Coexistence of brachiopod and bivalves in the Late Paleozoic of Western Argentina
Abstract The relationship between brachiopods and bivalves has been widely discussed in previous studies. Based on analyses at different temporal and spatial scales, several authors have argued either for the indifference or the interaction between the two clades. In this contribution we evaluate brachiopod and bivalve coexistence at two different spatial scales in a siliciclastic shelf developed during the Late Paleozoic in Western Argentina. At regional scale, bivalves were more diverse than brachiopods, although both had a similar total number and comparable distribution of occurrences. At local scale, however, multivariate analyses indicate that brachiopods and bivalves were segregated. Null models reinforce this pattern confirming the non-random co-occurrence pattern, and that the coexistence of brachiopods and bivalves was significantly low. In addition, multivariate analyses indicate that the biotic gradient neither followed the bathymetrical, nor a geographical or temporal gradient. The possibility that such segregation would have been caused by taphonomic (storm) reworking, was also disregarded considering multivariate analyses together with taphonomic evidence. The lack of environmental segregation between brachiopods and bivalves, coupled to analyses taking into account ecological guilds, indicates that possible factors controlling the segregation, such as turbidity, substrate or productivity were not relevant. As a whole, these results suggest a possible competitive interaction between brachiopods and bivalves at local scales. Interestingly, brachiopod–bivalve coexistence at regional scale did not foster local coexistence, indicating that the processes acting at these two scales are, at least partially, decoupled. Finally, the regional coexistence pattern suggests that the major transition between brachiopod and bivalve dominated communities was most probably related to processes acting at regional to biogeographic scales rather than to competition.