TENTACULITES ITACURUBIENSIS n.sp., DOS DEPÓSITOS REGRESSIVOS DO PARAGUAI ORIENTAL, SILURIANO INFERIOR DA BACIA DO PARANÁ

Resumo

Paraguayan tentaculitids from the Cariay Formation, the upermost unit of the ltacurubi Group, eastern Paraguay, have been previously assigned to T. crotalinus, T. jaculus, T. stubeli, and T. trombetensis. However, on the basis of the analysis of ring ornamentation, heterotransversal rings, growth angle, ornamentation, and size of the external shell molds, it is possible to place them within a distinct new species Tentaculites intacurubiensis, Evidence for a probable late Ordovician through Early Silurian age range is provided by faunal data from the Itacurubi Group. These data include Climacograptus inootatus brasiliensis, Diplograptus modestus, Monograptus gregarius, and Arthrophycus alleghaniensis besides homology of Paraguay trilobites to Eohomalonotus, and close biogeographic affinity of the associated fauna to the Silurian Malvinokaffric Realm. The stratigraphic position of the fossiliferous beds is also consistent with the chronostratigraphy suggested by the fossil assemblage. The Paraguayan tentaculitids clearly differ from those found in Devonian strata outcropping on the eastern flank of the Parana Basin. The above mantioned morphological features of the shell closely resemble those of forms occuring worldwide in the Ordovician and Silurian. T. itacurubiensis n.sp, occurs in the Monograptus gregarius zone, of Llandoverian age. Thus, the presence of T. intacurubiensis n.sp, in the Parana Basin and T. trombetensis CLARKE, 1899 in the Amazon Basin is suggestive of the downward extension of the range of the Order Tentaculitida in South America to the Llandoverian (Early Silurian).