Reappraising the early evidence of durophagy and drilling predation in the fossil record: implications for escalation and the Cambrian Explosion.

The Cambrian Explosion is arguably the most extreme example of a biological radiation preserved in the fossil record, and studies of Cambrian Lagerstätten have facilitated the exploration of many facets of this key evolutionary event. As predation was a major ecological driver behind the Explosion - particularly the radiation of biomineralising metazoans - the evidence for shell crushing (durophagy), drilling and puncturing predation in the Cambrian (and possibly the Ediacaran) is considered. Examples of durophagous predation on biomineralised taxa other than trilobites are apparently rare, reflecting predator preference, taphonomic and sampling biases, or simply lack of documentation. The oldest known example of durophagy is shell damage on the problematic taxon Mobergella holsti from the early Cambrian (possibly Terreneuvian) of Sweden. Using functional morphology to identify (or perhaps misidentify) durophagous predators is discussed, with emphasis on the toolkit used by Cambrian arthropods, specifically the radiodontan oral cone and the frontal and gnathobasic appendages of various taxa. Records of drill holes and possible puncture holes in Cambrian shells are mostly on brachiopods, but the lack of prey diversity may represent either a true biological signal or a result of various biases. The oldest drilled Cambrian shells occur in a variety of Terreneuvian-aged taxa, but specimens of the ubiquitous Ediacaran shelly fossil Cloudina also show putative drilling traces. Knowledge on Cambrian shell drillers is sorely lacking and there is little evidence or consensus concerning the taxonomic groups that made the holes, which often leads to the suggestion of an unknown 'soft bodied driller'. Useful methodologies for deciphering the identities and capabilities of shell drillers are outlined. Evidence for puncture holes in Cambrian shelly taxa is rare. Such holes are more jagged than drill holes and possibly made by a Cambrian 'puncher'. The Cambrian arthropod Yohoia may have used its frontal appendages in a jack-knifing manner, similar to Recent stomatopod crustaceans, to strike and puncture shells rapidly. Finally, Cambrian durophagous and shell-drilling predation is considered in the context of escalation - an evolutionary process that, amongst other scenarios, involves predators (and other 'enemies') as the predominant agents of natural selection. The rapid increase in diversity and abundance of biomineralised shells during the early Cambrian is often attributed to escalation: enemies placed selective pressure on prey, forcing phenotypic responses in prey and, by extension, in predator groups over time. Unfortunately, few case studies illustrate long-term patterns in shelly fossil morphologies that may reflect the influence of predation throughout the Cambrian. More studies on phenotypic change in hard-shelled lineages are needed to convincingly illustrate escalation and the responses of prey during the Cambrian.