INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Sumrall, C. D., Heredia, S., Rodríguez, M. C. y Mestre, A. The First Report of Edrioasteroid Echinoderms From South America.
SUMRALL, C; HEREDIA, S; RODRÍGUEZ, C; MESTRE, A
Congreso; Congreso Geológico Argentino 2008- Jujuy; 2008
Congreso Geológico Argentino
FiEdrioasteroids are a class of Paleozoic echinoderms that range from the Early Cambrian through Late Permian. Although they are known from most continents, the vast majority of taxa have been reported from North America and Europe with relatively few reports from Africa (Chauvel, 1966, 1978) and Australia (Holoway and Jell, 1983; Smith and Jell, 1990). No reports of edrioasteroids have been made from Asia - though one species is in being investigated from the Middle Cambrian Kaili Biota (Sumrall et al, in prep.) and no edrioasteroids are known from either Antarctica or South America up to now. Here we present a brief report on the first known edrioasteroid for South America from the Los Espejos Formation. This edrioasteroid belongs to an enigmatic group called Rhenopyrgids that are characterized by having a relatively small oral area placed on a long organized pedunculate zone and attached to soft substrates by a sac-like holdfast of small platelets (Holloway and Jell, 1983). These specimens were found at the upper part of Los Espejos Formation at the classical Loma de Los Piojos locality (S30º1820.7; W68º4558.8), near Jáchal (San Juan province). There, Los Espejos Formation is composed by siltstones, sandstones and several coquinoids lenses. Besides the edrioasteroid specimens, brachiopods, conodonts, trilobites, graptolites, gastropods and crinoids are common trough the middle and upper part of the section. The presence of Kokelella variabilis variabilis Walliser suggests the homonymous biozone from the Gorstian (Lower Ludlow). The edrioasteroid specimens described here are preserved as natural molds on a bedding plane of sandstone rock. Numerous specimens are randomly aligned on the surface and preserved in lateral aspect. This type of preservation is common for Rhenopyrgids preserved in siliclastic units as shown by specimens described by Bather (1915), Rievers, (1961), and Holloway and Jell (1983). The edrioasteroid examined here is a new species of the enigmatic genus Rhenopyrgus. The South American form has a small, highly domed oral surface with wide straight ambulacra and a single deltoid plate filling each of the interambulacral areas. The oral surface of the best preserved specimen comprises roughly 15 percent of the thecal height and is attached to a well organized pedunculate zone in which the highly imbricate pedunculatar plates are aligned into eight to ten columns of approximately 35 circlets of plates. Distally the pedunculate zone grades into an attachment structure plated with small irregularly positioned elements. Rhenopyrgids are small edrioasteroids with a small highly domed oral surface, an extremely long pedunculate zone and a small, multiplated holdfast. However, the morphology and evolutionary history of the group has been hampered by poor preservation, a lack of systematic treatment, and confusion between rhenopyrgids and pyrgocystids. Bather first described rhenopyrgids in a paper describing in detail the morphology of Pyrgocystis. He described the highly turreted Pyrgocystis grayae (a rhenopyrgid) from the Upper Ordovician of Girvan UK, noting that in comparison to Pyrgocystis sardensoni from the Early Ordovician Galena Group at St. Paul, MI, USA, Pyrgocystis grayae has a more domed oral surface, a longer pedunculate zone, and lacks spines on the oral surface. The comparison was made difficult by strongly different taphonomy typical polar thecal collapse is seen in P. sardensoni whereas P. grayae shows lateral flattening of an extended theca. Papers by Ruedmann (1925), Gekker (1939) and Regnéll (1945) described several rhenopyrgids from the Upper Silurian of New York, USA (in reality the Eifelian; G. A. McIntosh, pers. com.); the Middle Ordovician of the Leningrad district Russia, and Lower and Middle Silurian of Gotland, Sweden respectively. Dehm (1961) recognized the distinctness of Rhenopyrgids from Pyrgocystids and erected the subgenus Rhenopyrgus to accommodate Pyrgocystis (Rhenopyrgus) coronaeformis from the Hunsrück Slate of Germany. Holloway and Jell (1983) detailed Rhenopyrgus whitei from the Early Devonian of Victoria, Australia and recognized the distinctiveness of Rhenopyrgids by erecting a new family, Rhenopyrgidae, left unassigned to order. Smith and Jell (1990) allied rhenopyrgids with edrioblastoids whereas Guensburg and Sprinkle (1994) allied them to cyathocystids. Rhenopyrgids are a geographically widespread and temporally long-lived group of edrioasteroids. The oldest reports of rhenopyrgids are Middle and Upper Ordovician occurrences in the Leningrad Region of Russia and Great Britton (Bather, 1915; Gekker, 1939). By the Silurian Rhenopyrgids were widely distributed including the Middle Silurian of New York, USA, the lower and Middle Silurian Gotland, Sweden, and the new species from Argentina, South America (Regnéll, 1945; G. A. McIntosh, pers. com.). This pattern continues into the Devonian where occurrences are documented in the Lower Devonian of Germany, and Victoria, Australia (Rievers, 1961; Holloway and Jell, 1983), and undocumented reports from the Eifelian and Emsian of Morocco, the Eifelian of New Your State, USA (G. C. McIntoch, Pers. Com.) and the Givitian of northern Ohio, USA.