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Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution.

Zamora S, Rahman IA, Smith AB - PLoS ONE (2012)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Palaeontology, The Natural History Museum, London, United Kingdom.

ABSTRACT
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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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.
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pone-0038296-g007: 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.

Mentions: Figure 7 summarizes the homologies that we draw between cinctans, ctenocystoids (Ctenocystis, Courtessolea) and Ctenoimbricata. Three openings are identified in cinctans [21], but only two in ctenocystoids [23] and Ctenoimbricata. Whereas cinctans have two anterior openings close together, identified as exhalant (atrial) and inhalant (mouth) orifices [12], [21], [27], Ctenoimbricata and ctenocystoids have, in the same position, only one wide opening. This implies that in ctenocystoids and Ctenoimbricata the wide anterior opening accommodates both inhalant and exhalant flows and has the combined function of feeding and expelling water from the interior of the theca. In cinctans this flow has become partitioned and the left-hand inhalant flow channeled to the mouth (on the animal’s right-hand side) via the anterior groove. Ctenocystoids and cinctans are interpreted as pharyngeal basket feeders with internal gill slits, akin to tunicates [10], [23] and the same is likely true for Ctenoimbricata.


Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution.

Zamora S, Rahman IA, Smith AB - PLoS ONE (2012)

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.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3368939&req=5

pone-0038296-g007: 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.
Mentions: Figure 7 summarizes the homologies that we draw between cinctans, ctenocystoids (Ctenocystis, Courtessolea) and Ctenoimbricata. Three openings are identified in cinctans [21], but only two in ctenocystoids [23] and Ctenoimbricata. Whereas cinctans have two anterior openings close together, identified as exhalant (atrial) and inhalant (mouth) orifices [12], [21], [27], Ctenoimbricata and ctenocystoids have, in the same position, only one wide opening. This implies that in ctenocystoids and Ctenoimbricata the wide anterior opening accommodates both inhalant and exhalant flows and has the combined function of feeding and expelling water from the interior of the theca. In cinctans this flow has become partitioned and the left-hand inhalant flow channeled to the mouth (on the animal’s right-hand side) via the anterior groove. Ctenocystoids and cinctans are interpreted as pharyngeal basket feeders with internal gill slits, akin to tunicates [10], [23] and the same is likely true for Ctenoimbricata.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Palaeontology, The Natural History Museum, London, United Kingdom.

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus