EXPLORING SIMILARITY BETWEEN JURASSIC VERTEBRATE ASSEMBLAGES FROM CIMMARON COUNTY, OKLAHOMA

Outcrops of the Jurassic Morrison Formation in Cimmaron County, Oklahoma, bear vertebrate and invertebrate fossil assemblages. In the recent work of Richmond et al. (2020), the site OMNH V1694 is presented as older than other vertebrate localities and in a separate geological member. We explored whether it was statistically possible to test whether all the assemblages belong to the same local fauna.

The localities have not been collected uniformly, so to avoid collection bias we did not consider any fossil taxon that is known solely from screening of sediment. Much catalogued material from all sites is not identified to a level useful in testing faunal similarity. As a result, when we examined similarity using genera we had low statistical significance. We used the Raup-Crick statistic to determine which possible pairwise comparisons deviated from what could be obtained in a random draw. Due to the low number of identified genera from most sites, this was true of only five possible comparisons with p = 0.10.

In all the five significant pairwise comparisons assemblages are more dissimilar than would be expected from a random draw from the same fauna, as shown by use of the Simpson, Jaccard, and Sorenson-Dice coefficients. One of these comparisons is between OMNH V1694 and V92, which have no catalogued genera in common. Only two genera from V1694 were not excluded from the analysis, however. All other significant comparisons are between V97 and contemporaneous localities. No single genus is present in all localities. All coefficients calculated between V97 and OMNH localities V92, V93, V94, and V95 are below 0.25.

Richmond et al. interpreted V97 as a crevasse splay and other sites as watering holes. Our results are consistent with qualitative observations that V97 has an aquatic fauna with few terrestrial taxa, and with the interpretation that it is sampling a different environment.

V1694 also represents one or more high-energy deposits as evidenced by bone orientations, imbrication, and presence of mud rip-up clasts. The question remains whether as more V1694 fossils are identified V1694 will be significantly similar to V97 because of facies and taphonomic variables or whether it will reflect a different age.