Statistical testing for paleocommunity recurrence: Are similar fossil assemblages ever the same?

Observations of the recurrence of similar fossil assemblages through long intervals of geologic time punctuated by rapid changes in both the composition and structure of fossil assemblages has recently resulted in the concept of “coordinated stasis” to describe this pattern in the behavior of paleocommunities through time. Coordinated stasis may imply the existence of ecological mechanisms that actively maintain particular community structures and thus have a significant impact on the process of macroevolution. However, before such mechanisms can be invoked, it must be shown that ecological stasis as observed in the fossil record is more significant than the persistence of similar community types due to the repeated reinvasion of recurring habitats from a persistent species pool (the “null hypothesis” for paleocommunity recurrence). Analysis of the relationship between communities and paleocommunities and an expansion of the ecological hierarchy at the community/paleocommunity level allows the creation of a rigorous definition of the entities composing the local paleocommunity (the samples collected within a stratigraphic horizon at a single locality), paleocommunity (groups of samples shown to be statistically identical), and paleocommunity type (groups of samples that are similar but can be shown to be statistically different). These definitions permit the development of the null model for paleocommunity recurrence and establish a base-level of variability within a local paleocommunity that permits rigorous statistical comparisons to be made between paleoecological samples at larger temporal and geographic scales. Paleoecological samples from four marine tongues in the Middle Pennsylvanian Breathitt Formation are used to analyze species abundance variability at several spatial and temporal scales and to test for paleocommunity recurrence. Although recurrence of statistically identical local paleocommunities occurs at single localities and between localities within individual marine units, it is usually not detectable between marine tongues, suggesting that paleocommunity recurrence in the marine strata of the Breathitt Formation is the recurrence of paleocommunity types and does not falsify the null hypothesis.