Ventral

8 circumoral ring 15

9 esophagus 16

10 radial water vessel 17

11 ampulla 18

12 ocular pore 19

13 periproct 20

14 madreporite 21

gonopore gonad spine corona peristome buccal tube foot tube foot apical system ambulacral interambulacral area apical system ambulacral interambulacral area area

peristome periproct tubercle

Figure 15.11 Echinoid morphology: (a) internal anatomy in cross-section; (b) dorsal and (c) ventral views of Echinus. (Based on Smith 1984.)

peristome periproct ambulacral pores tubercle

Figure 15.11 Echinoid morphology: (a) internal anatomy in cross-section; (b) dorsal and (c) ventral views of Echinus. (Based on Smith 1984.)

area

The echinoid's various organs are suspended within the test and supported by fluid. The water vascular system copes with a number of functions. The stone canal rises vertically, from the central ring around the esophagus, to unite with the madreporite. Five radial water vessels depart from the central ring to service the ambulacral areas; smaller vessels are attached to each tube foot and its ampulla where variations in water pressure drive the animal's locomotory system.

The echinoid digestive system lacks a stomach and operates through the esophagus together with a large and small intestine; waste material is expelled through the rectum into the anus and externally by way of the periproct. An unsophisticated nervous system comprising a nerve ring and five radial nerves connects with the ambulacral pores where the nerve ends divide externally to form a sensory net.

The regular to irregular transition

Regular echinoids, the sea urchins, with a compact, symmetric morphology are most basal. Their shape contrasts with the irregulars, with marked bilateral symmetries and specialized for forward movement. The irregular echinoid morphotype evolved rapidly and apparently involved some large architectural changes to adapt the animal to burrowing. Plesiechinus hawkinsi is one of the first irregular echinoids, appearing early in the Early Jurassic (Sinemurian) with an asymmetric test, short numerous spines, large adapical pores and a posteriorly placed periproct together with presumed keeled teeth. Ten million years later, by the Toarcian, much of the "toolkit" of adaptations had evolved for a burrowing life mode. Secondary bilateral symmetry was superimposed on the existing pentameral symmetry to form a heart-shaped or flattened ellipsoidal test. The periproct migrated from a position on the apical surface to the posterior side of the test to eject waste laterally. By the Early Cretaceous, one of the ambulacral areas had become modified to form a food groove and a series of tube feet were extendable with flattened ends to assist respiration.

One of the earliest sand dollars, the clypeasteroid Togocyamus, appeared during the Paleocene, and some 20 million years later in the Eocene more typical sand dollars had evolved to command a cosmopolitan distribution. The flattened test was adapted for burrowing, whereas the accessory tube feet could encourage food along the food grooves and draped the test with sand. The highly accentuated petals helped respiration by providing an increased surface area for the tube feet, and the development of a low lantern with horizontal teeth signaled changes in feeding patterns.

Ecology: modes of life

The regular Echinus and the irregular Echino-cardium probably mark the ends of a spectrum of life modes from epifaunal mobile behavior to a number of infaunal burrowing strategies (Fig. 15.12). Mobile regular forms such as Echinus grazed on both hard and soft sub strates and in caves and crevices on the sea-floor; these sea urchins may have been omnivores, carnivores or herbivores. Irregular forms display a range of adaptations appropriate to an infaunal mode of life where burrows were carefully constructed in low-energy environments. Extreme morphologies were developed in the sand dollars or Clypeasteroidea, permitting rapid burial just below the sediment-water interface in shifting sands. Echi-noids generally lived in shallow seawaters, but some went deeper; the timing of this move offshore has been controversial (Box 15.6).

Life modes and evolution: microevolution of Micraster One of the classic case studies of evolutionary patterns in fossils is seen in Micraster, an infaunal, irregular echinoid. Paleobiologists have repeatedly used this example to test phyletic gradualist and punctuated equilibria models (see p. 121) and as the raw material for the rigorous statistical analysis of both ontogenetic and phylogenetic change. In the best-known lineage, M. leskeiM. decipiens-M. coranguinum, the following morphological changes occurred (Fig. 15.14):

1 The development of a higher, broader (heart-shaped) form associated with an increase in size and thickness of the test.

