Substrate affinities of higher taxa and the Ordovician Radiation

The Ordovician Radiation exhibited a global transition in dominance from the Cambrian evolutionary fauna (e.g., trilobites), to the Paleozoic and Modern faunas (e.g., articulate brachiopods and bivalve molluscs). Although its causes have yet to be determined definitively, the transition coincided with increased global tectonism. Erosion of source areas uplifted during orogenic activity increased the siliciclastic richness of marine substrates in many venues, and it has been hypothesized previously that higher taxa with affinities for siliciclastics diversified in association with these environmental changes, whereas higher taxa not exhibiting such affinities either failed to radiate or declined in diversity. Here, we provide an initial test of this substrate affinity hypothesis by evaluating the Ordovician affinities of trilobites and articulate brachiopods. Our analyses—at the class level for both trilobites and articulate brachiopods, and at the order level for orthid and strophomenid brachiopods—were based on the affinities of constituent genera for siliciclastic, carbonate, and mixed siliciclastic/carbonate settings. Individual genus affinities are calculated with a database of genus occurrences encompassing nine Ordovician paleocontinents. Using these values, we developed a standardized relative affinity (SRA) metric to compare the propensities of higher taxa, and to assess changes in relative affinities of individual higher taxa from series to series. A simple comparison of trilobites and articulate brachiopods for the Ordovician in aggregate does not appear to support the substrate affinity hypothesis: articulate brachiopods, which contributed increasingly to overall diversity through the period, exhibit an overall affinity for carbonates and an aversion to siliciclastics. However, a rather different view emerges when we consider the affinity trajectories of higher taxa through the period: articulate brachiopods exhibit a growing affinity for siliciclastics and a declining affinity for carbonates, whereas the opposite is the case among trilobites. Among constituent articulate brachiopod orders, the affinity trajectories of orthids and strophomenids mirror that of the class. Thus, the increasing dominance of articulate brachiopods in the Middle and Late Ordovician may have been linked to the affinity for siliciclastics of a diversifying subset of the group, but further investigation will be required to verify this claim