New Silurian craniopsids (Brachiopoda, Craniiformea) from the Precordillera basin of western Argentina and their associated faunas

Abstract

The craniopsid brachiopod Craniops is reported for the first time from the Silurian of the Precordillera basin of West-Central Argentina. It is represented by two new species, C. speculum, from the lower part of the Los Espejos Formation (lower Ludlow), and C. brussai, from the upper part of the same formation (Ludfordian-Pridoli). The fauna comes from laminated mudstones and bioturbated siltstones indicating outer-shelf deposition below storm base. Craniops species are associated mostly with ostracods, chonetids, the small rhynchonellid Harringtonina australis, and tentaculitid shells. Ostracods are characterized by the abundance of dimorphic palaeocopes, Hemsiella, and Zorotoxotis (Beyrichioidea), and to a lesser extent, by the binodicope Australobollia together with Aechmina in the lower level, and Bollia and Petrisigmopsis in the upper one. Larvae of the new Precordilleran species of Craniops probably settled on living or death brachiopod and/or tentaculitid shells, as the presence of a relatively large attachment cicatrix indicates, and as soon become detached they rested on the floor near their supporters forming well defined clusters. The two new species of Craniops described herein are associated with typical Afro-South American taxa (e.g. Australina, Clarkeia, Harringtonina, Castellaroina, Amosina). Setting aside the probable craniopsid described by Clarke (1899) from Brazil, this is the sole record of Craniops in such biogeographic entity. By the Wenlock-Ludlow the genus spreads into the relatively higher latitude Bohemian region, and by the Ludlow-Pridolian reached the cold waters of the Andean Gondwana Precordillera basin.