ONSHORE-OFFSHORE PATTERNS OF ORDOVICIAN DIVERSIFICATION IN THE BRACHIOPOD ORDER STROPHOMENIDA: A PHYLOGENETIC PERSPECTIVE

The Ordovician radiation is characterized by onshore-offshore patterns of diversification among major evolutionary faunas demonstrating an environmental template for global diversification. Sepkoski (1991) developed a simple mathematical model of onshore-offshore faunal change based on incumbency, differential extinction rate among clades, and ecological similarity of daughter species to their ancestors. The model replicated basic clade patterns of onshore-offshore change and suggested that global diversity reflects local ecological processes. Here we test some expectations of this model by combining a phylogenetic revision of the Strophomenida with paleoecological information on local and regional abundance and habitat occurrence of strophomenide genera. Such a combination of information we call “phylogenetic paleoecology”. Our phylogenetic revision identified two major Ordovician groups, the monophyletic Strophomenoidea and the paraphyletic “Plectambonitoidea” consisting of several monophyletic groups. Our phylogeny implies that these two groups radiated in the earliest Ordovician with the plectambonitoids radiating first, followed by the strophomenoids. Regional diversity patterns indicate that each group radiated into different onshore-offshore habitats. Basal plectambonitoids appeared initially in the midshelf, but then radiated in offshore environments. The phylogeny suggests that the strophomenoids arose from a nearshore plectambonitoid ancestor, and then radiated primarily in onshore environments. Gradient analyses of Middle and Late Ordovician assemblages indicate that local abundance mirrors these environmental distributions with strophomenoids dominating shallow environments and plectambonitoids dominating deeper environments. This more detailed perspective of diversification with the Strophomenida does not show strong onshore-offshore diversification patterns as described for other groups, rather it suggests that the environmental preferences of diversifying strophomenide clades was set early in the Ordovician radiation with little environmental exploration as the radiation continued.