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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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Related in: MedlinePlus

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.
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pone-0038296-g008: 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.

Mentions: The presence of a complete ring of dorsal and ventral ctenoid plates is a synapomorphy Courtessolea shares with other, more derived ctenocystoids. Courtessolea differs from more derived ctenocystoids, however, in lacking a double marginal ring of plates, and in having a bilaterally symmetrical skeletal frame. It also differs in having a narrow zone of tessellate plates between the dorsal ctenidial plates and the marginal frame plates, which we take to be homologous with the dorsal imbricate plates in Ctenoimbricata. We therefore place Courtessolea as sister group to all other ctenocystoids but more derived than Ctenoimbricata. Ctenoimbricata shares clear homologies with both ctenocystoids and cinctans as discussed above. All three have a frame of marginal ossicles encircling dorsal and ventral plated membranes. In Ctenoimbricata, Cincta and Courtessolea this frame is single, but in more derived ctenocystoids it is double and asymmetric. In Cincta the frame is also asymmetrical and extends to the posterior as a long stabilizing bar, an autapomorphy of that group. The periproct is posterior in Ctenoimbricata, Courtessolea and Ctenocystoidea, but opens through the dorsal membrane in the former two and through the marginal frame in ctenocystoids. In Cincta the periproct pierces the dorsal membrane, but is displaced to the left anterior indicating a U-shaped digestive tract. Ctenoimbricata has only spinose ctenidial elements ventrally and has a labral-like sheet of plates dorsally. Ctenoimbricata is thus more basal than Courtessolea, being either sister group to the Ctenocystoidea or sister group to all echinoderms. Figure 8 summarizes the phylogenetic position of these primitive echinoderms with respect to other deuterostomes.


Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution.

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

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.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0038296-g008: 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.
Mentions: The presence of a complete ring of dorsal and ventral ctenoid plates is a synapomorphy Courtessolea shares with other, more derived ctenocystoids. Courtessolea differs from more derived ctenocystoids, however, in lacking a double marginal ring of plates, and in having a bilaterally symmetrical skeletal frame. It also differs in having a narrow zone of tessellate plates between the dorsal ctenidial plates and the marginal frame plates, which we take to be homologous with the dorsal imbricate plates in Ctenoimbricata. We therefore place Courtessolea as sister group to all other ctenocystoids but more derived than Ctenoimbricata. Ctenoimbricata shares clear homologies with both ctenocystoids and cinctans as discussed above. All three have a frame of marginal ossicles encircling dorsal and ventral plated membranes. In Ctenoimbricata, Cincta and Courtessolea this frame is single, but in more derived ctenocystoids it is double and asymmetric. In Cincta the frame is also asymmetrical and extends to the posterior as a long stabilizing bar, an autapomorphy of that group. The periproct is posterior in Ctenoimbricata, Courtessolea and Ctenocystoidea, but opens through the dorsal membrane in the former two and through the marginal frame in ctenocystoids. In Cincta the periproct pierces the dorsal membrane, but is displaced to the left anterior indicating a U-shaped digestive tract. Ctenoimbricata has only spinose ctenidial elements ventrally and has a labral-like sheet of plates dorsally. Ctenoimbricata is thus more basal than Courtessolea, being either sister group to the Ctenocystoidea or sister group to all echinoderms. Figure 8 summarizes the phylogenetic position of these primitive echinoderms with respect to other deuterostomes.

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