Morphologic variation within and among populations of the camerate crinoid Agaricocrinus (Lower Mississippian, Kentucky and Tennessee): Breaking the spell of the mushroom

Abstract

Morphologic variation within and among populations of the common monobathrid camerate crinoid Agaricocrinus was investigated using multivariate and bivariate techniques for samples from 15 localities in the Lower Mississippian (late Osagean) Fort Payne Formation of south-central Kentucky and north-central Tennessee. Despite a wide range of variability, two morphotypes were distinguished at most localities in both autochthonous and allochthonous carbonate facies: Agaricocrinus americanus and the less common A. crassus. Morphologic characters that can be reliably used to separate these taxa include character of the anal area, width of the second primibrachial, and shape of the arm facets. Many other characters of calyx shape and plate proportions used previously for species recognition, such as depth of the basal concavity, height of the interbrachials, height of the tegmen, and number of arms, are widely variable within populations. Variation within species populations is ontogenetic, although a geographic component is recognized within A. crassus. The definition of A. crassus is emended to encompass a broader range of variation than heretofore realized. Eight species previously reported from the Fort Payne Formation are placed in synonymy: A. arcula, A. dissimilis, A. elegans, A. podagricus, A. ponderosus, A. profundus, and A. tugurium are junior synonyms of A. americanus. A. attenuata is a junior synonym of A. crassus.