Purujosa trilobite assemblage and the evolution of trilobite enrollment

Abstract

Localities bearing abundant enrolled Cambrian trilobites are extremely rare, but here we report a new and exceptional such assemblage with a high diversity of enrolled specimens from the middle Cambrian of Spain. This novel find demonstrates that encapsulated enrollment was a behavioral response available to many of the early benthic trilobite morpho­types. This result provides a new perspective for reviewing the evolution of trilobite enrollment through the Paleozoic. We find a significant and consistent increase in the proportion of enrolled trilobites within assemblages bearing abundant articulated trilobites. Similarly, the mean number of coaptative devices also increased progressively during the Paleozoic. Such devices likely provided later enrolled trilobites with greater structural integrity, and may support an escalatory model of trilobite defense evolution. Likewise, environmental circumstances favoring the preservation of enrolled trilobites may have become more common during post-Cambrian time.