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Echinoderm Symmetry

Echinoderms have always been noted for having a five-part symmetry to their bodies, which is most evident in the rays. However, this is a gross oversimplification to a much more complex and interesting body design. Echinoderms belong to a large group of animals called bilaterians and as such bear a basic bilateral symmetry - that is they have a right and left side. This bilateral symmetry, in turn, is overprinted by three-part symmetry, that in turn, is overprinted by five-part symmetry. This results in two rays branching from the right and left sides of the mouth and a single ray branching from the top of the mouth. This pattern, called the 2-1-2 symmetry, is primitive for echinoderms and is the common condition found in most groups. Because of bilateral symmetry the rays can be designated A through E starting at the anterior ray and proceeding clockwise.

Cooperidiscus from the Devonian of New York showing the 2-1-2 symmetry. Three rays leave the centrally located mouth – A, shared BC, and shared DE. The B, C, D, and E rays form by splitting the shared rays.

Where early growth stages have been documented, most echinoderms pass through a bilateral, triradial, and pentaradial stage. In edrioasteroids, the two shared rays develop first, forming bilateral symmetry, the anterior, or A ray, develops next forming the triradial symmetry, and last the shared rays divide forming the adult morphology with five rays.

The growth stages of the edrioasteroid Neoisorophusella lanei. 1) In very small echinoderms only two rays are present the shared BC ray on the right and the shared DE ray on the left. 2) In slightly larger individuals, the lateral rays split and the A ray develops. 3) With maturity, the distal A through E rays continue to grow. This results in three points during growth where ray development can be terminated i.e. formation of the lateral BC and DE rays, splitting of the laterally shared rays, and formation is the A ray. Echinoderms with aberrant ray counts result from truncations of growth at some combination of these growth stages.

My research in this area has concentrated on the reduction of ray count. It is now clear that these three stages of ray development - formation of the shared rays, formation of the A ray, and splitting of the shared rays - are tied to the reduction in ray count and symmetry via paedomorphism. In forms with five rays, growth proceeds as described above. In forms with four rays, however, the shared rays form and later split, but the A ray fails to develop. This can account for all four-rayed echinoderms seen in the fossil record. Forms with three rays grow the shared rays and the a ray, but the shared rays never split. Forms with two rays grow the shared rays, but the a ray fails to form and the shared rays fail to split resulting in mature individuals retaining only the shared rays. Forms with a single ray fail to grow the shared rays, but form the A ray. Since the shared rays never form, the do not split resulting in adults with a single A ray. Two other symmetry types also exist. Pseudo-five-fold symmetry results in a nearly complete loss of the shared rays and true five-fold symmetry results from a complete loss of all vestiges of the 2-1-2 symmetry. This is the rarest form of symmetry, but oddly is the form most prevalent among modern . These ideas are currently being written up with Dr. Gregory Wray at Duke University.

CS

Colin Sumrall

Department of Earth and Planetary Sciences
1412 Circle Drive
Knoxville, TN 37996-1410
Phone: (865) 974-0400
Email: csumrall@utk.edu


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