New data on the Early Ordovician stemmed echinoderm fauna from Argentina

SUMRALL, C.; CARRERA, M.; VACCARI, N.E.

Mendoza

Congreso; 4thInternational Palaeontological Congress: The History of Life: A view from the Southern Hemisphere.; 2014

International Palaeontological Association

Two new echinoderm faunas have recently been collected from Lower Ordovician rocks of western Argentina. These include the Lower Ordovician limestones of San Juan Formation (Tremadocian-Darriwilian), San Juan Province and the siliciclastic Suri Formation (Floian- Dapingian) in Famatina Basin, La Rioja and Catamarca Provinces. The San Juan fauna includes several species of glyptocystitoid rhombiferans, some isolated blastozoan columnals and a few crinoid stems and holdfasts. The glyptocystitoid material though incompletely preserved includes three distinct cheirocrinid taxa. The first species bears a large cylindrical theca with numerous disjunct dichopores. The summit is poorly documented, but the ambulacra are confined to the thecal summit. These specimens bear typical cheirocrinid stems that taper distally. The second species known only from a few isolate plates is a cheirocrinid with large numbers of confluient dichopores. These plates are poorly documented but include radial plates demonstrating that the ambulacra are confined to the thecal summit. The final species is known from a few isolate plates from one locality. These plates bear multidisjunct dichopores, a feature known from only a few taxa. Non-glyptocystitoid blastozoan material and crinoid material is fragmentary and includes isolated columnals and holdfast structures. The Suri Formation, Lower Ordovician (Floian-Dapingian) at the Río Cachiyuyo locality, La Rioja and Chaschuil locality, Catamarca, has produced echinoderms assignable to several echinoderm clades. A single, partial glyptocystitid rhombiferan assignable to a new species of Macrocystella is preserved including a nearly complete stem and thecal base. Although the plating cannot be fully documented, the presence of corrugated plate surfaces that lack any evidence of pectinirhombs is consistent with assignment to Macrocystella. Ornamentation differentiates this species from other Macrocystella plates described from South America. Also present, in the Chaschuil area, are several specimens of large diploporitans with elongate globular thecae that conform to the Perigondwanan taxon Ascocystites. Specimens are preserved as internal molds and do not provide detailed information about the oral area, appendages and periproctal region. Finally a few specimens of thin, isolated thecal plates assignable to Blastozoa are also present. While the Suri Formation has taxa that have Perigondwannan affinities, the glyptocystitoid fauna from the San Juan Formation has taxa that are common members of Laurentian faunas consistent with data recovered for other invertebrate groups.