Crinoid Biodiversity in the Lower Mississippian Lake Valley Formation, New Mexico

Crinoid faunal diversity within the Nunn Member of the Lower Mississippian Lake Valley Formation, New Mexico, differs from the faunal diversity of age-equivalent rocks across North America. The depositional environment of the Nunn Member was a narrow, relatively deep-water, low-energy carbonate ramp, with considerable topography due to the Waulsortian mounds found in the underlying Alamogordo Member. This unusual paleoenvironment appears to correlate with a lower diversity fauna than is present on wider, shallower carbonate platforms elsewhere in North America at this time. In particular, a relative paucity of cladid and flexible crinoids distinguishes the Nunn Member fauna from others in North America. Camerate are the most abundant clade on this carbonate ramp, with 23 genera comprising 74% of the total diversity. Monobathrid camerate genera outnumber diplobathrid camerates 20 to 3. Cladids are the next most diverse clade with 5 genera, and flexible and disparid genera number 2 and 1, respectively. Combined North American faunas are more diverse than the Nunn Member fauna, with 38 genera of camerates, 42 cladids, 8 flexibles and 5 disparids.

The Nunn Member is further typified by a smaller proportion of short-duration crinoid genera, and a higher proportion of long-duration genera than other North American faunas of similar age. The unique, deeper-water environment of the Lake Valley Formation was likely less hospitable, and populated with hardier, less specialized crinoid genera. Specialist genera, those adapted to thrive in specific ecological niches, are uncommon in the Nunn Member. Generalists, able to flourish under a broad set of environmental conditions, appear to have been better-suited to colonizing this habitat.