Class Ophiuroidea Research Articles

Equinodermos (Echinodermata) del Pacífico Mexicano

Echinoderms (Echinodermata) from the Mexican Pacific. A systematic list of echinoderms of the Mexican Pacific, based on museum specimens of the Coleccion Nacional de Equinodermos, Instituto de Ciencias del Mar y Limnologia, Universidad Nacional Autonoma de Mexico and the National Museum of Natural History, Smithsonian Institution, Washington, D.C., is presented. A total of 196 echinoderm species is recorded, distributed in 112 genera, 56 families and 20 orders. Eight new records for the Mexican Pacific are presented: one for Class Crinoidea: Hyocrinus foelli; six for Class Asteroidea: Echinaster (Echinaster) parvispinus, Henricia nana, Henricia seminudus, Rathbunaster californicus and Leptasterias pusilla, and one for Class Ophiuroidea: Amphiodia tabogae. Rev. Biol. Trop. 56 (Suppl. 3): 57-73. Epub 2009 January 05.

Skeletal homologies, phylogeny and classification of the earliest asterozoan echinoderms

Synopsis A thorough reappraisal of extant and fossil asterozoan plate systems is used to construct working hypotheses of primary plate homologies for Ordovician Asterozoa. Results indicate that inferomarginal ossicles of asteroids, somasteroids and the primitive ophiuroid Phragmactis are likely to be homologous with edrioasteroid marginals. The intermediate virgals and coverplates of somasteroids are identified as modified actinals and the sub‐laterals of certain stenurids are axial, here termed podial plates, with laterals in these taxa homologous to true lateral plates and adambulacrals of ophiuroids and asteroids, respectively. The nature of the asterozoan mouth frame is reinvestigated as a result of new study of the mouth frames of key taxa, with a reappraisal of existing developmental and symmetry data. It is argued that primitive asterozoan mouth angle plates were entirely ambulacral and probably not spine‐bearing, but that mouth spines evolved in asteroids and ophiuroids either on the existing first ambulacrals or by the superposition of first adambulacrals adoral to first ambulacrals. These new data are used to undertake a cladistic analysis of all known genera of Ordovician asterozoan echinoderms (38 taxa), based on a dataset of 150 morphological characters, rooted on the Middle Cambrian edrioasteroid taxa Stromatocystites and Cambraster. The resulting tree is fully resolved although some deeper branches show poor support. It reveals that Ordovician asteroids form a monophyletic group (defined by the presence of an unpaired axillary inferomarginal, opposing ambulacrals and blocky abutting adambulacrals) and are a sister group to the remainingasterozoans, including the somasteroids. Somasteroids are monophyletic and, with the primitive taxon Phragmactis, are a sister group to the remaining ophiuroids. The class Ophiuroidea is diagnosed by the near‐simultaneous appearance of apomorphies including an interradial disc with highly separated arms and aboral and oral interambulacral longitudinal musculature. A phylogenetic tree constructed for asterozoans based on current stratigraphic occurrence data indicates that at the time of their first appearance, asteroids and ophiuroid body plans were already established. This suggests a significant but unobserved pre‐Ordovician history. Comparison of observed and phylogenetic diversity together with the appearance of relatively young taxa close to the base of the tree point to a poor asterozoan record, particularly in the Lower Ordovician. A revised classification for stem‐group asterozoans is proposed based on well‐supported clades with the new taxon Eopentaroida erected. Mapping of character states on the most parsimonious solution suggests that whilst asteroid morphological evolution was conservative in the Ordovician, ophiuroids underwent a profound morphological diversification. Functionally, these changes are linked to the adoption of carnivory by derived Ordovician ophiuroids. In contrast, asteroids probably remained dominantly deposit‐feeders throughout the period. Character distribution on the cladogram refutes none of the hypotheses of primary plate homology, although it suggests that podial ossicles were developed in two clades independently and that the marginals of eopentaroids may not be homologous to those of other asterozoans and edrioasteroids.

Ophiuroid phylogeny and higher taxonomy: morphological, molecular and palaeontological perspectives

A cladistic analysis of the class Ophiuroidea at subfamily level is presented based on a morphological data base of 43 characters. The tree is rooted using the best known fossil member of the stem-group. This identifies Ophiocanopidae as sister group to other extant ophiuroids, which themselves fall into two major groups, the Ophiurida and the Euryalida. Although our most parsimonious solution is highly resolved, not all parts of the cladogram are well supported. Molecular data from the 5′ end of the large subunit ribosomal RNA molecule are presented for 10 taxa, with representatives from major higher groupings. This provides some support for the morphological tree, but leaves most of the deep branches unresolved. A phylogenetic tree is constructed for ophiuroids by selecting one of the cladograms as our working hypothesis and calibrating this against the stratigraphic record of the group. A revised taxonomy for the group is presented and the new taxon Ophiodermatina is erected.

Ophiuroid phylogeny and higher taxonomy: morphological, molecular and palaeontological perspectives

A cladistic analysis of the class Ophiuroidea at subfamily level is presented based on a morphological data base of 43 characters. The tree is rooted using the best known fossil member of the stem-group. This identifies Ophiocanopidae as sister group to other extant ophiuroids, which themselves fall into two major groups, the Ophiurida and the Euryalida. Although our most parsimonious solution is highly resolved, not all parts of the cladogram are well supported. Molecular data from the 5′ end of the large subunit ribosomal RNA molecule are presented for 10 taxa, with representatives from major higher groupings. This provides some support for the morphological tree, but leaves most of the deep branches unresolved. A phylogenetic tree is constructed for ophiuroids by selecting one of the cladograms as our working hypothesis and calibrating this against the stratigraphic record of the group. A revised taxonomy for the group is presented and the new taxon Ophiodermatina is erected.

The giant neurone system in Ophiuroids. II. The hyponeural motor tracts.

The radial nerve cords of members of the class Ophiuroidea consist of two parts, the ectoneural and the hyponeural tissues, which are separated by an acellular basal lamina. The hyponeural tissue is composed entirely of motor fibres. The cell bodies of the hyponeural neurones are arranged in ganglia, one to each segment of the arm, and each containing approximately one hundred cell bodies. Synaptic contact between the two tissues occurs across the basal lamina. Ultrastructural evidence shows that the majority of these synapses operate in the ectoneural to hyponeural direction. Three pairs of nerve bundles, each containing approximately thirty five large motor fibres arise from each ganglion and innervate the intervertebral muscles. The large motor fibres divide into a number of pre-terminal axons in the region in which the motor fibre enters the muscle block. The terminal axons run at right-angles across the muscle fibres and neuromuscular junctions are found at the points of contact between the two; each terminal axon makes contact with a large number of muscle fibres. The hyponeural axons also pass through the juxtaligamental tissue before they reach the muscle blocks and there is some evidence of synaptic contact with the juxtaligamental cells. The juxtaligamental tissue is thought to be associated with changes in the structural properties of the collagenous ligaments of the arm during arm autotomy (Wilkie 1979). Degeneration studies confirmed the layout of the hyponeural motor axons.