Zamora S et al. (2012), Plated Cambrian bilaterians reveal the earliest...

ECB-ART-48230

PLoS One 2012 Jan 01;76:e38296. doi: 10.1371/journal.pone.0038296.

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Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution.

Zamora S , Rahman IA , Smith AB .

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Echinoderms are unique in being pentaradiate, having diverged from the ancestral bilaterian body plan more radically than any other animal phylum. This transformation arises during ontogeny, as echinoderm larvae are initially bilateral, then pass through an asymmetric phase, before giving rise to the pentaradiate adult. Many fossil echinoderms are radial and a few are asymmetric, but until now none have been described that show the original bilaterian stage in echinoderm evolution. Here we report new fossils from the early middle Cambrian of southern Europe that are the first echinoderms with a fully bilaterian body plan as adults. Morphologically they are intermediate between two of the most basal classes, the Ctenocystoidea and Cincta. This provides a root for all echinoderms and confirms that the earliest members were deposit feeders not suspension feeders.

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	Figure 1. Radiate and asymmetric echinoderms from the Cambrian showing a selection of primitive echinoderm body plans. A, the ctenocystoid Ctenocystis; B, the cinctan Gyrocystis; C, the helicoplacoid Helicoplacus; D, the solute Coleicarpus; E, the eocrinoid Gogia; F, stromatocystitid edrioasteroid.
	Figure 2. Fossil locality and geological setting. A, Map of Spain showing the location of the Iberian Chains (IC). B, Map showing the Purujosa locality in the northern part of the Iberian Chains. C, Geological map of Purujosa with indication of the studied section (marked with a star). D, Composite stratigraphic section indicating the level from where Ctenoimbricata spinosa was collected.
	Figure 3. Natural mould and latex casts of the holotype of Ctenoimbricata spinosa gen. et sp. nov. (MPZ 2011/93) in dorsal (A, C) and ventral views (B, D).Latex casts were whitened with NH4Cl sublimate.
	Figure 4. Ctenoimbricata spinosa gen. et sp. nov. Cambrian Series 3, Stage 5, Purujosa, Spain.Computer models (A–G, I) and photograph with interpretive camera lucida drawing (H). (A–D, G–I) Holotype MPZ 2011/93. (E, F) Paratype MPZ 2011/94. (A, B) Dorsal and ventral views. (C) Oblique left view. (D) Lateral view of two marginal plates showing the articulation of the spines. (E) Marginal frame plates after correction of plate orientations. (F) Suroral plate in dorsal and lateral aspect. (G) Oblique right view with the dorsal ctenidium partially transparent to show the ventral ctenidial plates. (H) Left anterior part of the theca showing the arrangement of the dorsal ctenidial plates. (I) Frontal view. Abbreviations: ds (dorsal spines), icp (imbricate ctenidial plates), Lcp (lower ctenidial plates), LL (adoral left plate), M (marginal plate), RL (adoral right plate), scp (spiny ctenidial plates), sp (suroral plate), ui, li (upper and lower integuments).
	Figure 5. Reconstruction of Ctenoimbricata spinosa gen. et sp. nov.
	Figure 6. Courtessolea moncereti Domínguez-Alonso, 1999 (MNHN F.A45783).Cambrian Series 3, Stage 5, Ferrals-les-Montagnes, France. Dorsal view and interpretive diagram (note that the theca shows a small degree of post-mortem distortion). Abbreviations: ap (anal pyramid), LL (adoral left plate), M (marginal plate), mf (marginal frame), Mp (posterior marginal plate), RL (adoral right plate), stp (spiny ctenidial plates), sp (suroral plate), tcp (tessellate ctenidial plates), ui (upper integument).
	Figure 7. Diagram showing inferred homologies between ctenocystoids (Ctenocystis and Courtessolea), Ctenoimbricata and cinctans (Sotocinctus).The upper row illustrates dorsal surfaces, the lower row ventral surfaces; colors indicate plating series that are homologized. Reconstructions of ctenocystoids are modified from [34]. S = suroral plate; O = operculum.
	Figure 8. Cladogram showing some major events in deuterostome evolution.Relationships of living organisms are based on molecular data ([4]–[8]); fossils are placed using morphological homologies. 1, Dipleurula-type larva and tripartite organization of body coeloms; 2, Multiplated calcite skeleton with stereom microstructure; 3, Complete ctenidial ring; 4, Periproct non-terminal; 5, Water vascular system with single hydropore (asymmetric development of coeloms); 6, Adult body plan not bilateral (associated with larval attachment and torsion during metamorphosis in extant groups); 7, Tentacular feeding; 8, Radial organization of water vascular system.

References [+] :

Bottjer, Paleogenomics of echinoderms. 2006, Pubmed, Echinobase