Comparison of Early Mississippian Crinoid Generic Diversity Patterns Between the Lake Valley Formation of New Mexico and the Burlington Limestone of Iowa

The Early Mississippian Lake Valley Formation is exposed on either side of the Rio Grande Rift Valley in southern New Mexico. Crinoid faunal samples from the Nunn Member were investigated. Crinoid generic composition and diversity were compared to the age-equivalent Burlington Limestone in Iowa. Crinoid faunas in both areas are composed of similar genera, but the patterns of diversity differ. The Burlington Limestone shows high diversity, with 86 genera, and nearly equal proportions of monobathrid camerates and cladids accounting for 84% of the total genera. In contrast, the Lake Valley Formation has only 31 genera and is dominated by monobathrids, accounting for 65% of the total genera, whereas cladids are less common at 16%. Environmental differences between the two regions during the early Mississippian may account for the different patterns in diversity among crinoid populations. The Lake Valley Formation was located along a narrow shelf margin facing the open ocean. This environment produced a deeper ramp and the formation of Waulsortian mounds, which are associated with the Alamagordo Member that underlies the Nunn Member and suggest deposition in deeper water within a lower energy environment. The crinoids living in the Burlington Limestone were along the broad middle carbonate ramp in a shallow epeiric sea and were apparently subjected to episodic higher turbulence. Low current energy conditions may have been less favorable for feeding strategies used by cladids and more favorable for those of monobathrids. Because the monobathrids are prevalent in both carbonate environments, they appear to be generalized with regard to current energies, whereas the cladids seem to be restricted to higher current energies. Although these patterns may reflect a taphonomic bias against cladids, the lack of their disarticulated calyx plates suggests the diversity patterns in the Lake Valley Formation are real.