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344 FOSSIL INVERTEBRATES/Crinoids platycrinitid camerates and the Cretaceous to Holocene bourgueticrinids and their allies In these crinoids, the articulations (Figure 2C) are traversed by a prominent fulcral ridge flanked by two ligament pits, with the fulcral ridges on the proximal and distal faces of each columnal typically at an angle to each other The crinoid stem may be more or less permanently attached to a hard substrate by a cemented holdfast, may be rooted in sediment by irregular branches or outgrowths from the stem or may attach on a more temporary basis to hard or soft substrates using regular offshoots of the stem, known as cirri Cirri are the most versatile form of attachment and were adopted independently by several major crinoid groups, including the dominant extant group – the stemless comatulids The cup or calyx, located between the stem and the arms, houses the vital organs of the digestive, nervous, and water vascular systems Typically the cup consists of a regular series of interlocked circlets, each usually of five plates, often with one or two additional plates intercalated towards the top In the earliest known crinoid (the Early Ordovician Aethocrinus) there were four circlets, but in virtually all other crinoids the plates of the lowest circlet have been lost and the cup comprises either two or three circlets (see below) Typically the plates of each circlet are offset by 36 relative to the plates of adjacent circlets, imparting rigidity even with only two circlets The shape of the cup varies, from shallow and bowl shaped to almost globular, with the same basic designs having evolved repeatedly in different groups The calyx incorporates all of the plates between the top of the stem and the base of the free arms, including the cup, the tegmen (which covers the oral surface), and any parts of the arms that are incorporated into the tegmen In many crinoids the cup plates are connected only by ligaments and the tegmen is weakly constructed; hence, it disarticulates rapidly after death However, in the Palaeozoic subclass Camerata the calyx forms a relatively rigid and often globose structure, which may remain intact for some time after death Consequently camerate calyces are better represented as fossils than are those of the other crinoid subclasses (see below) The food grooves of the arms converge on the mouth, which is located on (or beneath in the case of camerates) the surface of the tegmen along with the anus The anal opening is commonly elevated on an anal tube or anal sac, forming a large and complex structure in some fossil taxa, presumably to avoid faeces entering the mouth The arms are the food-gathering parts of crinoids, although ultimately it is the tube feet, lining a groove on the oral side of the arms, that are directly involved in capturing food and moving it towards the mouth The arms increase the support area for tube feet but are not essential, as evidenced by a few crinoid taxa in which they are reduced in number or even absent Secondary functions of the arms include respiration and locomotion, but generally selection pressure is towards improvement of the foodgathering mechanism within the constraints imposed by other factors Flexibility of the arms is achieved in the same manner as flexibility of the stem, with brachial plates connected by ligaments and/or muscles Muscle tissue is present in all echinoderm tube feet but appeared in brachial articulations in only one mid-Palaeozoic group, which ultimately evolved into the post-Palaeozoic subclass Articulata As in the stem, specialised ligamentary articulations can be used to autotomize parts of the arms At their simplest, crinoids have five unbranched arms, each arising from one of the plates in the uppermost circlet of the cup Increasing the effective arm length, by branching, increases the number of tube feet that can be supported and hence increases the filtration efficiency The development of numerous small side branches, or pinnules, is one strategy that has evolved independently in several major groups The arms themselves show a wide range of branching patterns, from simple dichotomous (isotomous) (Figure 1) through to strongly endotomous (Figure 3) Endotomous branching appears to be the most efficient pattern, in terms of expenditure of materials versus food-gathering capabilities, and an analogy has been drawn with the arrangement of roads on banana plantations Intriguingly, only a few fossil crinoid taxa developed this pattern to a significant extent Phylogeny, Systematics, and Geological History With their morphologically complex multielement skeletons, crinoids are ideal subjects for phylogenetic analysis Each part of the skeleton – stem, cup, and arms - can provide a wealth of morphological data for use in descriptive and phylogenetic investigations However, all too often primary descriptions of genera and species are lacking in detail, particularly for the stem, while interpretations at higher taxonomic levels have been hindered both by this primary deficiency of data and, until the mid-1990s, by a rigid adherence to traditional interpretations of skeletal homologies At high taxonomic levels the structure of the calyx is considered to be of major phylogenetic significance
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