Benedetto—The strophomenide brachiopod Ahtiella in Gondwana
Famatina Range provides an invaluable frame to establish well- resolved phylogenies based on the fossil record. In this paper, evidence is presented suggesting that Ahtiella originated from the hesperonomiid orthoid Monorthis transversa Benedetto, 2003b, which always occurs in strata below those bearing Ahtiella famatiniana new species (described herein). A general trend of morphological change through time emerges from the compara- tive morphology of Ahtiella species and its putative ancestor Monorthis. Relevant for our phylogenetic hypothesis is the fact that the earliest species of Ahtiella recorded in Gondwana exhibit transitional characteristics between Monorthis and the typical Ahtiella species from younger strata of Cuyania and Baltica. Finally, a phylogenetic analysis is presented herein to investigate the evolutionary relationships among the Gondwanan species of Ahtiella and those from Cuyania, Baltica, and Avalonia.
Stratigraphic provenance and age
The early Paleozoic geodynamic history of southern South America involved three main sedimentary domains (Fig. 1), which were inhabited at different times by species of Ahtiella. They are: (1) the autochthonous Central Andean Basin devel- oped around the Brazilian craton through Peru, Bolivia, and northwestern Argentina; (2) the volcanosedimentary Famatina Basin, which together with the Puna volcanic arc developed peripheral to the active pre-Andean Gondwana margin; and (3) the Precordillera Basin developed on the Laurentian-derived Cuyania terrane, which accreted to Gondwana during the early Paleozoic (for a comprehensive review of the Ordovician basins of Argentina, see Astini, 2003).
Central Andean Basin.—The southern part of the large Central Andean Basin is widely exposed in the Cordillera Oriental of northwestern Argentina, where the uppermost Cambrian to Lower Ordovician siliciclastic succession of the Santa Victoria Group attains 3,800m in thickness. In the study area of Los Colorados (Fig. 2), it is overlain by a succession of reddish purple sandstones named the Alto del Cóndor Formation, which is succeeded by fossiliferous greenish mudstones and marls. This interval, which was referred to as the ‘Green Member of the Sepulturas Formation’ by Astini (1994) and as the Sepulturas Formation by Astini et al. (2004a), is well exposed at Quebrada Chamarra and Quebrada del Cardonal (Fig. 2). Brachiopods consist of Monorthis coloradoensis (reassigned herein to Ahtiella) and rare specimens of Paralenorthis sp., Dinorthis? sp., and small dalmanellids. Trilobites are represented by Neseuretus sp., a trinucleid of the Anebolithus-Incaia group (personal communication, B.G. Waisfeld, 2017), and a new species of Hoekaspis, the latter recorded elsewhere in the Sub- andean Ranges of northwestern Argentina from beds not older than the upper Darriwilian (Waisfeld and Vaccari, 2003). Albanesi and Astini (2002) reported from interbedded carbonate-rich layers a conodont assemblage consisting of Erraticodon, Erismodus, and Plectodina, as well as micro- remains of the agnathan Sacabambaspis considered of late Darriwilian age.
Famatina Basin.—The Famatina Range is characterized by a thick succession of sedimentary and volcanosedimentary rocks
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Figure 1. Map of central-western South America showing the main Ordovician sedimentary basins discussed in the text (gray shading).
deposited in a retroarc basin almost synchronously with the emplacement of crust-derived magmatism along the proto- Andean margin (Pankhurst et al., 2001; Dahlquist et al., 2005). Marine intervals are well exposed to the north of the basin in the Chaschuil area (Catamarca Province), and to the south in the Cachiyuyo-Saladillo rivers area (La Rioja Province) (Fig. 3). In the Chaschuil area, a regressive volcanosedimentary sequence acummulated on a high-gradient narrow platform flanking the volcanic chain (Mángano and Buatois, 1996, 1997). Its lower part, ~ 150m thick, was referred to the Loma del Kilómetro Member of the Suri Formation; it has been interpreted as deposited in a storm- and mass flow-dominated shelf, whereas the upper Punta Pétrea Member is a coarse-grained volcani- clastic wedge that records the progradation of a fan delta system onto the shelf sediments. The Dapingian age of the Loma del Kilómetro Member, containing Monorthis transversa (dis- cussed below) and other brachiopods (Benedetto, 1994), is based on conodonts of the Baltoniodus navis Biozone (Albanesi and Vaccari, 1994) and the Baltoniodus triangularis Biozone
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