First record of Lower Silurian conodonts from South America: biostratigraphic and palaeobiogeographic implications of Llandovery conodonts in the Precordillera of Argentina

A collection of conodonts, including Dapsilodus obliquicostatus (Branson & Mehl), Distomodus cf. kentuckyensis Branson & Branson, Ozarkodina oldhamens is (Rexroad), and Panderodus sp., has been obtained from brachiopod coquina s in the lower part of the La Chilca Formation in the Precordillera of west -central Argentina. This conodont fauna, the first to be recorded from the Llandovery in South America, can be referred to the D. kentuckyensis Biozon e, and is interpreted to represent an interval in the middle Rhuddanian to lower Aeronian stages. This age assignment is supported by graptolite data from the lower third of the La Chilca Formation beneath the conodont-bearin g interval. The brachiopods associated with the conodonts belong to the Afr o-South American Realm, but include a high percentage of North Atlantic and cosmopolitan genera. The conodont taxa present are widespread in the tropi cal zone (Laurentia, Baltica, etc.). This suggests that the pronounced cono dont faunal provicialism in the Upper Ordovician had disappeared in the Low er Silurian, where high- and low-latitude conodont faunas appear to be clos ely similar.