A MACROECOLOGICAL PERSPECTIVE ON THE CAMBRIAN ORIGINS OF RHYNCHONELLIFORM BRACHIOPODS

This study examines the environmental origins of the Rhynchonelliformea, a successful clade of brachiopods that dominated post-Cambrian benthic communities. Rhynchonelliform brachiopods were dominant in nearshore environments in the Early Ordovician and then spread offshore. Cambrian environmental distributions are less well known for this group. An important question is, how does the environmental distribution of rhynchonelliform brachiopods in the Cambrian relate to their post-Cambrian environmental distribution?

A database of approximately 200 Cambrian marine faunal assemblages in North America was compiled from the Paleobiological database, and additional literature sources. These data were vetted, and faunal assemblages were assigned to onshore-offshore depth-related habitat zones.

A preliminary analysis reveals Cambrian rhynchonelliforms were widely distributed along the onshore-offshore gradient in the early Cambrian but became more restricted in the Late Cambrian. The Obolellata occurred across the onshore-offshore gradient in the Early Cambrian but were restricted to slope and basin in the Middle Cambrian before going extinct. The Kutorginata occurred across the onshore-offshore gradient in the Early and Middle Cambrian, before going extinct. The Rhynchonellata occurred in low diversity in the Early Cambrian outer shelf and slope environments. In the Middle Cambrian they expanded into the middle shelf but were still most diverse in the slope and basin. In the Late Cambrian, they expanded and shifted their center of diversity into inner shelf environments. The Strophomenata occurred only in the Late Cambrian inner and middle shelf environments at low diversity. These preliminary patterns suggest that the rhynchonelliform brachiopods diversified rapidly to occupy the entire onshore-offshore gradient in the Early Cambrian. By the Late Cambrian the Obolellata and Kutorginata were extinct and the center of diversity of the remaining classes shifted to inner and middle shelf environments. This group included the Rhynchonellata and the Strophomenata, which went on to dominate Ordovician marine benthic communities. Thus, the onshore diversification of rhynchonelliform brachiopods in the Ordovician had its roots in the Cambrian.