ECOLOGICAL GRADIENT STRUCTURE IN THE MISSISSIPPIAN LODGEPOLE FORMATION, SOUTHWEST MONTANA

As part of the 2014 Stratigraphic Paleobiology Field Course, sponsored by the Paleontological Society, twelve graduate students measured section and collected faunal counts in the lower Mississippian Lodgepole Formation near Cardwell, Montana. From two localities, we collected 92 samples, containing 50 taxa and over 3,000 individuals. These data were used to characterize the stratigraphic architecture and the faunal gradients of this carbonate ramp.

Using detrended correspondence analysis, we identified two faunal gradients, which explain nearly half of the variation in the dataset. The first gradient, along axis 1 of the DCA, corresponds to water depth, with corals being most abundant in the foreshoal and brachiopods in the deep subtidal facies. The second gradient separated taxa by attachment, which we interpret as substrate affinity. This suggests that high axis 2 values had less cohesive substrates than low axis 2 values. A subset of data restricted to brachiopods is consistent with the hypothesis that productids are restricted to deeper water than spire-bearing brachiopods. Supporting our interpretation of axis 2, the attached spire-bearing brachiopods plot lower on axis 2 than the unattached productids.

Richness and evenness of samples are both greater in the deep subtidal than in shallower facies. With a Spearman rank correlation, we found that 34% of the richness can be explained by water depth. Niche response curves indicate that more taxa had their preferred environment in deeper-water facies, and that rare taxa have a greater variance in environmental tolerance than common taxa.

Thus, the major faunal changes in the Lodgepole are driven primarily by water depth, and secondarily by substrate consistency.