Elongated thoracic spines as potential predatory deterrents in olenelline trilobites from the lower Cambrian of Nevada

Repaired injuries offer the opportunity to study predation in the fossil record and test hypotheses concerning predation intensity, anti-predatory characters, and predator selectivity. Many studies have used the late Palaeozoic and Mesozoic records but quantitative explorations of early Palaeozoic predation, especially during the Cambrian, are lacking. Trilobites, a group of biomineralised arthropods with a high early Palaeozoic disparity and diversity, are an ideal study system for exploring repaired injury frequencies. The Ruin Wash Lagerstatte (Nevada, USA; Stage 4, Cambrian Series 2) provides a bulk sample of closely related but morphologically distinct taxa. The five most abundant taxa, Olenellus chiefensis, O. fowleri, O. gilberti, O. terminatus, and Nephrolenellus geniculatus differ in the spinosity of the cephalon, axial lobe of the thorax, and thoracic pleural lobes. A total of 26 repaired injuries on 330 articulated trilobite cephalothoraces reveal that O. terminatus has the highest incidence of repaired injuries (repair frequency = 0.18) and N. geniculatus has the lowest (repair frequency = 0.043). The role of enlarged spines on the third thoracic segment, which are most elongate in N. geniculatus, are suggested as predation deterrents. Predator selection for prey of a larger size may have played a minor role, though we did not find statistical support for this idea. Three cephala with repaired injuries were noted in a sample of 685 specimens: a much lower repair frequency compared to thoraces. This indicates that cephalic attacks were more often fatal when compared to thoracic attacks. Our study provides the first quantitative support for species specific predation on Cambrian trilobites.