TOUGH BRACHS TO CRACK: IDENTIFYING PREDATION AS A DRIVER OF DEVONIAN BRACHIOPOD COMMUNITY STRUCTURE

Studies of marine paleocommunities routinely identify depth as the primary driver of species distribution and community structure; this is likely owing to the association between depth and multiple physical and chemical variables. Modern studies, however, observe that other – often ecological – processes drive communities. This disconnect may be due to scale: paleocommunity studies are sampled on large geographic and temporal scales while modern studies operate at very fine scales. If ecological processes are also important over broader time intervals, they should be detectable in the fossil record and a logical first step is to study fine scale paleocommunities. Here, we used both NMDS and POLAR ordinations to test whether water depth and predation were drivers of brachiopod community structure in the Devonian Cerro Gordo Member of the Lime Creek Formation in Iowa. This unit spanned <2Myrs and was a period of shallowing and warming in the Iowa Basin. As shallowing was approximately constant throughout the interval, stratigraphic height was used as a proxy for depth. Predation on brachiopods was frequent and traceable in this unit making it a useful ecological variable of interest. Predation was measured by: 1) repair frequency, a measure of failed attacks in a sample, and 2) WMO (weighted mean ornament), a measure of the relative ornamentation of the members of each sample – for brachiopods, shell ornament is a significant contributor to overall defensive capabilities. For the NMDS ordination, repair frequency (r=0.705, p=0.010) and WMO (r=0.641, p=0.025) correlated with the first axis while water depth (r=0.703, p=0.011) correlated with the second axis. The POLAR ordination produced similar results, although the correlation between repair frequency and the first axis was not significant (r=0.511, p=0.090), and both WMO (r=0.713, p=0.009) and depth (r=0.617, p=0.033) correlated with the second axis. These results suggest that while water depth still has an influence, ecological processes may be more significant in understanding community structure at finer scales in the fossil record. Further studies of fine scale paleocommunities may be able to inform both fossil and modern ecologists on the function and evolution of ecosystems through physical and ecological change.