2 The peristome (mouth) moved anteriorly and the posteriorly situated periproct (anus) had a lower position on the side of the test with a broader subanal fasciole.

3 The madreporite increased in size at the expense of the adjacent specialized plates.

4 More tuberculate and deeper anterior ambulacra evolved.

5 More granulated periplastronal areas developed.

These morphological changes are associated with life in progressively deeper burrows. But there is a lack of associated trace fossils that might prove this. On the other hand, the adaptations may have been geared to greater burrowing efficiency, probably in shallow depths in the chalk where such traces were destroyed by reworking of the sediments.

Microevolutionary trends have been tested in other echinoid lineages. The irregular Discoides occurs abundantly through an Upper Cretaceous section at Wilmington, south Devon, England. The height and diameter of

Lower Jurassic Lower Tertiary

Hettangian Sinemurian Toarcian Paleocene

Lower Jurassic Lower Tertiary

Hettangian Sinemurian Toarcian Paleocene

Sand Dollar Dorsal Vertal

flattening of test -

elongation of test -

decrease in size of tubercles and spines increase in number of tubercles and spines shifting of anus posteriorly -

shifting of peristome anteriorly-

decrease in size of peristome -

flattening of test -

elongation of test -

decrease in size of tubercles and spines increase in number of tubercles and spines shifting of anus posteriorly -

shifting of peristome anteriorly-

decrease in size of peristome -

regular debris-covered irregular, flat, shallow, echinoids regular echinoids rapidly burrowing echinoids regular debris-covered irregular, flat, shallow, echinoids regular echinoids rapidly burrowing echinoids

Mellita Quinquiesperforata
Figure 15.12 Echinoid life modes: (a) transition from the sea urchins through the heart urchins to the sand dollars; (b) habits and modes of life of echinoids. (a, based on Kier, P. 1982. Palaeontology 25; b, based on Kier, P. 1982. Smithson. Contr. Paleobiol. 13.)

the echinoids change through the section, but this seems to be related to the grain size of the sediment, where high, narrow forms favor fine sediment. The case history is available at http:// www.blackwellpublishing.com/paleobiology/.

Evolution

The first echinoids had appeared by the Mid Ordovician but it is only in Lower Carboniferous rocks that echinoids become relatively

Box 15.6 Into the deep: but not until the Late Cretaceous?

A number of animal groups common in the Paleozoic evolutionary fauna, such as the brachiopods and crinoids, are common in deep-water environments. This reinforces the view that the deep sea is some sort of refuge for archaic taxa that have been forced down the continental slope by predation or unsuccessful competition on the shelf. Andrew Smith and Bruce Stockley (2005) have developed a quite different model, however, based on molecular clock estimates and the phylogeny of echinoid taxa. Results show that the modern deep-sea omnivore fauna appeared gradually over the last 150200 myr; detritivores, however, were in place during a much shorter time span between 75 and 50 Ma (Fig. 15.13). This 25 myr window of seaward migration appears to be associated with marked increases in seasonality, continental sediment discharge and surface productivity. The increased availability of organic carbon and nutrients in the deep sea provided the means for habitat expansion rather than an escape from competition and predation on the shelf.

Heart Shape Echinoids
Figure 15.13 Events in the deep sea: cumulative frequency polygons for maximum and minimum times of origin of 38 clades of extant, carnivore and detritivore deep-sea echinoids (Smith & Stockley 2005). K/T, Cretaceous-Tertiary boundary; OAEs, oceanic anoxic events.

time

Turonian

Coniacian - Santonian sea bottom time

Turonian

Coniacian - Santonian sea bottom

M. coranguinum

Figure 15.14 Evolution of the Late Cretaceous heart urchin, Micraster. (Based on Rose, E.P.F. & Cross, N.E. 1994. Geol. Today 9.)

M. coranguinum

Figure 15.14 Evolution of the Late Cretaceous heart urchin, Micraster. (Based on Rose, E.P.F. & Cross, N.E. 1994. Geol. Today 9.)

abundant. The sparse early record of the group might reflect a relatively fragile skeleton that quickly disintegrated after death; on the other hand the echinoids were probably not a common element of the Paleozoic benthos. The enigmatic Bothriocidaris, described from the Ordovician of Estonia and from southwest Scotland, has been variously classified as an echinoid, cystoid or holothu-rian. Some authorities consider that Bothriocidaris and Eothuria might be unclassifiable echinoids, hopeful monsters that arose during the rapid Ordovician radiation of the group.

