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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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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.
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pone-0038296-g001: 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.

Mentions: Fortunately, echinoderms have left behind an excellent fossil record that illuminates some of the initial steps involved in the assembly of their unique body plan [12]. Fossil echinoderms from the Cambrian include both radiate and asymmetric forms [13], [14] (Figure 1). Their identity as total group echinoderms is in no doubt because all possess a skeleton composed of stereom, an autapomorphy for the clade [15]. The radiate forms (e.g. helicoplacoids, stromatocystitids, gogiids) have between two and five primary ambulacral rays. Because only a single asymmetrically-placed hydropore is ever present in these forms and their ambulacral construction is closely similar, we deduce that their water vascular system must be like that of extant echinoderms and elaborated from a single coelom. Therefore these are derived morphologies that provide little help in understanding the pre-radial history of echinoderms, although they do reveal the great range of subsequent diversification that took place once radiality had been achieved [16]. More interesting are the echinoderms that show no evidence of radiality and that have long been interpreted as more primitive [12], [17]–[19]. These include forms that have a single asymmetrically positioned ambulacral ray and hydropore (solutes and, according to some interpretations, e.g. [20], stylophoroans), and those with asymmetrically paired marginal grooves and an anterolateral mouth (cintans) [21]. Possibly most basal of all are the weakly asymmetric ctenocystoids [22]. These are almost bilateral in their organization but are constructed with differing numbers of marginal plates on left- and right-hand sides of the body, especially evident in the ventral marginal ring, e.g. [23]. Surprisingly, despite both phylogenetic relationships and the larval development of extant echinoderms pointing to a bilateral ancestry for echinoderms, there has, until now, been no convincing fossil record of this evolutionary stage.


Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution.

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

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.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3368939&req=5

pone-0038296-g001: 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.
Mentions: Fortunately, echinoderms have left behind an excellent fossil record that illuminates some of the initial steps involved in the assembly of their unique body plan [12]. Fossil echinoderms from the Cambrian include both radiate and asymmetric forms [13], [14] (Figure 1). Their identity as total group echinoderms is in no doubt because all possess a skeleton composed of stereom, an autapomorphy for the clade [15]. The radiate forms (e.g. helicoplacoids, stromatocystitids, gogiids) have between two and five primary ambulacral rays. Because only a single asymmetrically-placed hydropore is ever present in these forms and their ambulacral construction is closely similar, we deduce that their water vascular system must be like that of extant echinoderms and elaborated from a single coelom. Therefore these are derived morphologies that provide little help in understanding the pre-radial history of echinoderms, although they do reveal the great range of subsequent diversification that took place once radiality had been achieved [16]. More interesting are the echinoderms that show no evidence of radiality and that have long been interpreted as more primitive [12], [17]–[19]. These include forms that have a single asymmetrically positioned ambulacral ray and hydropore (solutes and, according to some interpretations, e.g. [20], stylophoroans), and those with asymmetrically paired marginal grooves and an anterolateral mouth (cintans) [21]. Possibly most basal of all are the weakly asymmetric ctenocystoids [22]. These are almost bilateral in their organization but are constructed with differing numbers of marginal plates on left- and right-hand sides of the body, especially evident in the ventral marginal ring, e.g. [23]. Surprisingly, despite both phylogenetic relationships and the larval development of extant echinoderms pointing to a bilateral ancestry for echinoderms, there has, until now, been no convincing fossil record of this evolutionary stage.

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