Quantitative paleoecology of marine faunas in the lower Hamilton Group (Middle Devonian, central New York): Significance for probing models of long-term community stability

Beautifully fossiliferous strata in the Hamilton Group (Middle Devonian, central New York) constitute a rich "ecological archive"sufficient to probe and test foundational concepts in paleontology. The evident community stability of Hamilton faunas over 4-6 m.y. - including two proposed mechanisms for coordinated stasis - has ignited controversy. Resolving community structure and both taxonomic and ecological temporal persistence within the Hamilton Group thus becomes critical to testing whether these Hamilton communities are stable and whether they are ecologically "locked."Toward this end, we conducted multivariate analyses (cluster and correspondence analysis) of marine faunas in 81 large samples (~300 specimens each) in shallowing-upward sequences of the Cardiff and Pecksport Members (Marcellus Subgroup, Oatka Creek Formation) of the Hamilton Group. Eight statistically and ecologically distinctive benthic communities characterize the vertical gradient, from depauperate, deeper-water dark shales below to species-rich shelf siltstones above. These communities correlate strongly with grain size, bioturbation intensity, bedding thickness, density of fossils, and faunal and ecological diversity. Species richness varies inversely with weight percent organic matter. We characterized taxonomic distributions using multivariate statistics; these statistical analyses were based on percentages of 50 taxa. In order of decreasing depth, the communities are: Cephalopod-Pterochaenia, Pterochaenia- Eumetabolotoechia, Eumetabolotoechia, Emanuella, Eumetabolotoechia-Ambocoelia, Arcuaminetes-Eumetabolotoechia, Arcuaminetes- Ambocoelia, and Mucrospirifer- Ambocoelia. The Cephalopod-Pterochaenia community represents a mixed benthicpelagic fauna associated with the deepest and finest-grained facies. The Pterochaenia-Eumetabolotoechia, Eumetabolotoechia, and Emanuella communities have low to moderate species richness and are dominated by epifaunal, active suspension feeders, especially the small epibyssate bivalve Pterochaenia fragilis, and the pedunculate brachiopods Eumetabolotoechia multicostata and Emanuella subumbona. The Pterochaenia-Eumetabolotoechia community is an opportunistic fauna that developed when the substrate first became favorable for colonization by benthic organisms. To a lesser extent, this probably also holds true for the Eumetabolotoechia assemblage. Communities near the top of the shallowing-upward cycle - Eumetabolotoechia- Ambocoelia, Arcuaminetes- Eumetabolotoechia, Arcuaminetes-Ambocoelia, and Mucrospirifer-Ambocoelia - have higher taxonomic and ecological heterogeneity, with a more diverse array of trophic and locomotory groups than their counterparts in the finer-grained, and inferred deeper, facies. Cluster significance tests applied to all pairs of communities known from adequate numbers of samples demonstrated that the communities are statistically valid and distinctive. Multivariate means of all communities were significantly different; furthermore, most pairs of communities were drawn from populations that showed no overlap in terms of rectangular distributions. The community sequence and an ordination derived from the first two axes of the correspondence analysis provided relative depth curves. Our communities, with two exceptions, do not have clear counterparts among upper Hamilton Group faunas. The ecological locking model proposed to explain the stability of Hamilton faunas is not supported by our quantitative tests to date.