Echinoderms

echinoderms are a phylum of spiny-skinned, exclusively marine animals that first appear in the Early Cambrian. Modern examples include starfish, sea urchins, crinoids ("sea lillies"), and sea cucumbers. All modern examples have five-fold or pentameral radial symmetry (like a five-pointed star), but this is not the case for many fossil echinoderm groups. Echinoderms are most easily recognized as fossils by their distinctive skeleton composed of plates. Echino-derm plates qualify as sclerites, but individual plates in some echi-noderms (especially specialized, modern sand dollars) are fused together to such an extent that it is difficult or impossible to make out the outlines of the individual plates. In these cases it is better to refer to the entire echinoderm skeleton as a "test" rather than as a scleritome. Echinoderm skeletons and skeletal plates are always calcareous. Each plate is a single porous crystal of calcite. These porous plates are unmistakable when viewed in thin section.

Three very different examples of echinoderms are known from

Www Filter Feeding Echinodermata
FIGURE 4.15. Laudonia, a Lower Cambrian trilobite with conspicuous me-tagenal spines. Greatest width 3 cm. (From M. McMenamin 1989)

the Early Cambrian: eocrinoids, edrioasteroids, and helicoplacoids. All three of these echinoderms are considered by Sprinkle and Kier (1987) to have been suspension feeders or filter feeders. The oldest echinoderm fossils known come from the Montenegro Member of the Campito Formation in the White-Inyo Mountains of eastern California (Durham 1971). These fossils are disarticulated plates that probably belonged to helicoplacoids (P. W. Signor, personal communication). Fossil eocrinoid plates occur slightly higher in the White-Inyo Mountains' stratigraphic succession. Eocrinoids (figure 4.16) are early stalked echinoderms, in which the main body of the animal is held above the sea floor by a supporting stalk. Despite their name, eocrinoids were not directly ancestral to the familiar crinoids which became extremely abundant 200 million years later (Sprinkle and Kier 1987). The stalk of the eocrinoid is composed of many small plates. This stalk supports the theca or main body of the eocrinoid. The theca is a sac-like or box-like body made of sutured or imbricate

FIGURE 4.13. Cephalon of the Lower Cambrian olenellid trilobite [opposite above), judomia. Note its large eyes and long genal spines. Width of specimen (spine tip to spine tip) 3.6 cm. (From M. McMenamin 1988)

FIGURE 4.14. Cephalon of Nevadia ovalis, {opposite below), another large-eyed trilobite from the Lower Cambrian. Scale bar = 5 mm. (After M. McMenamin 1987b)

plates enclosing and protecting the main body. Thecal plates are larger than stalk plates and often have notches along their edges (figure 4.16), forming pores which ran into the interior of the theca. These sutural pores were used for respiration (Sprinkle and Kier 1987). Extending from the theca are eight or more arms (also composed of plates), which were used by the animal for filter feeding. In the eocrinoid Gogia spiralis, each arm is tightly spiralled (Durham 1971). The inner edge of each arm contains a food groove (or ambu-lacral groove). This food groove conducted trapped food particles to the eocrinoid's mouth, which was located at the top of the theca near the base of the arms.

Edrioasteroids also first appear during the Early Cambrian. Ed-rioasteroids have a biscuit-shaped, discoid, or globular theca that was cemented to the seafloor (or, occasionally, to another organism's shell). On the upper surface of the theca were five curving or straight food grooves. These food grooves make edrioasteroids appear similar

FIGURE 4.16. The Cambrian eocrinoid, Gogia spiralis. The name of this species refers to its unusual spiralled arms. An eocrinoid's stalk is make of many calcitic plates, while its calyx or "head" is made of beadlike plates. Despite the name, eocrinoids are probably not ancestral to true crinoids or "sea lillies." Length of specimen 5.3 cm. (From Robison 1965)

FIGURE 4.16. The Cambrian eocrinoid, Gogia spiralis. The name of this species refers to its unusual spiralled arms. An eocrinoid's stalk is make of many calcitic plates, while its calyx or "head" is made of beadlike plates. Despite the name, eocrinoids are probably not ancestral to true crinoids or "sea lillies." Length of specimen 5.3 cm. (From Robison 1965)

to Tribrachidium with its three radiating arms (Stanley 1976), but many paleontologists consider these similarities to be superficial. Gehling (1987) recently described, from the classic Ediacaran strata of Australia, a globular fossil with five rays on its surface that looks very suggestive of an edrioasteroid. This fossil, Aikaiua adami, (figure 4.17) is presented by Gehling (1987) as the oldest known echi-noderm, but its similarities to later echinoderms may prove superficial.

The most bizarre of the Early Cambrian echinoderms was Helicoplacus, a spiralled, spindle-shaped fossil restricted to Lower Cambrian sediments of western North America (figure 4.18). The whole test could be expanded and contracted; expansion apparently was accomplished by inflation from the inside. In one species, there are spines on some of the plates. Helicoplacus had one or more food grooves like the eocrinoids, except that instead of being on the arms, the grooves wound around the body of the animal, following the spiralled rows of plates. These grooves in Helicoplacus were presumably for filter-feeding, but this cannot have been a very efficient way to feed, with so little food groove surface area exposed. Indeed, there is controversy concerning the position of the mouth in Helicoplacus. Durham and Caster (1963) reconstructed the mouth position as being on the top of the pear-shaped or fusiform body. Derstler (1982) disagrees, arguing that the mouth was on the side of the spindle. Another possibility is that Helicoplacus had no mouth at all—perhaps it absorbed dissolved nutrients directly from sea water. Whatever their feeding strategy, helicoplacoids were a very short-lived group. They did not survive the Lower Cambrian.

FIGURE 4.17. Arkarua adami, described as the oldest known echinoderm. Its affinities to true, skeleton-bearing echinoderms remain to be proven. Diameter of fossil 6 mm. (After Gehling 1987)

FIGURE 4.18. Helicoplacus gilberti, a Lower Cambrian echinoderm. Heli-coplacoid food grooves wound in a spiral around the body of the animal parallel to the rows of plates, but the position of its mouth (where the food grooves joined together) is a matter of debate. Length of specimen 2.5 cm. (After Durham and Caster 1963)

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  • kim
    How did the sea cucmber survie the cambrian explosion?
    2 years ago
  • melissa walker
    Did sa cucmbers survive the cambrian explosion?
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