TAKING CENTER STAGE: EXPLORING ECHINOCONCHOID BRACHIOPOD EVOLUTION THROUGH FUNCTIONAL PERFORMANCE

Functional morphology is often used to test for adaptation. A relatively novel technique, the performance space, provides an integrated approach by testing the performance of multiple taxa, lineages and traits against environmental and cladistic data. Performance spaces quantify performance, similar to the way morphospaces quantify morphology. This study examines the evolutionary morphology of the North American members of the brachiopod superfamily Echinoconchoidea (Order Productida), which radiated during the Carboniferous. Echinoconchoids were concavo-convex and lacked pedicles, relying on rigid spines and body shape to interact with sediment and ambient current Models of eight genera (2 primitive, and 3 each from 2 derived subfamilies) were constructed using laser scanning and 3D printing. Niche-differentiation, a major influence on adaptive radiations, was tested between subfamilies using performance as a proxy. A performance space of 5 axes was constructed to measure hydrodynamic, metabolic and defensive performance. Three axes were defined by performance in experiments that quantified stability in mobile sands, buoyancy in soft muds and feeding efficiency. The other two axes were mathematically derived through shape measurements as proxies for respiration/metabolic efficiency and effective size/defense. The results were converted to z-scores and projected on a PCA. Performance was primarily driven by size. Smaller genera were less resistant to transport, settled slower, had greater ratios of lophophore size to body volume, and decreased passive flow. Larger genera were more resistant to transport, settled faster, and had lesser ratios of lophophore size to body volume, and increased passive flow. This performance trend corresponds with a general increase in echinoconchoid size from the Devonian to the Permian, as the clade expanded into new niches. Additionally, spine length affected performance; in three genera of similar size and convexity, longer spined genera were more stable in mobile substrates and had longer settling times. The most derived taxa in our study from different subfamilies performed similarly despite morphological dissimilarity, demonstrating independent adaptive solutions to the same functional problems, suggesting possible niche overlap.