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.
Highlights
Echinoderms are the animal phylum that has departed most radically from the ancestral bilaterian body plan [1], [2]
There is uncertainty as to whether pterobranchs are sister group to enteropneusts or a derived clade nested within enteropneusts [2], [8], [9], making the ancestral body plan of hemichordates ambiguous
Clues to the origins of the echinoderm body plan organization can be gained from the ontogeny of extant taxa: an initially bilaterally symmetrical larva undergoes an asymmetric metamorphosis that involves a complete body-axis shift, eventually giving rise to a pentaradiate adult with five ambulacral areas [10], [11]
Summary
Echinoderms are the animal phylum that has departed most radically from the ancestral bilaterian body plan [1], [2]. Molecular data show that the sister group to echinoderms are the hemichordates [4,5,6,7], a clade that includes both the deposit feeding enteropneusts and suspension feeding pterobranchs This helps little in our understanding of the origin of the echinoderm body plan or the mode of life of the earliest echinoderms. The adult pentaradiality of ambulacral rays is derived from elaboration of a single larval coelom, originally one of a pair. This implies that echinoderm evolutionary history proceeded first through a bilateral and an asymmetrical phase before arriving at the ubiquitous pentaradiate morphology shown by all crown group echinoderms
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