Paleoautecology and systematic re-evaluation of Camptocrinus alabamensis (Mississippian: Chesterian) from northern Alabama

Abstract

Camptocrinus alabamensis Strimple and Moore, from the Gasperian (Mississippian: Chesterian) part of the Monteagle Limestone and overlying Bangor Limestone in northern Alabama, is the only species of Camptocrinus known to have more than 10 arms. Camptocrinus alabamensis exhibits morphological features (basals that are taller on their posterior ends than on their anterior ends, flattened primanal, shorter posteriormost secundibrachs on C ray and D ray) indicative of a tightly coiled life position, with most of the stem lying along the sediment. Unlike other coiled crinoids, Camptocrinus alabamensis had a double-folded stem and more than 10 arms, which we interpret to have been compromise solutions to the problems of protection from high current energy and predation pressure coupled with the need to lift the animal into a feeding posture. In addition, C. alabamensis is the only dichocrinid exhibiting highly modified pinnules, which may have been used as rheo/chemosensory organs. Camptocrinus alabamensis was a leeward passive suspension feeder that, in times of danger, could fold in its arms and coil tightly against the sediment.Camptocrinus alabamensis is removed from synonymy with C. cirrifer and the diagnosis of Camptocrinus is modified. Species of the genera Camptocrinus and Neocamptocrinus should be used only for specimens in which calyces are known; therefore, the plethora of stem-species erected over the years should all be assigned to stem genera unless adequate calyx descriptions are available.