A noteworthy transition in global biodiversity of benthic marine faunas was initiated during the mid-Paleozoic, in which burrowing deposit feeders (BDF) began to increase in taxonomic richness, while, simultaneously, immobile suspension feeders that lived on soft substrates (ISOSS) began to decrease. The factors governing such large-scale faunal patterns have long been subjects of debate among paleontologists—do they result from biological interactions, responses to physical transitions, or both? The Middle Devonian Hamilton Group of New York State provides an ideal backdrop for addressing this question because of its diverse, well-preserved benthic marine fossils belonging to both groups (BDF and ISOSS) contained within strata that recorded changes in sedimentation associated with the Acadian orogeny.

Faunal censuses were taken at sixteen localities on a west to east transect from Buffalo to Albany, New York, stratigraphically spanning the Ludlowville and Moscow Formations of the upper Hamilton Group. Genus-level data subjected to multivariate statistical analyses reveal a strong lithofacies control on the biota. Limestones and calcareous mudstones and shales contain diverse ISOSS faunas including crinoids, bryozoans, corals, and a variety of articulate brachiopods. Siltstones and sandstones are more heavily bioturbated based on numbers of burrows and trace fossils (Zoophycos), and contain more nuculoid bivalves, all recognized as BDF. Additionally, there is a clear geographic constraint on faunal variation that correlates with the configuration of the Appalachian Basin of the Middle Devonian. ISOSS organisms were profuse on the carbonate ramp, west of the central trough, while BDF were most abundant in the eastern siliciclastic-rich environments. The strong correlation between the fauna and sediments in which they are found suggests that, as the Catskill wedge prograded westward through the Upper Devonian, so did BDF-dominated communities. Data are being assembled for Upper Devonian facies to further test this possibility. On these spatial and temporal scales, the decrease among ISOSS organisms would not be caused by biological interactions, but rather the loss of suitable environments.