Pelmatozoan echinoderms from the Cambrian-Ordovician transition of the Iberian Chains (NE Spain): Early diversification of anchoring strategies

Abstract

Three new echinoderm taxa representative of biodiversification across the Cambrian-Ordovician transition in the Iberian Chains are described in this paper. A new type of pelmatozoan columnal is described (in open nomenclature) from the Furongian Valtorres Formation. The primitive rhombiferan, Macrocystella aff. pauli is described based on disarticulated thecal plates and stem columnals, from the Tremadocian Santed Formation; it is a free-living organism with a flexible stem lying on a clay-rich seafloor allowing the theca to be held upright. A complete revision of the pelmatozoan Oryctoconus (characterized by its holdfast and nodal and internodal columnals) is made based on the description of the species O. lobatus and O. josopaiti n. sp. found in the Early Ordovician Valconchán and Santed Formations. The number and degree of development of anterior appendages in the Oryctoconus holdfasts (from absent to lobate and long spinose extensions), the length/diameter-ratio of the basal shields (ranging from amphora- to bowl-like morphologies), and the size (from slender millimetre-sized to robust centimetre-sized holdfats) are considered as the main anchoring strategies controlled by substrate consistency, grain size (ranging from clayey to sandy), and turbulence. The occurrence of nodal columnals mimicking the shape and size of holdfasts may represent distinct adaptations to soft substrates under high pulses of sedimentation rates. As a result, the morphological modifications of the reported holdfasts and nodal columnals are not necessarily phylogenetically related, but primarily controlled by substrate palaeoecological features. © 2009 Birkhäuser Verlag, Basel.