Environmental patterns in the origin and diversification loci of Early Cambrian skeletalized Metazoa: Evidence from the Avalon microcontinent

The Cambrian Radiation was expressed as major changes in marine communities. High-diversity skeletalized metazoan faunas appeared and persisted with little change in the most proximal onshore habitats and show little expansion into the offshore through much of the long (ca. 24 m.y.) pretrilobitic Placentian Epoch on the Avalon microcontinent. These small shelly fossil communities and a large number of coelomate burrowers that appeared earlier in the Placentian comprise the Placentian Ecologic Evolutionary Unit (new)—the initial stage of the Cambrian Evolutionary Fauna. Early members of a number of high-level metazoan groups were concentrated in onshore habitats vacated by the demise of the Ediacaran fauna, where they diversified and, possibly, originated no earlier than latter part of the Placentian Epoch (543–ca. 519 (m.y.a.). Development of unstable substrates and predation in open shelf habitats occupied coelomate trace producers may have been a key factor in the restriction of diverse skeletalized metazoan faunas to peritidal habitats, where mineralized skeletons may have served as protection from desiccation and UV-damage. In contrast, the oldest trilobites are most diverse and abundant in offshore habitats, and their appearance in habitats dominated by large trace producers suggests a protective role of their mineralized integument.