NEW UPPER CAMBRIAN-TREMADOC RHYNCHONELLIFORMEAN BRACHIOPODS FROM NORTHWESTERN ARGENTINA: EVOLUTIONARY TRENDS AND EARLY DIVERSIFICATION OF PLECTORTHOIDEANS IN THE ANDEAN GONDWANA

Journal of Paleontology, Mar 2007 by Benedetto, Juan L

ABSTRACT-

New rhynchonelliformean brachiopods are documented from Upper Cambrian and Tremadoc beds of the Cordillera Oriental and Puna regions, which are representative of the initial radiation of Plectorthoidea on the Andean (South American) shelves of Gondwana. Protorthisina n. gen., so far the oldest known representative of the family Euorthisinidae, and Lesserorthis n. gen. (Euorthisinidae?) are described, as well as the new species Euorthisina? nazarenensis, Kvania mergli, Kvania lariensis, Kvania azulpampensis (with two subspecies), Kvania? primigenia, and Nanorthis calderensis (with two subspecies). Protorthisina, Kvania, and Nanorthis form a stratigraphically continuous series of species displaying well-defined evolutionary trends involving size, shell ornament, and internal features. In the cardinalia, a transition from a V-shaped septalium (typical of Protorthisina) through almost parallel brachiophore plates (typical of Kvania) to the orthoid notothyrial platform of Nanorthis is documented. The species Nanorthis purmamarcaensis Benedetto and "Nanorthis" grandis (Harrington) are considered as the end members of the lineage. The heterochronic (peramorphic) origin of this trend is supported by ontogenetic evidence from species of Kvania and Nanorthis calderensis. The origin of the 'plectorthoid' cardinalia of Kvania from a euorthisinid configuration present in the stem group, followed by the appearance of a orthoidlike configuration in Nanorthis, pose an intricate systematic problem. The preliminary phylogenetic analysis of the northwestern basin taxa shows that the Upper Cambrian Protorthisina simplex n. gen. and sp. is the stem group of both the nanorthid and euorthisinid clades. On the basis of the new evidence, the genus Nanorthis is removed from the Orthoidea to the Plectorthoidea, and the diagnosis of the families Euorthisinidae and Nanorthidae are revised.

INTRODUCTION

UNLIKE OTHER Gondwanan basins, the stratigraphic record in the Central Andean basin of northwestern Argentina and Bolivia is continuous from the Upper Cambrian to the mid-Arenig. These successions, dominated by shelfal clastic deposits, often over 3,500 m thick, are unique in their abundance of fossils-especially trilobites, brachiopods, and bivalves-and form an excellent basis for studying the early stages of brachiopod diversification on the temperate to cold water platforms of western Gondwana (Benedetto, 1998; Benedetto and Sánchez, 2003; Sánchez and Benedetto, 2004). Recent field work in the Upper Cambrian-lower Tremadoc beds of northwestern Argentina has yielded a suite of small, well-preserved orthidines which constitute the oldest record of rhynchonelliform brachiopods in the Central Andean basin. They include the new genera Protorthisina and Lesserorthis and new species of Kvania Havlicek, 1994 and Nanorthis Ulrich and Cooper, 1936. All these genera are representative of the initial orthide radiation in western Gondwana. They may be ascribed to the so-called 'Dalmanella-shaped small brachiopods' (Havlícek, 1994), which include the Early Ordovician orthids Renorthis Öpik 1939, Nothorthis Ulrich and Cooper 1938, Nanorthis, and the dalmanellid Nocturnellia Havlicek, 1950. These taxa are thought to lie at the base of the differentiation of a number of orthide lineages such as plectorthids, giraldiellids, nanorthids, ranorthids and draboviids (Havlícek, 1977, 1994). Apart from draboviids, the other families are well represented in the Lower Ordovician beds of western Argentina. The genera Nothorthis and Ranorthis, for instance, are abundant in the upper Tremadoc-lower Arenig assemblages of the Precordillera terrane, and available evidence suggests that both taxa are phylogenetically related (Benedetto, 2001, 2002b; Benedetto et al., 2003).

The purpose of this paper is, firstly, to document thoroughly new taxa from Upper Cambrian and Tremadoc beds of the Cordillera Oriental and western Puna region, which include the oldest known representatives of the families Euorthisinidae and Nanorthidae. The second objective is to explore the role played by heterochrony in directing morphological changes documented in the Protorthisina-Nanorthis lineage. Finally, the phylogenetic relationships between the early plectorthoideans from northwestern Argentina and related taxa were evaluated on the basis of morphologic and stratigraphic evidence. As will be shown, the taxa described in this work are phylogenetically related and demonstrate the initial radiation of plectorthoidean brachiopods in the proto-Andean (South American) shelves of Gondwana. Likewise, the inferred phylogenetic links of Nanorthis species with the basal plectorthoideans provide an opportunity to clarify the origin and systematic placement of nanorthids among the suborder Orthidina.

FOSSILIFEROUS LOCALITIES, STRATIGRAPHY, AND AGE

The brachiopods described in this paper come from the lower part of the Santa Victoria Group exposed in the Cordillera Oriental, and from the volcanic-sedimentary Las Vicuñas Formation exposed in the western Puna region (Fig. 1). The Santa Victoria Group unconformably overlies the Cambrian Meson Group, which consists of Skolithos-rich reddish sandstones and siltstones deposited in tide-dominated environments (Mángano and Buatois, 1999). The lowermost unit of the Santa Victoria Group is a 20-30 m thick sandstone interval with large scale cross bedding named the Padrioc Formation. It is conformably overlain by the Lampazar Formation, which is formed of fossiliferous black shales and mudstones interbedded with thin layers of fine-grained calcareous sandstones bearing trilobites and scattered brachiopods. Trilobites belong to the Parabolina (Neoparabolina) frequens argentina Zone ( = Parabolina argentina Zone defined by Harrington and Leanza, 1957). Originally, this biozone was considered early Tremadoc in age, but later its lower half was referred to the Upper Cambrian on the basis of the trilobite association recovered from the Lampazar Formation at Sierra de Cajas (Benedetto, 1977). Subsequent studies on conodonts led to the assignment of the entire Lampazar Formation to the Upper Cambrian. In the Sierra de Cajas sections, for instance, Rao and Hunicken (1995a, 1995b) recognized the C. proavus (H. hirsutus Subzone), C. caboti, and C. intermedius zones, while conodonts recovered from the upper part of the Lampazar Formation at Angosto del Moreno indicate the C. proavus Zone (Moya and Albanesi, 2000). Tortello (2003) and Tortello and Esteban (2003) described a varied olenid-agnostoid fauna from the same formation at Sierra de Cajas and recognized the PseudorhaptagnostusGymnagnostus Subzone, which spans the lower half of the Parabolina (Neoparabolina) frequens argentina Zone.