Box Columnal classification

The majority of crinoid assemblages are represented by disarticulated ossicles. Conventional taxonomy based on a description of complete, articulated specimens is thus not possible. Nevertheless, ossicles have many distinctive features, arguably with more well-defined characteristics than many groups of macrofossils (Fig. 15.3). Single stems consist of many ossicles with a central canal or lumen usually carrying nerve fibers. Both the ossicles and lumens have distinctive shapes that are the basis of a form taxonomy of the group. Form taxonomy helps us classify the shapes of fossils, in the same way that we can classify nuts and bolts. It is a useful method of organizing our data, but since it is not biologically meaningful, cannot be used in phylogenetic analyses. Stems may be either homeo-morphic, composed of similarly shaped ossicles, or heteromorphic with a variety of different-shaped ossicles. Moreover stems may be subdivided into zones that may be internally homeomorphic or heteromorphic. Columnal taxonomy has proved useful in describing taxa (so-called col. taxa) of pelmatozoan, particularly crinoid, ossicles of stratigraphic significance.

Figure 15.3 Some crinoid ossicle types. (a) Articular facet of a columnal of the bourgueticrinid Democrinus (?) sp., with a fulcral ridge of the synarthrial articulation; the lumen opens at the bottom of the "8"-shaped depression (x15). (b) Cirral scar on a nodal of the isocrinoid Neocrinus with well-preserved stereom microstructure and knob-like synarthrial fulcrum (x18). (c) Articular facet of a columnal of the isocrinoid Neocrinus with symplectial articulation around the five petal-like areola areas (x9). (Courtesy of Stephen Donovan.)

Figure 15.3 Some crinoid ossicle types. (a) Articular facet of a columnal of the bourgueticrinid Democrinus (?) sp., with a fulcral ridge of the synarthrial articulation; the lumen opens at the bottom of the "8"-shaped depression (x15). (b) Cirral scar on a nodal of the isocrinoid Neocrinus with well-preserved stereom microstructure and knob-like synarthrial fulcrum (x18). (c) Articular facet of a columnal of the isocrinoid Neocrinus with symplectial articulation around the five petal-like areola areas (x9). (Courtesy of Stephen Donovan.)

units within a deep-water mudstone sequence. Entire crinoids, cystoids, echinoids and cal-cichordates were carried downslope and rapidly buried on the unstable slopes of a submarine fan system.

Crinoidea_

Although the crinoids, famously called "sea lilies", look more like plants than animals, there is no doubt they are animals, and echi-

noderms at that. They are usually sessile, with characteristic echinoderm pentameral symmetry, rooted by a stalk, for at least part of their life cycle, to the seabed; but some forms after a short fixed stage are entirely free living. Modern forms live in dense clusters or "forests" ranging from the warm waters of the tropics to the icy conditions of polar latitudes. The "feather stars" prefer the clear-water conditions of the continental shelf, living in nooks and crevices, and emerging at night to perch on ridges. The fixed sea lilies occupy the deep-water environments of the continental slope. The majority of fossil forms were almost certainly part of the shallow-water sessile benthos. The success of the cri-noids may be measured by the fact we know over 6000 fossil species and an age range from the Early Ordovician to the present day.

Morphology and life modes

The crinoids consist of a segmented stalk or stem composed of columnals or ossicles (Box

15.3) fixed to the seabed by root-like structures or holdfasts. Attached to the top of the stalk is the case containing the main functional part of the animal called alternatively the calyx, aboral cup or theca. The calyx is built of two rings of calcitic plates - the basals and the overlying radials in a monocyclic configuration. In a number of taxa, the dicyclic forms, a second circle of smaller plates, the infrabasals, interface between the basals and the stem, providing further articulation. The upper, oral surface of the calyx is covered by a flexible membrane or tegmen and houses a number of important structures. These are the mouth, which is usually situated centrally at the convergence of five radially arranged feeding grooves; the anus, which is sited posteriorly with the outlet often modified by an anal tube enhancing the efficiency of waste disposal; and the arms or brachials, which extend upwards from the calyx and together form the crown.

As already noted, two main life strategies were pursued by the crinoids (Fig. 15.4). The majority of fossil crinoids and about 25 Recent basals-

infrabasal basals-

infrabasal

Seelilie Aufbau
anal tube
Sagenocrinites
(b)

Figure 15.4 (a) Morphology of the Ordovician Dictenocrinus. (b) Two main crinoid life strategies, fixed and mobile. (Redrawn from various sources.)

genera are stalked forms, attached to the seabed. Modern oceans, however, are dominated by mobile comatulids that move about like pneumatic umbrellas, pumping their long arms in unison. Antedon is one of nearly 100 non-stalked genera that, after a short fixed stage, are free to crawl and swim with the aid of flexible arms and cirri.

