A Middle Ordovician (Darriwilian) Dysaerobic Brachiopod Assemblage from the Precordillera Terrane of Argentina: Implications for Early Colonization of Deep Waters

Abstract

A rapid increase of water depth associated with dysoxic bottom conditions took place in the central part of the Precordillera basin during the middle Darriwilian. The peak of transgression and drowning of the carbonate ramp are marked by the transition from the uppermost carbonate beds of the San Juan Formation to a rhythmic alternation of graptolitic black shales and marlstones (base of the Los Azules Formation). This lithofacial change is associated to a drastic faunal shift involving a renewal of the ecologic structure. The carbonate facies dominated by filter-feeding organisms were rapidly replaced by new communities characterized by benthic and pelagic trilobites and low-richness assemblages of minute brachiopods. Two main associations are recognized in the Los Azules Formation: a basal ephemeral Sivorthis-Mendolaspis Association and a more persistent Chilcorthis-Palaeoglossa Association inhabiting deeper and more dysoxic sea-bottoms. The new plectorthoid family Chilcorthidae is established to include the new genus Chilcorthis. Five new species are described:

Middle Ordovician (Darriwilian) linguliform and craniiform brachiopods from the Precordillera (Cuyania Terrane) of west-central Argentina

“…As Popov et al (2013) pointed out, the Middle Ordovician is the best known interval in terms of the biogeographical distribution of lingulate brachiopods. Besides the fauna studied herein, the only other assemblage described from the Precordillera Basin comes from the dysoxic black shales and marls of the Los Azules Formation, of lower-mid Darriwilian age (Benedetto, 2015). It consists of the obolids Palaeoglossa minima Benedetto, 2015, and Anomaloglossa ?…”

“…A rapid sea level rise associated with tectonic activity led to a diachronous drowning of the carbonate ramp. In the central part of the basin, this event took place around the Dapingian/ Darriwilian transition, leading to deposition of a rhythmic alternation of graptolitic black shales and marlstones named Los Azules Formation (Astini, 2003; Carrera et al, 2013; Benedetto, 2015).…”

“…Among brachiopods, most work has been directed toward the study of rhynchonelliforms (Herrera and Benedetto, 1989; Benedetto and Herrera, 1993; Benedetto, 2001, 2007; Benedetto et al, 2003; Benedetto and Cech, 2006; among others), with little attention being paid to the linguliforms. Recently, Benedetto (2015) described a Middle Ordovician brachiopod assemblage from the black shales and marls (Los Azules Formation) overlying the San Juan Formation (Fig. 2), which includes minute rhynchonelliforms, obolids, acrotretids, and the craniid Philhedra .…”

BroeggeriaWalcott and other upper Cambrian and Tremadocian linguloid brachiopods from NW Argentina

“…For instance, Palaeoglossa minima Benedetto integrates together with Anomaloglossa ? and Cyrtonotreta , a deep‐water dysaerobic community in Darriwilian black shales of the Precordillera of western Argentina (Benedetto, ).…”

“…The Australian P. yarrimbahensis Percival and Engelbretsen, , clearly differs from P. elongata in its elongate ovoid shell outline, much shorter ventral pseudointerarea, and absence of dorsal median septum. The only other record of Palaeoglossa in Argentina is P. minima Benedetto, , from Darriwilian black shales of the Precordillera basin. The Precordilleran species can be distinguished from P. elongata in having considerably smaller shell (mean length 4.7 mm), less conspicuous concentric ornament, slightly shorter ventral pseudointerarea, parallel‐sided pedicle groove and smaller ventral umbonal muscle scars.…”