The genus Dalmanella (Brachiopoda) across the end-Ordovician extinction event in the Precordillera of western Argentina

BENEDETTO, J.L.; LEONE, FLORENCIA

San Luis

Jornada; Reunion Anual de Comunicaciones APA; 2017

Asociacion Paleontologica Argentina

Dalmanella testudinaria (Dalman, 1828) is one of the more typical taxa of the widespread cold/cool water Hirnantia Fauna. In the Argentine Precordillera this species was reported thirty years ago from the Hirnantian Don Braulio Formation, but this assignment was cast in doubt in a recent revision of the species. Through a detailed morphological analysis of a large collection of specimens, including average shell ratios, ribbing pattern, and internal key features, the attribution of the Precordilleran material to D. testudinaria is now confirmed. The overlying shallow-water basal sandstones of the La Chilca Formation, of well constrained Rhuddanian age (P. acuminatus?C. cyphus graptolite biozones; D. kentuckyensis conodont biozone) yielded abundant specimens of a dalmanellid which was previously referred to Dalmanella aff. testudinaria. These specimens differ from the Swedish type material of D. testudinaria mainly in their smaller size and slightly more transverse shell outline, but since in the Baltoscandian material these and other features (e.g. number of costellae) are quite variable we consider them as intraspecific variations. Thus, available evidence indicates that in the Precordillera basin D. testudinaria survived with minor changes the second pulse of the end-Ordovician mass extinction reaching at least the mid or late Rhuddanian. The only other Early Silurian record of D. testudinaria is from Tongzi, SW China, where it occurs through the N. persculptus, A. ascensus, and possibly the lower P. acuminatus biozones. In the La Chilca assemblage Hirnantia is lacking, like in China, and D. testudinaria is associated with species of Eostropheodonta, Anabaia, and Heterorthella.