Study of interspecific competition in the fossil record is problematic, but morphological information can provide evidence for such interactions. Ecological theory predicts a limit to morphological similarity among competing species, ideally manifested as a constant (Hutchinsonian) ratio of mean sizes among successive pairs of competing species. However, this pattern breaks down in many modern ecological systems because the effects of competition are mitigated by nonequilibrium factors such as predation and spatial heterogeneity. To test whether a consistent limit to similarity exists in a competition-dominated system, we analyzed the morphology of four coexisting brachiopod species in an Ordovician paleoenvironment in which nonequilibrium factors were negligible.
Analysis of feeding-structure lengths using probabilistic models demonstrates that (1) ratios between means of successive species are unexpectedly constant (p < 0.03); and (2) species distributions overlap significantly less than expected (p < 0.03). These results suggest that a limit to similarity was reached, and that morphospace occupation by the four species was driven by interspecific competition. Demonstration of limiting similarity in this ancient community suggests that similar approaches could be used to assess the relative influence of interspecific competition on community diversity and structure through deep time.