Classification and evolution

The oldest reported crinoid, Echmatocrinus brachiatus from the Middle Cambrian Burgess Shale, has uniserial or single brachials, in contrast to the biserial arms of contemporary eocrinoids. Echmatocrinus has few other unequivocal crinoid characters and there is wide agreement that it is actually an octocoral. More recognizable crinoids with more typically constructed cups and columnal-bearing stems, such as Dendrocrinus, appear some time later during the Tremadocian. A major expansion in the Early Ordovician tropics marked a period of intense morphological experimentation and many adaptive radiations.

Virtually all Paleozoic crinoids were stalked, and traditionally have been grouped into three subclasses, the Inadunata, Flexibilia and Cam-erata (Fig. 15.5). Inadunate crinoids comprise a large and varied group, originating in the Early Ordovician and continuing until the Triassic. They have a rigid calyx with either free or loosely attached brachials, and mono-cyclic or dicyclic calyx bases. Camerate cri-noids are characterized by large cups with both monocyclic and dicyclic plate configurations. The uniserial or biserial brachials, decorated with pinnules, are firmly attached to the cup and the tegmen is heavily plated, obscuring the food grooves and mouth, but developed laterally with an anal tube. The Flexibilia, comprising some 60 genera, have a dicyclic plate configuration comprising three infra-basals. The brachials are uniserial and lacking pinnules, and the tegmen is flexible with a mosaic of small plates. Their stems have circular cross-sections and lack cirri. These groups are especially well known in the Carboniferous (Box 15.4).

The fourth subclass of crinoids, the Articu-lata, with the exception of some Triassic ina-dunates, includes all post-Paleozoic crinoids. A few Paleozoic forms with articulate similarities such as Ampelocrinus and Cymbiocri-nus may be stem-group articulates. Over 250

genera are recognized with almost two-thirds of known genera extant. Microcrinoids are a highly specialized crinoid morphotype developed within both the Inadunata, during the Paleozoic, and the Articulata, during the Mesozoic. Microcrinoids are minute, never more than 2 mm in size; they may be pedo-morphic forms living together with more typical crinoid communities.

Blastozoans_

Blastozoans are an informal grouping that includes three of the more minor, yet nevertheless important, echinoderm groups that are all extinct: the cystoids, blastoids and eocri-noids. These pelmatozoans were usually equipped with a short stem but often lacked brachia or arms. Blastomorphs were probably high-level filter feeders, particularly characterized by pores or brachioles punctuating the thecal plates. Eocrinoids are included by some authors in the cystoids, appearing near the base of the Cambrian and becoming extinct during the Silurian. The eocrinoids, however, probably included ancestors to both the cys-toids and the crinoids.

Cystoids

Mid Paleozoic blastozoans with respiratory pore structures modifying the thecal plates have been traditionally placed within the Cys-toidea. This mixed bag includes two classes, the Diploporita and the Rhombifera, that became very widespread during the Mid Paleozoic. They had spherical or sac-like thecae, commonly with 1000 or more irregularly arranged plates. Moreover the group has brachioles lacking pinnules and characteristically the plates are usually equipped with distinctive pore structures. A variety of such pore structures have been recognized in the cys-toids (Fig. 15.7), and they are fundamental in the higher-level classification of the group.

Diploporita The diploporites had thecal plates punctuated by pairs of pores either covered with soft tissue (diplopores) or a layer of stereom with the pore pairs joined by a network of minor canals (humatipores). These pores probably held a bulbous respiratory bag and allowed for the efficient entry and exit of celomic fluid. Both stalked and non-stalked forms are present in this group, suggesting a

Ark WeldingChladocrinus

Figure 15.5 Some crinoid genera: (a) Dimerocrinites (Silurian; Camarata), (b) Cupalocrinus (Ordovician; Indunata), (c) Sagenocrinites (Silurian; Flexibilia), (d) Chladocrinus (Jurassic; Articulata) and (e) Paracomatula (Jurassic; Articulata comatulide). Magnification approximately x1 (a, c), x2 (b, d, e). (From Smith & Murray 1985.)

Figure 15.5 Some crinoid genera: (a) Dimerocrinites (Silurian; Camarata), (b) Cupalocrinus (Ordovician; Indunata), (c) Sagenocrinites (Silurian; Flexibilia), (d) Chladocrinus (Jurassic; Articulata) and (e) Paracomatula (Jurassic; Articulata comatulide). Magnification approximately x1 (a, c), x2 (b, d, e). (From Smith & Murray 1985.)

