Late Cambrian and Early Ordovician trilobites from the Peruvian Altiplano and their significance to the Central Andean region of South America

Abstract

Early Paleozoic trilobites from the Umachiri Inlier of the Peruvian Altiplano, ~ 100 km northwest of Lake Titicaca, comprise two assemblages, one Cambrian and the other Ordovician. The former assemblage comes from the arkosic upper member of the recently defined Llallahue Formation and represents the oldest record of Cambrian trilobites in the Central Andean Basin. The assemblage consists of transported sclerites of aphelaspidids (Aphelaspis sp. indet. 1; Aphelaspididae gen. indet. sp. indet.) and indeterminable parabolinoidids, indicative of a Paibian–early Jiangshanian (= early Furongian) age. The Ordovician trilobites come from the lower Cunahuiri Member of the overlying Umachiri Formation and include some widespread taxa (Neseuretus Hicks, 1873; Annamitella Mansuy, 1920) that are scarcely geographically diagnostic, plus an asaphid species—Suriaspis? cf. Suriaspis trumpyi (Harrington and Kay, 1951)—that is closely related to material previously described in the Early Ordovician of Colombia. Despite the low diversity of both trilobite assemblages, the Cambrian record is comparable to early Furongian cosmopolitan taxa described primarily in Gondwana (Antarctica), Laurentia, and other regions. The scarce Ordovician specimens, recorded from siltstones and conglomerates, include forms that are more clearly Gondwanan to peri-Gondwanan. These new Cambrian and Ordovician Central Andean Basin assemblages on the Arequipa Terrane belong to separate tectonostratigraphic environments separated by a regional unconformity. The Cambrian assemblage has some affinities to Antarctic taxa that can be explained by the existence of wide back-arc basins along a continuous Terra Australis margin of Gondwana that contributed to effective dispersal of cosmopolitan taxa; in contrast, the Ordovician basin was more restricted and contained trilobites that were endemic to western Gondwana, which is consistent with brachiopod taxa reported from the same Ordovician strata.