A Sea Without Fish by Davis Richard Arnold.;Meyer David L.;

Author:Davis, Richard Arnold.;Meyer, David L.;
Language: eng
Format: epub
Publisher: Indiana University Press

Stylophorans

For many paleontologists, stylophorans surpass even the cyclocystoids as some of the most bizarre fossil echinoderms. For a minority of specialists, stylophorans are considered not even to be echinoderms, but rather ancestors of the vertebrates belonging to a group called calcichordates. (The interested reader is referred to Gee [1996] for a blow-by-blow account of this debate.) This controversy, coupled with their exceeding rarity in Cincinnatian strata, make the stylophorans one of the most intriguing fossils ever to be found in the Cincinnati region. A single genus, Enoploura, is found in the Cincinnatian, with two species, E. popei Caster in the Maysvillian (Figure 12.17) and E. balanoides (Meek) in the Maysvillian and Richmondian (Caster 1952; Parsley 1991).

Enoploura is typical of the stylophorans, an extinct echinoderm class that ranges from the Middle Cambrian through the Early Pennsylvanian, with about eighty genera in all. Enoploura has a flattened, plated theca that is roughly rectangular; the plating has no radial or pentameral symmetry whatsoever, but rather is bilaterally symmetrical. A pair of spines is attached at one end, and at the opposite end, a single, segmented appendage is attached. This appendage has a very complex structure and its function has been vigorously debated. It was once thought to be a stalk-like holdfast, but those favoring the echinoderm affinity of the stylophorans consider it to be a feeding appendage, called the aulacophore. The basal part of the aulacophore is made up of a series of thin elements each consisting of four components; it was probably flexible. Following this flexible region is the so-called styloid, bearing a pair of bladelike flanges that presumably dug into the sediment. The terminal part of the aulacophore has a tapering series of sharply keeled ossicles bearing a groove with covering plates. Two possible feeding positions have been postulated for the aulacophore: held up into the water or arched slightly above the substratum. In any case the food consisted of very fine organic particles. Particles taken in along a food groove entered an internal mouth, and wastes were emitted through an anal opening between the spines. Proponents of the calcichordate interpretation of stylophorans regard the appendage to be a true, wriggling tail, with the mouth located at the opposite end of the theca. Study of well-preserved skeletal microstructure in Cincinnatian Enoploura by Carlson and Fisher (1981) revealed close similarity to typical echinoderm stereom, further supporting the classification of stylophorans with echinoderms. Some enigmatic fossils called machaeridians might also be related to stylophorans (see chapter 10).

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