342 FOSSIL INVERTEBRATES/Crinoids Crinoids M J Simms, Ulster Museum, Belfast, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Crinoids are one of five extant classes of echinoderm (the others being echinoids, asteroids, ophiuroids and holothuroids (see Fossil Invertebrates: Echinoderms (Other Than Echinoids))) with a rich fossil record extending back nearly 500 Ma to the Early Ordovician They are exclusively marine suspension feeders and, with a few exceptions, sessile benthos Crinoids reached a peak of taxonomic diversity in the Late Palaeozoic but experienced a catastrophic decline around the Permo-Triassic boundary, from which they subsequently rediversified More than 700 extant species and over 15 000 fossil species have been described Like other echinoderms, crinoids have a multielement endoskeleton of high-magnesium calcite ossicles connected by soft tissue and enclosed by a thin veneer of living tissue Each ossicle is a single optically continuous calcite crystal with a stereom structure – a labyrinthine network of cavities permeated with soft tissue called stroma In the basic crinoid design the endoskeleton can be divided into three morphological sections The stem or column is a usually slender flexible elongate structure for attachment to the substrate and elevation above it The stem is surmounted by the cup, which is a fairly rigid structure, usually constructed of two or three circlets of plates, containing organs of digestion, movement, and nervous control Arising from the upper circlet of the cup are elongate flexible arms involved in food gathering Typical crinoids have a superficial similarity to flowers, giving rise to their popular name of ‘sea lilies’ and indeed to the name ‘crinoid’ itself (from the Greek krinos ¼ a lily) Stalked crinoids are very much in a minority among extant taxa, with fewer than 100 species In the remaining more than 600 species a stem is present only at the larval stage; the stemless adults are often termed ‘feather stars.’ The soft parts of crinoids comprise a small and inconspicuous proportion of the total mass They are virtually unknown in fossil crinoids, although their presence and functions can be inferred from the structure of the preserved hard parts and by analogy with extant crinoids The digestive system is contained entirely within the cup, with both mouth and anus opening onto the upper surface In common with all echinoderms, and unique to them, crinoids possess a network of fluid-filled tubes called the water vascular system Branching canals extend radially into the arms from a central ring canal housed in the cup and terminate in the tube feet – small tentacle-like structures that detect and capture food particles before passing them to ciliated food grooves, which run down the arms to converge on the mouth A further unique echinoderm character, almost certainly present in fossil taxa, is catch connective tissue, which is a type of ligament that can change its properties from pliable to rigid, enabling parts of the skeleton, such as the stem or arms, to ‘lock’ into position for prolonged periods with minimal energy expenditure Catch connective tissue is noncontractile and cannot contribute to active movement Only muscles are able to this; they are restricted to the arms of only some taxa and are entirely absent from the stem Morphology and Functional Interpretations A crinoid’s morphology is determined by the interaction of three distinct factors: inherited characteristics, i.e the morphology of its immediate ancestor; architecture, i.e physical properties of the materials; and evolutionary selection pressure from the external environment, i.e ecological factors The skeleton of most crinoids is readily divisible into three distinct sections – the stem, cup, and arms (Figure 1) – a basic morphology that was inherited from a pre-crinoid ancestor and subsequently modified within limits imposed by ancestry, architecture, and ecology In a few specialist taxa the stem or arms may be greatly reduced or absent The stem’s main functions are attachment to the substrate and, especially, elevation above it Most fossil crinoids have a stem ranging from a few centimetres to perhaps a metre or so in length, similar to that of extant stalked crinoids, although in some taxa it is very much longer while in others it may be reduced or absent (particularly in free-living crinoids such as the comatulids) Typically, the stem comprises a stacked series of ossicles, or columnals, pierced by a central canal containing extensions of the coelom and nervous system Each columnal is typically a single ossicle, but a few Early Palaeozoic taxa have what are known as meric columnals constructed from several ossicles or meres Each columnal is connected to adjacent columnals by catch connective tissue Short