Echinoderms

Clearly we stood among the ruins of some latter-day South Kensington! Here, apparently, was the Pal&ontological Section, and a very splendid array of fossils it must have been. . . . The place was very silent. The thick dust deadened our footsteps. Weena, who had been rolling a sea urchin down the sloping glass of a case, presently came, as I stared about me, and very quietly took my hand and stood beside me. And at first I was so much surprised by this ancient monument of an intellectual age, that I gave no thought to the possibilities it presented. Even my preoccupation about the Time Machine receded a little from my mind.

H. G. Wells (1898) The Time Machine

Echinoderms today are one of the most abundant marine animal groups, and as fossils they can sometimes be rather robust, as Weena from The Time Machine found. Sea urchins are common in many intertidal environments, and out in the deep sea the ocean floors are covered by brittle stars and sea cucumbers. The phylum Echinodermata has an unusual five-fold symmetry and is uniquely equipped with a water vascular system in which water is forced around the plumbing by muscular action, while tube feet, extending from the system, are often modified for food processing, locomotion and respiration. The 6000 or so living echinoderm species include familiar forms such as sea lilies, sea urchins, sand dollars, starfish and sea cucumbers (Fig. 15.1). Although many species today live in the intertidal or subtidal zones, the group is most diverse in the deep sea. Echinoderms also occupied a wide range of marine environments and pursued a variety of life strategies in the geological past. Fossil echinoderms are relatively common, and because many echinoderm skeletons disintegrate rapidly after death, many limestones are packed with the distinctive skeletal debris of calcitic plates.

Apart from the water vascular system, echi-noderms have a number of other distinctive features. All members of the phylum have a mesodermal skeleton constructed from porous plates of calcite; each plate is usually a single crystal of calcite and easy to recognize in thin sections. In addition, the plates have a unique ultrastructure of rods linked to form a three-dimensional lattice. This network, or stereom, is permeated by finger-like pieces of soft tissue that occupy the spaces, or stroma, in the lattice. Finally, five-rayed or pentameral symmetry, occasionally modified by a secondary bilateral symmetry, is typical of the echino-derms. The phylum is generally split into the mobile, non-stalked eleutherozoans and the mainly fixed, stalked pelmatozoans (Box 15.1), but the earliest forms are hard to classify (Box 15.2).

The multiplated echinoderm skeleton disintegrates very rapidly after death; although individual plates or ossicles have high preservation potential, the complete skeletons do not. Nevertheless, occasionally rapid burial or transportation into anoxic conditions may result in the preservation of complete echino-derm skeletons. Starfish beds, usually characterized by accumulations of complete echinoderms, occur sporadically throughout the fossil record. The Leintwardine Starfish Bed of the England-Wales border area con

Low-level epifaunal

Edrioasteroids

Helicoplacoid

Holothurian

Low-level epifaunal

Edrioasteroids

Helicoplacoid

Holothurian

Mobile infaunal and epifaunal

Figure 15.1 Life modes of the main echinoderm body plans. (Based on Sprinkle 1980.)

High-level epifaunal

Blastoid

High-level epifaunal

Blastoid

Mobile epifaunal

Mobile infaunal and epifaunal

Mobile epifaunal

Figure 15.1 Life modes of the main echinoderm body plans. (Based on Sprinkle 1980.)

tains Late Silurian echinoderms within finegrained turbidites, and the Lower Jurassic Starfish Bed of South Dorset, England is dominated by ophiuroids suddenly buried by a thick layer of sandstone. However, one of the most remarkable echinoderm Lagerst├Ątte occurs in the Upper Ordovician succession of the Craighead inlier, north of the Girvan valley, southwest Scotland. Here, the Lady Burn Starfish Bed is one of several sandstone

0 0

Post a comment