CAMBRIAN CANNIBALS: AGNOSTID TRILOBITE ETHOLOGY AND THE EARLIEST KNOWN CASE OF ARTHROPOD CANNIBALISM

Largely due to their diminutive size, agnostid trilobites have defied attempts to properly interpret their affinities, environmental preferences, ethology, and feeding strategies. Results from a suite of 44 separate slabs bearing specimens of Peronopsis interstricta provide new data concerning agnostid behavior. All samples were obtained from the Middle Cambrian Wheeler Formation, Millard County, Utah. Seven samples out of the entire suite (16%) contain juxtapositions of large (>4 mm in length) and small (<4 mm) specimens. The small individuals of the pairs frequently appear to be damaged or partly ingested. In some cases a small individual is overridden by the cephalon of a larger animal, in what does not appear to be a merely chance association. These associations are interpreted here as evidence of attack by the larger member of the conspecific pair. Of the samples preserving multiple trilobites, 58% show evidence for cannibalism. These results suggest that Peronopsis interstricta was a predator. The predator interpretation is supported by putative tiny bite scars on the pygidium of a specimen of Peronopsis interstricta, damage that may be the result of intraspecific attack. An alternative to the predatory attack interpretation is that the attacks represent an expression of intraspecific territoriality or mating competition. Scavenging and accidental juxtaposition interpretations must also be considered. The cannibalism explanation, however, seems best supported by available evidence. As such, these encounters represent the earliest known examples of arthropod cannibalism, and thus add to the accumulating evidence indicating that the Cambrian biosphere experienced an unprecedented increase in benthic marine predation pressure. Paleoecological reconstructions of the Cambrian sea floor must now render agnostid trilobites as predators. Cannibalism should also be considered as a potential contributing factor to the appearance of widespread Cambrian predators. The behavioral tools associated with macropredation may have been refined within a single species before being unleashed on the rest of the biosphere. As the agnostid Peronopsis interstricta was evidently blind, these predatory trilobites presumably relied on senses other than sight to locate and capture their prey.