A NEW VIEW OF RHENOPYRGID EDRIOASTEROIDS – EVIDENCE FROM THE SILURIAN OF ARGENTINA

Rhenopyrgids are an unusual, and poorly understood clade of edrioasteroids that range from the Middle Ordovician through Middle Devonian with a world-wide distribution. They are characterized by a small, globular theca dominated by wide, short, straight ambulacra, place atop an extremely long and flexible peduncular stalk. A newly discovered population of approximately 150 individuals of a new species from the Precordillera of Argentina in the upper part of the Los Espejos Formation (Lower Ludlow: Silurian) near Jáchal, Argentina provides insight into the morphology, ontogeny, and ecology of these unusual animals. Morphologically this species is similar to other described species, but differs by bearing a noted constriction between the oral surface and the pedunculate zone. This constriction is seen in Rhenopyrgus from the Devonian of Australia, but not the type species from the Devonian of Germany or from Ordovician material from England. This constriction bears disorganized and highly imbricate plates of much different character to the well organized plates of the pedunculate zone. The collection includes a well sampled ontogeny with specimens between 6 and 30 mm in thecal height. Juvenile specimens have a proportionately shorter and wider pedunculate zone and more conical oral surface. Lengthening of the pedunculate zone is accomplished by the addition of peduncular plate circlets probably at the base. However, the organization of the pedunculate zone into eight alternating columns is invariant in the material. Of the approximately 150 specimens in the collection, none show the attachment structure affixed to skeletal debris suggesting attachment in loose sediment as suggested in previous studies. The presence of numerous specimens with the highly imbricate peduncular stalk evenly curved in the basal region but little variation in its preserved length suggests that it is used for reorientation of the oral surface rather than for telescoping. On the largest slab, specimens do not show current alignment. The construction of the oral surface with enlarged interradial plates and simple ambulacral cover plates, coupled with the elongate imbricate stalk strongly suggests edrioasterid/edrioblastoid affinities.