Trilobites de la Formación Volcancito (miembro Filo Azul, Cámbri, co Tardío) El Sistema de Famatina, la Rioja, Argentina

TORTELLO, M.F.; ESTEBAN, SUSANA BEATRIZ

AMEGHINIANA

Asociación Paleontológica Argentina

Lugar: Buenos Aires; Año: 2007 p. 597 - 620

0002-7014

  The Filo Azul Member of the Volcancito Formation (Famatina range, La Rioja, Argentina) consists of a lower part (30 m) of thinly laminated marls and calcareous shales with intercalations of fine-grained sandstones, a middle part (85 m) of massive middle to thick-grained calcareous sandstones, and an upper part (50 m) of shales with interbedded layers of fine sandstones. Previous studies on trilobites, graptolites and conodonts assigned the section to the Upper Cambrian-Lower Ordovician. The lower portion of the member (Parabolinafrequensargentina Biozone, lower part) contains a diverse trilobite fauna that was studied by Harrington and Leanza in the 1950s. Since then, new collections have been obtained, which are the subject of the present study. New polymerid trilobites, originally described on the basis of scarce material, are re-illustrated (Onychopygeriojana Harrington, Asaphellusriojanus Harrington and Leanza, Rhadinopleuraeurycephala Harrington and Leanza, Plicatolinascalpta Harrington and Leanza) and the assignations of several taxa are reconsidered (Angelina cf. hyeronimi (Kayser), Parabolinella sp. 1, Parabolinella sp. 2, Pseudokainellatoselliisp. nov., Conophrys sp.) (since the agnostoids have been recently revised, they are not included in this study). The fauna comprises several endemic species. On the other hand, the occurrence of Parabolina(Neoparabolina)frequens (Barrande) permits the outcrop to be correlated with other Upper Cambrian sections. The abundance of nutrients in the sea bottom must have favoured the development of a diverse trilobite community, which included different morphotypes and feeding habits. Several taxa were especially adapted to live under low-oxygen conditions, probably in symbiosis with sulfur bacteria.