study could convincingly be interpreted as resulting from predation. This study demonstrates the importance of using multiple criteria to interpret trace fossil evidence.