A collection of conodonts, including Dapsilodus obliquicostatus
(Branson & Mehl),
Distomodus cf. kentuckyensis Branson & Branson, Ozarkodina
oldhamensis (Rexroad), and Panderodus
sp., has been obtained from brachiopod coquinas 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
Biozone, 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-bearing
interval. The brachiopods associated with the conodonts belong to the Afro-South
American
Realm, but include a high percentage of North Atlantic and cosmopolitan
genera. The conodont taxa
present are widespread in the tropical zone (Laurentia, Baltica, etc.).
This suggests that the pronounced
conodont faunal provicialism in the Upper Ordovician had disappeared in
the Lower
Silurian, where high- and low-latitude conodont faunas appear to be closely
similar.