Ecologic Evolution of
Nigel C. Hughes
Skeletonized Cambrian trilobites are both varied and abundant and provide potential proxies for understanding the evolution of nonskeletonized arthropod groups. Soft- and hard-part morphology suggests that Cambrian Trilobita pursued a variety of feeding habits, ranging from predator-scavenger activity to sediment ingesting and suspension feeding. They occupied habitats ranging from infaunal to probably pelagic and lived in ecosystems that were structured in a manner comparable to those of marine habitats today. The range of ecologic diversity among skeletonized Cambrian trilobites is similar to that exhibited by nonskeletonized Cambrian arthropods. Data on taxonomic, morphologic, and size diversity, in combination with information about abundance and occurrence, suggest that considerable ecologic diversity was established by the appearance of trilobites in the fossil record. Species richness and the absolute abundance of individuals increased during the remainder of the Cambrian, but in at least some biogeographic provinces the rate of morphologic diversification was constrained after the Early Cambrian. This constraint may have been related to the demise of carnivorous redlichiid trilobites and the radiation of primitive libristomate trilobites with a primary consumption feeding mode. Many of the phylogenetic and ecologic components of Ordovician trilobite communities appeared no later than the Middle Cambrian but did not rise to dominance until the establishment of the Paleozoic fauna.
THE BIOMASS OF TRILOBITES in scientific collections far exceeds that of all other Cambrian metazoans put together. This fact reflects the volumetric and taxonomic abundance of trilobites in a wide range of Cambrian sediments, their intricate and labile morphology, and their occurrence throughout the majority of Cambrian time. These attributes have given the group unrivaled utility as zonal fossils in Cambrian strata, and as the principal faunal element used to assess Cambrian paleobiogeogra-