Lower Silurian trilobites from the northern Patagonian Sierra Grande formation.

RUSTÁN, J.J.; CINGOLANI, CARLOS; SICCARDI, ARÓN; URIZ, NORBERTO

Congreso; Reunión de comunicaciones de la Asociación Paleontológica Argentina, Córdoba; 2013

Silurian-Devonian fossils from Patagonia have only been cited from the Sierra Grande Formation, a mainly siliciclastic unit cropping out to the northeastern Río Negro Province. According pioneering reports by Müller in 1965 (then revised by Manceñido and Damborenea in 1984), these poorly known records included marine faunas preserved in bluish mudstones with reddish oxides, exposed at ?Loma de los Fósiles? locality. In spite of Silurian indicators as the brachiopods Clarkeia and Heterorthella, interpretations on Devonian Malvinokaffric affinities were supported by the calmoniid trilobite Bainella and the conulariid Conularia quichua. A recent survey of this greatly overlooked site, has significantly improved the paleontological record, which is composed of brachiopods, trilobites, gastropods, bivalves, nautiloids, hyoliths, corals, crinoids, conularids, and ichnofossils. Trilobites are dominated in abundance by odontopleurids (frequently articulated and exhibiting dorso-ventral flexures), accompanied by disarticulated dalmanitids, homalonotids and, tentatively, by phacopids, proetids and acastoids. A taxonomic approach suggests the odontopleurid is probably the cosmopolitan Late Ordovician-Early Silurian odontopleurine Eoleonaspis. This assignment, for separating Eoleonaspis from its closelly allied genera Primaspis, Leonaspis and Kettneraspis, is supported by four tight-based pygidial spines between major ones, slender base of the genal spine, and occipital ring lacking spines or a single stout tubercle. In addition, there are no evidences for recognizing Bainella among available records. Thus, trilobites indicate an Early Silurian age for this stratigraphic interval, suggesting that the Afro South American paleobiogeographical context should be taking into account for interpreting Early-Middle Paleozoic paleogeographical implications for northern Patagonian basins, rather than Devonian Malvinokaffric ones.