SHAPE ANALYSIS OF THE ORDER ORTHIDA (PHYLUM BRACHIOPODA) DURING THE ORDOVICIAN RADIATION

The connection between taxonomic radiations and increased morphological disparity has been well studied in the fossil record. However, the environmental and ecological drivers of increased morphological disparity during certain radiations are not fully understood. During the Great Ordovician Biodiversification event, the brachiopod order Orthida was one of the first to radiate, becoming the dominant brachiopod clade throughout the rest of the Ordovician. Several orogenic events during the Ordovician increased the siliciclastic input into previously carbonate-dominant basins, fundamentally changing habitats and producing environmental stress on shallow-water marine organisms. The morphological adaptations which allowed the othides to radiate, potentially in response to this period of increased siliciclastic input, are understudied.

This study performs a simple morphometric analysis on orthides, using data collected from the figures of type specimens of Ordovician genera from the Treatise on Invertebrate Paleontology (2000, 2002). We assessed valve convexity by measuring the height of the valves from the commissural plane to the maximum height of the valve, and dividing them by the length. Interarea descriptions for each genus were organized into categories based on the interarea classification provided in the Treatise, and orthide environmental occurrence data during the Ordovician were collected using the Paleobiology Database.

Orthide interarea categories increased in number and their valve convexity morphospace expanded, with the ventral valve showing the largest increase in disparity. At the same time as these new features radiated, the orthides experience an increased affinity towards more siliciclastic-rich environments (r = 0.929, p = 0.007). The concurrent association of the generic radiation, morphospace expansion, and siliciclastic affinity infers that the orthides expanded their convexity and interarea categories in response to the increased siliciclastic input. The increased morphological disparity allowed the orthides to expand their niche space, leading to the taxonomic radiation.