Cambrian predators: possible evidence from boreholes

Abstract

Boreholes in Cambrian shells are rarely reported but are a potentially significant source of information on levels of predation in early metazoan communities. This paper documents boreholes in a wide variety of Cambrian organisms. Particular attention is devoted to two species of the inarticulate brachiopod Linnarssonia(informally L.sp. A and L.sp. B) from the Middle Cambrian of southern Sweden (Brantevik, Forsemölla) and Bornholm (Læså, ⊘leå). Both successful and abortive holes occur, and both brachial and pedicle valves were attacked. Almost 20 percent of the valves of Linnarssonia sp. B from ⊘leå have successful boreholes, whereas in L.sp. A from Brantevik only 1.4 percent of the valves are bored. The proportion of successful bores in brachial and pedicle valves also varies widely, from 0.8:1 in L.sp. A (Forsemölla) to 5.8:1 in L.sp. B (Læså), although taphonomic bias caused by preferential removal of pedicle valves of L.sp. B may have distorted this ratio. The abundance and distribution of abortive holes also shows variation, with a maximum of 16.3 percent of L.sp. B from Forsemölla and a minimum of 1 percent in L.sp. B from Læså. Preference for brachial or pedicle valves among abortive holes is variable, ranging from 0.3:1 in L.sp. B (Læså) to 6.9:1 in the same species from ⊘leå. With one exception (L.sp. B from Forsemölla) successful bores always outnumber abortives, but the ratio in brachial and pedicle valves varies widely with a maximum in favor of successful bores being reached in brachial valves of L.sp. A from Læså (6:1). Boreholes are nonrandomly distributed. In L.sp. A and L sp. B successful attacks on brachial valves were concentrated in a central zone. Attacks were more scattered in the pedicle valves, although in L.sp. A boreholes tended to occur on the left-hand side. Abortive and successful bores in the enigmatic fossil Mobergella holsti,from the Lower Cambrian of Skäggenäs, southern Sweden, show a conspicuous concentration in the apical region. In addition, boreholes in Lower Cambrian material from Australia (the pseudobrachiopod Aroonia,the tommotiid Micrina) and eastern Siberia (brachiopods, and possibly echinoderms and the tommotiid Lapworthella) are reported. All these bores are attributed to the activity of predators, whose systematic affinities remain uncertain. Holes in tubicolous torellellids from northern Tamdytau, however, may alternatively represent attachment scars of other torellellids.