Box 15.4 The age of crinoids: an Early Carboniferous diversity spike

Early Carboniferous (Mississippian) crinoids were abundant and diverse, so much so that this interval is often called the "Age of crinoids". Limestones of this age often consist of over 50% pelmato-zoan debris, and are known as encrinites. Why then were crinoids so abundant at this time? Two factors seem to have contributed to these extensive shoals of crinoids (Kammer & Ausich 2006). Firstly, five major groups were in various states of recovery after the Frasnian-Famennian extinction event, particularly the advanced cladids (Fig. 15.6). Secondly, with the disappearance of the shelf-edge coral-stromatoporoid buildups at the end of the Devonian, platform geometries were quite different. There was improved and unimpeded water circulation, which promoted stenohaline conditions that encouraged the growth of crinoid communities. With new ecospace and a lack of predation pressures, crinoid diversity exploded. Sadly, the good times came to an end with regression and the cooler-water conditions associated with the Late Carboniferous glaciation. Crinoids were never again so diverse.

Figure 15.6 Diversity of Early Carboniferous crinoids. (From Kammer & Ausich 2006.)

wide range of strategies from a fixed sessile mode to free-living recumbent styles. The dip-loporites were very widespread from the Early Ordovician to the Early Devonian and probably evolved from a Late Cambrian blasto-zoan ancestor.

Rhombifera The rhombiferans appeared during the Late Cambrian equipped with bra-chioles and distinctive rhombic patterns of respiratory pores crossing thecal plate sutures (Fig. 15.7). They are classified according to the pattern and shape of their pores; these separate the order Dichoporita from the Fis-tulipora. The rhombiferans became common during the Early Ordovician and continued with a near cosmopolitan distribution until the Late Devonian, and were probably replaced by the better adapted blastoids during the Silurian and Devonian.

Blastoids

The extinct blastoids were small, pentamer-ally symmetric animals with short stems and hydrospires adapted for respiration (Fig. 15.8). They are represented by over 80 genera in rocks of Silurian to Permian age. The blastoid cup or theca is usually globular and composed of a ring of three basal plates, surmounted by a circle of five larger radial plates. The mouth is often surrounded by five large openings or spiracles associated with the respiratory system. Although relatively rare,

Echinosphaerites rhombiferan
Sphaeronites diploporite
Pleurocystites

Haplosphaeronis diploporite

Figure 15.7 Some Ordovician cystoid genera: Echinosphaerites and Sphaeronites, (xü.75), Haplosphaeronis and Pleurocystites (x1.5). (Based on Treatise on Invertebrate Paleontology, Part S. Geol. Soc. Am. and Univ. Kansas Press.)

Pleurocystites rhombiferan

Haplosphaeronis diploporite

Figure 15.7 Some Ordovician cystoid genera: Echinosphaerites and Sphaeronites, (xü.75), Haplosphaeronis and Pleurocystites (x1.5). (Based on Treatise on Invertebrate Paleontology, Part S. Geol. Soc. Am. and Univ. Kansas Press.)

lateral view aboral view oral view

Timoroblastus Permian lateral view aboral view oral view

Timoroblastus Permian

lateral v

lateral view lateral v oral view

Pentremites Carboniferous lateral view

Schizoblastus Carboniferous-Permian

Figure 15.8 Some blastoid genera. Magnification x0.6 for all. (Redrawn from various sources.)

Figure 15.8 Some blastoid genera. Magnification x0.6 for all. (Redrawn from various sources.)

Figure 15.9 (a) An eocrinoid, and (b) a paracrinoid. (Based on Treatise on Invertebrate Paleontology, Part S. Geol. Soc. Am. and Univ. Kansas Press.)

a few horizons are packed with blastoids, particularly when the diversity of the group peaked in the Early Carboniferous. Visean reefal facies in northern England yield abundant blastoids, as do the Permian limestones on the island of Timor where, for example, Timoroblastus and Schizoblastus occur.

The blastoids first appeared during the Silurian, probably evolving from an Ordovi-cian ancestor with brachioles and a reduced number of plates. They initially competed, ecologically, with the rhombiferan cystoids. The evolutionary history of the group was marked by changes in the shape of the theca and variations in the length of the ambulacra. Two main groups are recognized: the more basal Fissiculata characterized by hydrospire folds, and the Spiraculata with, as the name suggests, well-developed spiracles.

Eocrinoids

The eocrinoids were the earliest of the brachi-ole-bearing echinoderms. They had a huge range of thecal shapes with primitive holdfasts and an irregular to regular arrangement of plates (Fig. 15.9a). Sutural pores rather than thecal pores, along the joins between the plates, were characteristic of the earliest eocri-noids; in others there is a total lack of respiratory structures. Eocrinoids differ from the crinoid groups in having biserial brachial appendages. Over 30 genera have been described from rocks of Early Cambrian to Late Silurian age. The origins of the other blastozoan classes are probably to be found within this heterogenous group; for example the aberrant Late Cambrian eocrinoid Cam-brocrinus has been cited as an ancestor for the rhombiferan cystoids. Whereas many eocri-noids were high-level suspension feeders with the first columnal-constructed stems, some lay reclined or recumbent on the seabed. The Ordovician Cryptocrinus, for example, has a globular theca with a more irregular arrangement of plates.

