OF ROUND PEGS AND SQUARE HOLES: MATCHING BRACHIOPOD MORPHOLOGY TO ENVIRONMENTAL GRADIENTS IN MIDDLE DEVONIAN ASSEMBLAGES

The link between form and function is well established, as is the link between species morphology and environment. Most morphologic studies are conducted at the species level, and more rarely at the community level. Here, we examine the relationship between environmental gradients and community morphology using Middle Devonian brachiopod assemblages from Nevada. The abundance and preservation potential of brachiopods make the group an ideal focus for morphological studies. Within the well-established framework of a carbonate-platform slope/ramp, temporally constrained by conodont zones, we compare community assemblage morphology across a shelf break for multiple time intervals. Stratigraphic, environmental, and assemblage data were taken from Johnson (1990, Journal of Paleontology). Morphological characters (including width, length, and valve inflation) were measured based on the representative species of each genus as presented in the revised Invertebrate Treatise of Paleontology.

Non-metric multi-dimensional scaling (NMDS) was used to ordinate the brachiopod assemblages, allowing comparison of samples and species. Unsurprisingly, each environmental category (lithology and position relative to platform) clustered together in the ordination space. To compare morphology, the mean and weighted mean (based on abundance within an assemblage) of each morphological character was mapped onto the ordination. Both absolute size and relative measures (ratios) track the environmental gradients; deeper water assemblages were bigger, more inflated, and more elongate than those in shallower water (p < 0.05). Unequal valve inflation was most pronounced in intermediate water depths, with more equal valve heights in shallow and deep water assemblages, suggesting a transition between flatter shallower assemblages and more globose deeper assemblages. These results present a clear relationship between environment and morphology at the community level; this pattern encourages further work exploring community morphology.