THE EARLY RADIATION AND PHYLOGENY OF ECHINODERMS

Abstract

Summary: 1. Living echinoderms are characterized by an extensive water vascular system developed from the larval left hydrocoel, a complex, multi‐plated endoskeleton with stereom structure, and pentamery. Fossil evidence shows that stereom evolved before pentamery, but both were acquired during the Lower Cambrian.2. Cladistic analysis of Lower Cambrian genera reveals very few characters in common between carpoids and true echinoderms, and that the split between them was the first fundamental evolutionary dichotomy within the Dexiothetica.3. Helicoplacoids are stem group echinoderms with spiral plating and three ambulacra arranged radially around a lateral mouth. They are the most primitive echinoderms and the first to show a radial arrangement of the water vascular and ambulacral systems. Unlike later echinoderms, their skeleton shows no dorsal/ventral (aboral/oral) differentiation. They were probably sedentary suspension feeders.4. Camptostroma is the most primitive known pentaradiate echinoderm and, in our view, possibly a common ancestor of all living groups. It had a short conical dorsal (aboral) surface with imbricate plating, a ridged lateral wall and a slightly domed ventral (oral) surface with five curved ambulacra in a 2‐1‐2 arrangement inherited from the triradiate pattern of the helicoplacoids. Interambulacral areas bore epispires and the CD interambulacrum contained the anus, hydropore and/or gonopore. All parts of the theca had plates in at least two layers.5. All other echinoderms belong to one of two monophyletic subphyla, the Pelmatozoa and the Eleutherozoa.6. Stromatocystites is the earliest known eleutherozoan and differs from Camptostroma in having a test with only one layer of plates and having lost the dorsal elongation. In Stromatocystites the dorsal surface is flat and the plating tesselate. Stromatocystites was an unattached, low‐level suspension feeder.7. The lepidocystoids are the earliest known pelmatozoans. They differ from Camptostroma in having an attached dorsal stalk which retained the primitive imbricate plating, and by developing erect feeding structures along the ambulacra. In Kinzercystis, the ambulacra are confined to the thecal surface and erect, biserial brachioles arise alternately on either side. Lepidocystis has a similar arrangement except that, the distal part of each ambulacrum extends beyond the edge of the theca as a free arm.8. Pelmatozoans diverged more or less immediately into crinoids, with multiple free arms composed of uniserial plates, and cystoids sensu lato, which retained brachioles. Gogia (Lower to Middle Cambrian) is the most primitive known cystoid and differs from Kinzercystis principally in having all plating tesselate, while Echmatocrinus (Middle Cambrian) is the most primitive known crinoid and differs from Lepidocystis in lacking brachioles and in having more than five free arms with uniserial plates.9. Post Lower Cambrian differentiation of pelmatozoan groups proceeded rapidly, exploiting the primitive suspension‐feeding mode of life. Maximum morphological diversity was reached in the Ordovician, but thereafter crinoids progressively displaced cystoid groups and reached their peak diversity during the Carboniferous. The eleutherozoans were slower to diversify, but by the Arenig the earliest ‘sea‐stars’ (in reality, advanced members of the eleutherozoan stem group) had reversed their living orientation and had begun to exploit a deposit‐feeding mode of life. These in turn led to the ophiuroids, echinoids and holothuroids.10. The basic echinoderm ambulacrum was already present in the helicoplacoids. It had biserial, alternate flooring plates and complexly plated sheets of cover plates on either side. The radial water vessel lay in the floor of the ambulacrum, external to the body cavity, and gave rise ventrally to short, lateral branches (fore‐runners of tube feet) that were used to open the cover plate sheets, and dorsally was connected to internal compensation sacs which acted as fluid reservoirs (and were preadapted for a role in gaseous exchange). Plating on the cover plate sheets was organized and reflected the positions of the lateral branches from the radial water vessel. In Camptostroma, the cover plate sheets had biserially aligned rows of cover plates associated with the lateral branches.11. Brachioles arose by extension of the lateral branches of the radial water vessel and associated serially aligned cover plates found in Camptostroma. They bear a single alternate series of cover plates. In Lepidocystis the ambulacra extended beyond the edge of the oral surface as true arms. Brachial plates of arms are homologues of primary ambulacral flooring plates, and arms bear multiple series of cover plates. Uniserial ambulacral plating is a derived condition and evolved independently in crinoids, paracrinoids and isorophid edrioasteroids. Pinnules in crinoids arose independently in inadunates and camerates by a progressively more unequal branching of the arms. Thus all parts of the subvective system in crinoids are internally homologous, whereas in cystoids, brachioles and arms (or ambulacra) are not homologous structures.12. The position of the hydropore is the best reference point in orientating echinoderms. Carpenter's system of identifying ambulacra by letters, arranged clock‐wise in oral view with the A ambulacrum opposite the hydropore, is consistent in all echinoderm classes. In all Lower Cambrian pentaradiate echinoderms the anus, gonopore and hydropore lie in the CD interambulacrum and this is accepted as the primitive arrangement. In helicoplacoids we tentatively suggest that the A ambulacrum spiralled down from the mouth while the two ambulacra that spiralled up represent the B + C and D + E ambulacra combined.13. The pelmatozoan stem arose from a polyplated stalk, via a meric stem to a true column with holomeric (single piece) columnals. This happened independently in the crinoids and the cystoids.14. Our analysis of echinoderm phylogeny leads us to recommend the following changes to the higher level classification of echinoderms: The phylum Echinodermata includes only those groups with radial symmetry superimposed upon a fundamental larval asymmetry. It has a stem group that contains the triradiate helicoplacoids and a crown group to which all other (pentaradiate) echinoderms belong. The crown group contains two monophyletic subphyla, the Pelmatozoa and the Eleutherozoa, and the Pelmatozoa contains two superclasses, the Crinoidea which are extant and the Cystoidea, which are extinct.