New Siluro-Devonian Anomalocystitids (Echinodermata, Stylophora) from Bolivia and Canada, and a Reevaluation of Skeletal Homologies in Mitrates

AbstractTwo new mitrate stylophorans are described from the upper part of the Santa Rosa Formation (Lochkovian) in the Central Subandean Zone of Bolivia and from the Jupiter Formation (Cybèle Member, late Llandovery) of Anticosti Island (Canada). Both stylophorans are identified as allanicytidiine anomalocystitids. The new Bolivian mitrate, Perikefalea racheboeufi gen. and sp. nov. is characterized by a strongly reduced number of plates (five) on its upper thecal surface and a strong spike on its posterior median marginal. The morphology of P. racheboeufi suggests relatively strong affinities with Occultocystis, from the Lower Devonian Talacasto Formation of Argentina. Although incompletely preserved, the Anticosti mitrate, Perikefalea? cybeleae sp. nov., represents the first stylophoran described in the Silurian of North America and the first report of post-Ordovician Allanicytidiinae outside of (peri) Gondwanan regions. Skeletal homologies in mitrates (marginals, infracentrals, posterior spines, adorals, supracentrals) are reevaluated, so as to describe the plate pattern of Perikefalea. This revision was prompted by new fossil evidence from the Furongian–Tremadocian interval suggesting that (1) mitrates derive from “Phyllocystis” jingxiensis-like cornutes with wide marginals, a posteriorly open marginal frame (loss of M5 and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{M}}_{5}^{'}$$\end{document}), a zygal plate in central position, and confluent, sutured M3 and G; (2) anomalocystitids derive from Mitrocystites-like mitrocystitidans by modification of two marginals (digital and glossal) into posterior spines. Two distinct terminologies are tentatively proposed for supracentrals in Allanicytidiinae and Anomalocystitinae, because the highly standardized plate patterns of their Siluro-Devonian representatives were possibly acquired independently in the two clades.