Aulechinus from the Upper Ordovician of southwest Scotland is one of the most primitive echinoids and the first with only two plate columns in the ambulacral areas. During the Paleozoic there was generally an increase in the number and size of ambulacral areas and the sophistication of Aristotle's lantern, although most genera remained relatively small (Fig. 15.15).

There was a significant decline in echinoid diversity during the Late Carboniferous. By the Permian only half a dozen species are known, and they belonged to two primary groups: detritus feeders and opportunists. Large proterocidarids were highly specialized detritus feeders, and the small omnivorous Miocidaris and Xenechinus were opportunists. Two lineages, including Miocidaris, survived the end-Permian extinction event to radiate extravagantly during the early Meso-zoic, thus ensuring the survival of the echi-noids. Following the end-Permian extinctions the regular echinoids diversified during the Late Triassic and Early Jurassic with more advanced regulars dominating the early Meso-zoic record. The irregulars appeared during the Early Jurassic and substantially increased in numbers during the period. Diversity was severely reduced by the Cretaceous-Tertiary extinction event but both the regulars and irregulars recovered rapidly during the early Cenozoic.

Asteroidea_

Starfish are common on beaches today, and their biology has made them hugely successful. Some feed by preying on shellfish and other slow-moving shore and shallow-marine animals. Their feeding mode is unusual but deadly: they simply sit on top of their chosen snack, turn their stomachs inside out and absorb the flesh of their victim. The majority are benthic deposit feeders that ingest prey or filter feed. Starfish are also unusual in that they have eyes at the ends of their arms - these are actually light-detecting cells, not true eyes, but the adaptation is novel nonetheless.

Asteroids appeared first during the Early Ordovician. The subphylum contains two main groups: the asteroids or starfish and the ophiuroids or brittle stars. These animals have a star-shaped outline with usually five arms radiating outwards from the central body or disk. The water vascular system is open. The mouth is situated centrally on the underside of the animal on the oral or dorsal surface whereas the anus, if present, opens ventrally on the adoral surface. The asterozoans are characterized by a mobile lifestyle within the benthos, where many are carnivores. Astero-zoan skeletons disintegrate rapidly after death due to feeble cohesion between the skeletal plates. Thus, recognizable fossils are relatively rare. Nevertheless there are a number of starfish Lagerst├Ątten deposits where asterozoans are extremely abundant and well preserved.

Distribution and ecology of the main groups

Three classes of asterozoans have been recognized: the basal Somasteroidea, the Asteroidea or starfish and the Ophiuroidea or brittle stars. The Somasteroidea include some of the earliest starfish-like animals, described from the Tremadocian of Gondwana. These echino-derms have pentagonal-shaped bodies with the arms initially differentiated from around the oral surface. In some respects this short-lived group, which probably disappeared during the Mid Ordovician, displays primitive starfish characters intermediate between a pelmato-zoan ancestor and a typical asterozoan descendant. Typical asteroids have five arms radiating from the disk, which is coated by loosely fitting plates permitting considerable flexibility of movement (Fig. 15.16). Additional respiratory structures, called papulae, project from the celom through the plates of the upper surface. This backup system aids the high metabolic rates of these active starfish.

The first true starfish were probably derived from the somasteroids during the Early Ordo-vician and were relatively immobile, infaunal sediment shovelers. Some of the first starfish, for example Hudsonaster, from the Middle Ordovician, have similar plate configurations to the young growth stages of living forms such as Asterias. Although relatively uncommon in Paleozoic rocks, the group was important during the Mesozoic and Cenozoic and is now one of the most common echinoderm classes.

