LOW ENDEMISM OF EARLY OSAGEAN (MISSISSIPPIAN) CAMERATE CRINOIDS IN NORTH AMERICA

Early Mississippian seas over North America provided extensive habitat space for crinoids by extending across most of the continent. By comparing the camerate crinoid faunas at the genus level from the lower Burlington Limestone (IA), the Lake Valley Formation (NM), the Redwall Limestone (AZ), and the Anchor Limestone (NV) differences and patterns can be discerned. Similarities were evaluated using presence-absence data with the Jaccard Coefficient, and with rarefaction curves.

The Lake Valley and lower Burlington faunas are nearly equal in number of genera (33 and 35 respectively) with a similarity of 0.70. Although the Lake Valley is known for its deep water facies and Waulsortian mounds, the greatest numbers of crinoids are found in shallow water settings that are akin to the Burlington shelf. This may explain why the two are so alike despite the distance between them (1600 km). Conversely, the Redwall and the Anchor have approximately half the number of genera of the lower Burlington and Lake Valley. Although geographically close, the Redwall and the Anchor faunas are very dissimilar to one another (similarity of 0.18). The only genera that these two formations have in common are the five genera that are found in all four formations.

The differences between the Redwall and the Anchor as compared the Lake Valley and the lower Burlington are likely a taphonomic artifact as the camerates are thought to be from shallow water in both formations (Brower, 1970; Webster and Lane, 1987). The camerates are poorly preserved in the Redwall, likely due to early diagenesis that produced chert and dolomite. It is thought that the camerates found in the Anchor were transported from a shallow platform by carbonate debris flows (Webster and Lane, 1987), which probably diminished preservation of the crinoids. Neither formation has a significant number of unique genera (one each) and all other genera are found to be a subset of the lower Burlington fauna. Rarefaction curves comparing the Anchor, Redwall, and Lake Valley show that the differences between the three are likely due to incomplete sample sizes. As the seas covered most of North America during this time, the four areas were connected, allowing for camerate crinoids to widely disperse.