The brachiopod Kvania, a biostratigraphic marker across the Cambrian-Ordovician boundary in Andean Gondwana

Abstract

Abstract. The plectorthoid brachiopod Kvania is widely distributed in Upper Cambrian and Lower Ordovician rocks of the Central Andean basin of northwestern Argentina and southwestern Bolivia. In this contribution, Kvania lariensis Benedetto is reported for the first time in the Eastern Cordillera (Quebrada de Humahuaca area) of northwestern Argentina. Studied material comes from the Alfarcito Member of the Santa Rosita Formation (Santa Victoria Group) of well constrained earliest Tremadocian age (Tr1). The presence of Kvania lariensis in different localities of the Altiplano-Puna region and the Eastern Cordillera of Bolivia and Argentina allows proposing this species as a reliable biostratigraphic marker for the lowermost Ordovician of the region. On the basis of the phylogenetic hypothesis supporting the Protorthisina-Kvania-Gondwanorthis lineage, four phylozones are recognized across the Cambrian-Ordovician boundary. The basal member of the lineage is Kvania primigenia, which co-occurs with Parabolina (Neoparabolina) frequens argentina, is proposed here as a biostratigraphic marker for the upper Furongian (Stage 10). The base of the time slice Tr1 is indicated by the first appearance of Kvania lariensis, which is almost time-equivalent to the Jujuyaspis keideli trilobite biozone, whereas the descendent species Kvania azulpampensis is almost coeval with the Kainella andina trilobite biozone. Gondwanorthis calderensis calderensis, a more derived member of the lineage, indicates the upper part of the lower Tremadocian (Tr1), and is equivalent to the Kainella meridionalis trilobite biozone. The specimens of Kvania lariensis from the Alfarcito Member provide additional information about its phenotypic variablity, confirming the trend of increasing costellae number through ontogeny observed in the type material, and also corroborate that the lineage as a whole experienced a defined evolutionary trend to increasing both shell size and costellae number.