Cryptic behaviour in trilobites: Cambrian and Silurian examples from Canada, and other related occurrences

Trilobites exhibiting cryptic (hiding) behaviour are described from the Middle Cambrian Burgess Shale of British Columbia and the Llandovery Jupiter Formation of Anticosti Island, Québec. Other examples of cryptic behaviour in trilobites are discussed. It is suggested that certain trilobites entered partly enclosed spaces in order to hide, while moulting or scavenging. The Cambrian examples are the caracasses of the small trilobites Ptychagnostus praecurrens and Pagetia bootes found inside otherwise empty, horizontal-lying tubes of the priapulid worm Selkirkia Columbia. The anterior ends of all of these Cambrian trilobites face towards the narrow ends of the Selkirkia tubes. They provide evidence in support of a benthic life mode for these trilobites. The Silurian examples are of the phacopid trilobite Acernaspis orestes occurring inside a variety of burrows. One of the rock specimens has at least 25 moulted trilobite exoskeletons inside a single burrow. We interpret this specimen to be trilobites hiding in a burrow to escape predation while moulting. Other rock specimens with Acemaspis occurring in burrows contain both moults and carcasses. These rock specimens suggest that A. orestes entered a variety of burrows. Some trilobites are found in cavities much more frequently than others, and entering cavities appears to have been a species-specific behaviour.