Paracrinoids

The paracrinoids (Fig. 15.9b) are a small, odd group of arm-bearing echinoderms that have globular thecae and numerous irregularly-arranged plates together with two to five armlike, food-gathering structures. They are so different that some scientists have suggested that they represent a separate subphylum. The group is restricted to North America, where they are common in the Middle Ordovician.

Echinoidea_

Echinoids, the well-known sea urchins and sand dollars, have robust, rigid endoskeletons, or tests, composed of plates of calcite coated by an outer skin covered by spines. The tests are usually either globular or discoidal to heart-shaped (Smith 1984). Echinoids are most common in shallow-water marine environments where they congregate in groups as part of the nektobenthos. Their classification (Box 15.5) is based on the arrangement of plates and their mouth structures.

Echinoids have a long history from their first radiation in the Ordovician (Paul & Smith 1984). Two of the most significant evolutionary events in the history of the subphy-lum were marked by sudden divergences from the regular morphology to generate irregular burrowing echinoids. The first, in the Jurassic, led subsequently to a range of irregular bur-rowers, and the second, during the Paleocene, to the quasi-infaunal sand dollars. Both events were probably rapid and permitted major adaptive radiations of parts of the group into new ecological niches.

Basic morphology

The exoskeleton or test of most regular echi-noids, for example the common sea urchin Echinus esculentus, is hemispherical and displays all the main features of the group (Fig. 15.11). The lower, adapical or oral, surface is perforated by the mouth whereas the upper, apical or aboral, surface has the anal opening. The sea urchin is part of the active mobile benthos, in contrast to the sand dollars which were quasi-infaunal.

The test is built of a network of many hundreds of interlocking calcite plates organized into 10 segments, radiating from the oral surface and converging on the aboral surface. Five narrower segments or ambulacral areas (ambs) carry the animal's tube feet and are in contact with the ocular plates. The ambs alternate with the wider interambulacral areas (interambs), are armed with spines and abut against the genital plates. Together the ambs and interambs comprise in total 10 areas and 20 columns, which make up the corona - the majority of the test.

The central part of the aboral surface has a ring of five genital plates, each perforated by a hole to allow the release of gametes; the madreporite is commonly larger than the other genital plates and has numerous minute pores interfacing, beneath, with the water vascular system. These alternate with the ocular plates, terminating the ambulacral areas, and each houses further outlet holes for the water vascular system. This part of the apical system surrounds the periproct, or anal opening, which is partially covered by a number of smaller plates attached to a membrane. On the underside of the test, the peri-stome, containing the mouthparts, is also covered by a membrane coated with small plates. The mouth holds a relatively sophisticated jaw apparatus comprising five individual jaws each with a single, curved, saber-like tooth, operating like a mechanical grab and forcing particles into the animal's digestive system. The great ancient Greek naturalist Aristotle, who described the structure first, compared it to a "horn lantern with the panes of horn left out", and the echinoid jaw is often called Aristotle's lantern. In crown-group forms, muscles attached to the lantern are anchored to the perignathic girdle, developed around the edge of the peristome.

Box 15.5 Echinoid classification

The traditional split of the class into regular and irregular forms is no longer considered to reflect the true phylogeny of the echinoids. Whereas the irregular echinoids are probably monophyletic, arising only once, the regular echinoids do not form a clade. The group was traditionally subdivided into three subclasses (Fig. 15.10) - the Perischoechinoidea, Cidaroidea and Euechinoidea - the first, however, has been shown to be polyphyletic and the term stem-group echinoids is preferred.

Stem-group ECHINOIDEA

• Regulars with ambulacra in more than two columns, interambulacra with many columns; in total the test is composed of over 20 columns. Lantern with simple grooved teeth and lacking a peri-gnathic girdle

• Upper Ordovician to Permian

Crown-group ECHINOIDEA

Subclass CIDAROIDEA

• Regulars with test consisting of 20 columns of plates; two columns in each ambulacra and inter-ambulacral areas. Interambulacral plates have large tubercle. Teeth are crescentic to U-shaped and the perignathic girdle includes only interambulacral elements

• Lower Permian to Recent

Subclass EUECHINOIDEA

• Post-Paleozoic taxa, both regular and irregular. Both ambulacra and interambulacra with twin columns. Perignathic girdle composed on ambulacral projections

• Middle Triassic to Recent crown-group echinoids Euechinoidea Acroechinoidea

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