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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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Reconstruction of Ctenoimbricata spinosa gen. et sp. nov.
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pone-0038296-g005: Reconstruction of Ctenoimbricata spinosa gen. et sp. nov.

Mentions: Ctenoimbricata is a small (20 mm), disc-like animal with a clearly defined anterior–posterior axis and with skeletal elements arranged bilaterally symmetrically along that axis (Figures 3, 4, 5, Movies S1, S2). A uniserial marginal ring of stout plates frames the body, comprising four elements at the anterior forming part of the ctenidium (Fig. 4E, plates M0, M1), four on either side (plates M2–M5) and a single posterior element (plate Mp). Dorsal and ventral plated membranes cover the centre of the disc. At the anterior, there is a wide opening framed by marginal plates and covered dorsally by a sheet of imbricate plates. This dorsal roof is formed of several superimposed series of thin, flat plates that imbricate to the posterior. A row of very small spinose plates forms the outermost dorsal row. The dorsal ctenidium formed a single unit with limited flexibility. Ventrally, the opening is lined anteriorly by 14 spinose elements. The four median ones are anterior extensions of marginal frame plates M0 and M1. The remaining 10 are free elements that articulate with the outer edge of marginal plates. Distally, these plates taper, becoming knife-like, and they overlap from posterior to anterior. The periproct is not seen but certainly does not pass through the marginal ring, as this is unbroken. It must therefore be situated in the dorsal membrane, and the only part of that structure missing from our specimen is the very posterior. By comparison with Courtessolea (see below), therefore, we conclude that the periproct must have opened in the posterior part of the dorsal membrane, close to plate Mp.


Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution.

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

Reconstruction of Ctenoimbricata spinosa gen. et sp. nov.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0038296-g005: Reconstruction of Ctenoimbricata spinosa gen. et sp. nov.
Mentions: Ctenoimbricata is a small (20 mm), disc-like animal with a clearly defined anterior–posterior axis and with skeletal elements arranged bilaterally symmetrically along that axis (Figures 3, 4, 5, Movies S1, S2). A uniserial marginal ring of stout plates frames the body, comprising four elements at the anterior forming part of the ctenidium (Fig. 4E, plates M0, M1), four on either side (plates M2–M5) and a single posterior element (plate Mp). Dorsal and ventral plated membranes cover the centre of the disc. At the anterior, there is a wide opening framed by marginal plates and covered dorsally by a sheet of imbricate plates. This dorsal roof is formed of several superimposed series of thin, flat plates that imbricate to the posterior. A row of very small spinose plates forms the outermost dorsal row. The dorsal ctenidium formed a single unit with limited flexibility. Ventrally, the opening is lined anteriorly by 14 spinose elements. The four median ones are anterior extensions of marginal frame plates M0 and M1. The remaining 10 are free elements that articulate with the outer edge of marginal plates. Distally, these plates taper, becoming knife-like, and they overlap from posterior to anterior. The periproct is not seen but certainly does not pass through the marginal ring, as this is unbroken. It must therefore be situated in the dorsal membrane, and the only part of that structure missing from our specimen is the very posterior. By comparison with Courtessolea (see below), therefore, we conclude that the periproct must have opened in the posterior part of the dorsal membrane, close to plate Mp.

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