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Science 10 November 2006:
Vol. 314. no. 5801, pp. 956 - 960
DOI: 10.1126/science.1132310
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Review
Paleogenomics of Echinoderms
David J. Bottjer1*,
Eric H. Davidson2,
Kevin J. Peterson3 and
R. Andrew Cameron2
1 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089–0740, USA.
2 Division of Biology 156-29, California Institute of Technology, Pasadena, CA 91125, USA.
3 Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA.
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Fig. 1. Stereom formation in modern and fossil echinoderms. (A) A spine of the modern sea star Asterias with soft tissue removed, constructed of meshlike stereom. (B) An occular plate of the modern sea star Asterias with soft tissue removed, constructed of meshlike stereom. (C) Median cross section of the stylocone from the Middle Cambrian stylophoran ?Ceratocystis, showing stereom construction. (D) Detail of stereom from the inner face in external view of the stylocone from the Middle Cambrian stylophoran ?Ceratocystis. (E) Larval spicule of S. purpuratus with biomineralized stereom (calcite) dissolved away, showing the distribution of spicule matrix proteins. [(A) and (B) are used with permission from C. Sumrall; (C) and (D) are reprinted by permission from Macmillan Publishers Ltd. (Nature) (11), copyright (2005); (E) is reprinted from (27), copyright 1983, with permission from Elsevier.] Scale bars in (A) to (C), 500 µm; scale bar in (D), 100 µm; magnification of (E) is x3000.
[View Larger Version of this Image (126K GIF file)]
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Fig. 2. Evolutionary history of the major echinoderm groups. Deuterostomia consists of three major groups: the chordates, hemichordates, and echinoderms, all with fossil representatives in the Cambrian. Cambrian echinoderms are recognized by the possession of stereom, but the phylogenetically most basal groups (such as stylophorans) lack the water vascular system, are highly asymmetrical, and possess gill slits. Pentameral symmetry is seen in two major Early Cambrian lineages, the edrioasteroids and eocrinoids; a third Early Cambrian taxon, the helicoplacoids, have an unusual threefold symmetry thought to be derived from the ancestral pentameral arrangement (10). All stem-group echinoderm lineages became extinct by the Carboniferous (indicated with crosses). Crown-group echinoderms, indicated by the yellow circle, consist of the five major extant lineages in addition to numerous extinct lineages not shown. Most class-level crown groups first appear in the latest Paleozoic–early Mesozoic, including echinoids. The lineage leading to echinoids, and hence to S. purpuratus, is indicated in purple. Known stratigraphic ranges are shown with thick lines, and inferred range extensions are shown with thin lines.
[View Larger Version of this Image (68K GIF file)]
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Fig. 3. Stem-group (A to E) and crown-group (F to H) echinoderms. (A) Thestylophoran Cothurnocystis bifida (Middle Cambrian, Utah, USA). The putative gill skeletons as viewed from the back side are indicated with an arrowhead. M is the putative mouth. The arrow indicates the posterior appendage. (B) The solute Coleicarpus sprinklei (Middle Cambrian, Utah, USA). The arrow indicates the posterior appendage, and the double arrow points to the single ambulacrum. (C) The helicoplacoid Helicoplacus (Early Cambrian, California, USA). The double arrow points to one of the ambulacral grooves. (D) The eocrinoid Gogia spiralis (Middle Cambrian, Utah, USA). The double arrow points to one of the five arms. (E) The edrioasteroid Edriophus bigsbyi (Ordovician, Ontario, Canada). It displays conspicuous pentameral symmetry; one of the arms is indicated by the double arrow. (F) The crinoid Dorycrinus mississippiensis (Mississippian, Indiana, USA). (G) The asteroid Furcaster palaeozoicus (Devonian, Budenbach, Germany). (H) The echinoid Bothriocidaris (Ordovician, Estonia) [reprinted with permission from A. B. Smith, from (30)]. Scale bar, 0.5 cm in (A); 0.75 cm in (B) to (E); 2 cm in (F); 1.3 cm in (G); and 0.15 cm in (H). Part of a penny is shown for scale in (C).
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