THE INFLUENCE OF SAMPLE SIZE ON THE PERCEPTION OF FOSSIL COMMUNITY DIFFERENCES BETWEEN IN-PLACE AND SURFACE COLLECTED SAMPLES

The in-place, bulk collection of fossil-bearing rock/sediment is an intensive process, but is believed to better represent original fossil communities than the surface collection of individual, weathered-out specimens. Forcino and Stafford (2017) found disparate communities between in-place and surface collected samples from the Finis Shale of Jacksboro, Texas. This may have been driven by the much larger sample size of the surface samples. Here, we compared two 25% subsamples to the 100% surface sample (and to the equivalent in-place samples) to determine if the larger sample size influenced the observed differences between fossil communities collected in-place and on the surface.

We collected 3 in-place 4 L samples at 1 m stratigraphic intervals and 3 corresponding surface samples. An experienced paleontologist collected surface samples by exhaustively picking all specimens to fill a 2 L bag. Forcino and Stafford (2017) found median sample sizes of 220 and 607 for in-place and surface samples, respectively. To produce surface subsamples of a similar size to the in-place samples, we randomly mixed, then twice halved the three surface samples, producing two 25% subsamples each. The fossils were sorted, identified to the genus level, and counted.

The median sample size for the 25% subsamples was 201, similar to the in-place sample size. The median richness in the subsamples was 25, closer to the in-place samples (25) than the 100% surface samples (45). Conversely, the median evenness for the subsamples was 0.80, closer to the 100% surface samples (0.74) than the in-place samples (0.58). The diversity metrics indicate the 25% subsamples may not contain all rare taxa found in the 100% surface samples, but the abundance distribution is preserved. In NMDS ordination, surface samples separate from in-place samples, regardless of sample size. The relative abundances of brachiopods and mollusks are more similar between the 25% and 100% surface samples than the in-place samples. Overall, the 25% subsamples are not similar enough to the in-place samples to suggest that sample size drove the observed differences between the 100% surface and in-place samples. The differences are substantial enough that paleontologists should base their collection method on what the study demands, rather than what is easier or more cost effective.