The ophiuroids first appeared during the Early Ordovician (Arenig), but the group, as presently defined, may be paraphyletic. Classification is based on arm structure and disk plating. The ophiuroid body plan is distinctive, with a subcircular central disk and five

Crinoid Dorsal MarsMicraster Fossil Aboral And Oral View

Figure 15.15 Aboral, oral and lateral views of some echinoid genera: (a-c) Cidaris (Recent; regular), (d-f) Conulus (Cretaceous; irregular), (g-i) Laganum (Recent; sand dollar) and (j-l) Spatangus (Recent; heart urchin). All approximately natural size. (From Smith & Murray 1985.)

Figure 15.15 Aboral, oral and lateral views of some echinoid genera: (a-c) Cidaris (Recent; regular), (d-f) Conulus (Cretaceous; irregular), (g-i) Laganum (Recent; sand dollar) and (j-l) Spatangus (Recent; heart urchin). All approximately natural size. (From Smith & Murray 1985.)

Laganum Laganum Body Part

long, thin, flexible arms. The mouth is situated centrally on the lower surface of the disk. Most of the disk is filled by the stomach and, in the absence of an anus, waste products are regurgitated through the mouth. The arms consist of highly specialized ossicles or vertebrae. Ophiuroids are common in modern seas and oceans, preferring deeper-water environments below 500 m. Their basic architecture differs little from some of the first members of the group, for example Taeniaster from the Middle Ordovician of the United States.

A few modern starfish are vicious and voracious predators enjoying a diet of shellfish. Asteroids can prize apart the shells of bivalves with their sucker-armored tube feet far enough to evert their stomachs through their mouths and into the mantle cavity of the animal, where digestion of the soft parts takes place. Stephen Donovan and Andrew Gale (1990) suggested that this predatory life mode significantly inhibited the post-Permian diversification of some brachiopod groups. The strophomenides, the most diverse Permian brachiopods, largely pursued a reclined, quasi-infaunal life strategy and they may have presented an easy kill for the predatory asteroids.

Carpoidea_

The carpoids include some of the most bizarre and controversial fossil animals ever described.

Variably described as carpoids, homalozoans or calcichordates, depending on preference, most authorities consider the group to be very different to the radiate Echinodermata; indeed, carpoids show some puzzling similarities to the chordates.

The carpoids were marine animals ranging in age from Mid Cambrian to possibly Late Carboniferous, with a calcitic, echinoderm-type skeleton lacking radial symmetry (Fig. 15.17). Two main types of carpoid are recognized: the cornutes and the mitrates. The cor-nutes were often boot-shaped and appear to have a series of gill slits on the left side of the roof of the head, whereas the mitrates, derived from a cornute ancestor, were more bilaterally symmetric with covered gill slits on both sides.

It might seem unexpected, but the carpoids have featured at the center of a long-running and heated debate that has hit the headlines over the past 50 years. After much careful study, Richard Jefferies (1986) presented detailed evidence that carpoids and chordates share many characters, the so-called "cal-cichordate hypothesis". He based his conclusion on painstaking studies of their anatomy and the anatomy of embryos of modern echi-noderms and chordates. A chordate-implied reconstruction of carpoids suggests that the body consists of a head and a tail used for locomotion (Sutcliffe et al. 2000) (Fig. 15.18).

Moreover, Jefferies (1986) described structures indicating a fish-like brain, cranial nerves, gill slits and a filter-feeding pharynx similar to that in tunicates (often known as the sea squirts). In the calcichordate hypothesis, Hemichordata is identified as a sister group to Echinodermata + Chordata, a clade that Jefferies called Dexiothetica. Reappraisals of the anatomy of carpoids have shown,

Calcichordate

anus penproct arm hydropore gonopore ridge penproct arm hydropore gonopore ridge

central plate peripheral hinge line hind tail central plate peripheral hinge line hind tail

Figure 15.17 Morphology of the carpoids: (a) dorsal and (b) ventral surfaces. (From Jefferies & Daley 1996.)

however, that they may be interpreted rather more convincingly as echinoderms, and that the calcichordate hypothesis fails (Box 15.7). Further, when these redescriptions of the fossil material are combined with new molecular evidence on phylogeny, the case is lost (Ruta 1999). Molecular phylogenetic analyses (Winchell et al. 2002; Delsuc et al. 2006) show that Hemichordata is the sister group of Echinodermata, forming together the Ambu-lacraria, and that Ambulacraria is the sister group of Chordata. Dexiothetica does not exist.

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