SPATIAL VARIATION OF LATE PENNSYLVANIAN PALEOCOMMUNITIES: ECOLOGICAL AND SAMPLING IMPLICATIONS

The bulk of community paleoecological research has examined taxonomic relationships through time. Few studies have examined high-resolution, complete marine invertebrate fossil communities through space. Here, we examine spatial community variation in the Pennsylvanian Finis Shale of Texas. Our goals are to 1) quantify taxonomic variation of one time at various spatial scales, 2) provide a baseline for future temporal studies to ensure observed temporal variation is not an artifact of spatial variation, and 3) determine the most appropriate spatial sampling protocol for paleocommunity research.

Seven localities were sampled along a NE-SW axis, at 0 km, 3.7 km, 3.8 km, 3.9 km, 6.6 km, 10.5 km, and 94 km from Jacksboro Cemetery Road. At each locality, 3 L bulk sediment samples were collected every 1 m stratigraphically for a total of 33 samples consisting of a total of 5371 specimens in 73 genera.

1) Polar and NMDS Ordinations, using Sorenson distance measurement (PCOrd v. 5), separate the samples into three clusters: a) lower stratigraphic, Crurithyris-dominated samples, b) upper stratigraphic, Rhipidomella-dominated samples, and c) mollusk-dominated samples from the farthest locality. Ordination axis-one scores are correlated with each sample’s stratigraphic position (r² = 0.46, p < 0.001). In addition, there are consistent stratigraphic trends in richness and evenness at the six most proximal localities. Thus, axis-one, which accounts for the greatest variation (27%) in the data, corresponds with an environmental variable that is changing through time. Ordination axis-two scores, which account for 20% of the variation, correlate with the sample distance from Cemetery Road (r² = 0.86, p < 0.001), and so corresponds with an environmental variable that is changing through space.

2) There is more community variation through 1-2 m.y. than 10 km of space. When conducting temporal studies through the late Pennsylvanian, spatial variation of the Finis Shale would not distort the temporal community signal.

3) Despite the existence of spatial variation, the overall community signal is strong enough that two samples at one locality are sufficient to capture the community signal. This result differs from previous studies which observed that multiple, lateral samples must be collected to capture a community signal.