PALEONTOLOGY/EVOLUTION: Prickly Paleozoic Animals

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Science  09 Feb 2001:
Vol. 291, Issue 5506, pp. 947E
DOI: 10.1126/science.291.5506.947E

Were the flattened, asymmetric Paleozoic animals known as stylophorans in fact early chordates. This calcichordate hypothesis requires that chordates originally possessed calcite skeletons made of stereom (a microscopic mesh embedded in mesodermal tissues). But, traditionally, stereom has been seen as diagnostic of the echinoderms, and most paleontologists have interpreted stylophorans accordingly. Because they and the other homalozoans lack the fivefold symmetry so characteristic of living echinoderms (such as sea urchins), all four classes of homalozoans usually are placed together near the base of the phylum Echinodermata.

Recently, researchers have combined ontogenetic, embryologic, and molecular evidence to identify body-wall homologies among extant echinoderms. David et al. have applied these findings to reinterpret homalozoan morphologies (in particular, the thecal walls, stems, arms, and brachioles). They conclude that homalozoans comprise independently evolved groups of echinoderms, whose asymmetry is derived rather than representative of the phylum's original morphology. Three classes possess brachioles and are placed within the blastozoans (an extinct Paleozoic clade). The controversial stylophorans are most closely releated to crinoids (sea lilies). This new phylogeny is more congruent with the known fossil record. — SJS

Acknowledgments

Paleobiology 26, 529 (2000).

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