772


Journal of Paleontology 92(5):768–793


that in the Chaschuil area the levels of A. famatiniana n. sp. are interbedded with isotopically dated volcanic rocks (see above), a middle-late Dapingian age seems most likely.


Precordillera Basin (Cuyania terrane).—There is a general agreement that the up to 2,500m thick, passive-margin carbo- nate succession started to accummulate during the Cambrian Series 2 on the Laurentian continental margin, a segment of which subsequently rifted from the Ouachita Embayment ori- ginating the Cuyania terrane (Astini et al., 1995; Thomas and Astini, 1996, 2003; Astini, 1998; Benedetto, 1998a, 2004; Ramos, 2004; but see Finney, 2007, for a para-autochthonous Gondwanan hypothesis). Cuyania became part of the Gond- wana continent since its accretion to the proto-Andean margin by Middle Ordovician or more probably Late Ordovician times. Depositional environments evolved from tidal flats, shoals,


shallow subtidal settings, and restricted subtidal settings during the Cambrian −early Tremadocian, to open shelf settings by late Tremadocian-Darriwilian times (Cañas, 1999; Keller, 1999, 2012; Gómez and Astini, 2015). The carbonate succession referred to the San Juan Formation consists of ~ 350m of burrowed skeletal wackestones and packstones capped by a 25− 30m thick interval of mid-outer ramp nodular limestones bearing a rich benthic fauna dominated by rhynchonelliform brachiopods and sponges, with trilobites, bryozoans, gastro- pods, crinoids, and linguliforms as secondary components (Carrera, 2003; Waisfeld et al., 2003; Sorrentino et al., 2009; Carrera and Ernst, 2010; Lavié and Benedetto, 2016) (Fig. 4). The brachiopod association from these beds encompasses the Ahtiella argentina Biozone, which is the uppermost of the six biozones recognized through the San Juan Formation (Herrera and Benedetto, 1991; Benedetto, 2002, 2007). This interval is particularly well exposed along the western slope of Cerro Viejo, ~ 20 kmnortheast of the city of San José de Jáchal, where the San Juan Formation forms a westward-dipping homoclinal succession. The A. argentina specimens described herein were collected mainly at Quebrada Los Gatos and the adjacent Quebrada Honda stratigraphic sections from a 10 −12m thick package of nodular limestones lying immediately below the contact with the Los Azules Formation black shales (Fig. 4). The age of this interval is well constrained by conodonts of the Paroistodus horridus Subzone within the Lenodus variabilis Biozone (Albanesi and Ortega, 2002; Ortega et al., 2007) and the lower part of the succeeding Yangtzeplacognathus crassus Biozone (Mestre and Heredia, 2013; Serra et al., 2015). According to the time-slices schema proposed by Bergström et al. (2009), the A. argentina beds fall mostly within Dw1 reaching the lower part of Dw2. At Sierra de la Trampa, near 40km south of the city of San


José de Jáchal, a ~ 60m thick succession of nodular limestones crops out at Quebrada Las Chacritas and Quebrada La Tuna (Fig. 4). This package was originally referred by Espisúa (1968) to the ‘upper member’ of the San Juan Formation, then to the ‘Las Tunas calcareous unit’ by Carrera (1997), and finally to the Las Chacritas Formation by Astini (1998), which was formally defined by Peralta et al. (1999). The thin bedded wackestones, bioclastic grainstones, and mudstones of the Las Chacritas Formation have yielded rich poriferan assemblages (Carrera, 1997), excellently silicified trilobite larval stages (Waisfeld


et al., 2001, and references therein), and numerous brachiopods not yet described, including Skenidioides? sp. and Ahtiella tunaensis new species (this paper). Several conodont studies led to the recognition the Y. crassus Zone in the lower part of the Las Chacritas Formation, the Eoplacognathus pseudoplanus Biozone from 36m above the base, and the Eoplacognathus suecicus Biozone near the top (Albanesi and Astini, 2000; Heredia et al., 2011; Mestre and Heredia, 2012, 2013; Serra et al., 2015). Accordingly, A. tunaensis n. sp. can be confidently dated as middle Darriwilian (Dw2).


Remarks on the biogeography of Ahtiella and related ahtiellins


The earliest known representative of the genus is Ahtiella zarelae Villas in Gutiérrez-Marco and Villas, 2007 from the upper Floian San José Formation of southern Peru (Gutiérrez-Marco et al., 2008). The slightly younger A. famatiniana n. sp. occurs in vol- caniclastic rocks of the Famatina Range of middle-lateDapingian age. In Anglesey (northwesternWales), Ahtiella is represented by A. quadrata Bates, 1969, from the Expansograptus hirundo Biozone, which in the Atlantic Province encompasses the Dapingian and the base of Darriwilian (Zalasiewicz et al., 2009), and A. concava Bates, 1969, from the slightly younger Bob Dei- niol Formation. Ahtiella paucirugosa Neuman, 1976 has been reported from the lower Darriwilian Summerford Group of New World Island, central Newfoundland. These volcaniclastic rocks were interpreted as recording intra-Iapetus volcanic islands related to the Avalonian paleocontinent (Neuman, 1976, 1984; Neuman and Harper, 1992; Harper et al., 1996). Monorthis coloradoensis occurs in the Cordillera Oriental of northwestern Argentina from beds probably not older than mid-Darriwilian.Ahtiella is common in the carbonate or mixed carbonate-clastic rocks of Baltica and Cuyania. In Sweden and Estonia, Ahtiella encompasses the Kunda and Asseri regional stages, the latter reaching the middle Darriwilian Pterograptus elegans Biozone and E. suecicus Zone (Tolmacheva et al., 2003; Suyarkova and Koren, 2009). As sta- ted above, in the Precordillera basin, Ahtiella ranges from the lower Darriwilian (A. argentina) to the middle Darriwilian (A. tunaensis n. sp.) but does not reach the E. suecicus Zone. Such a distribution led Gutiérrez-Marco and Villas (2007) to infer that Ahtiella migrated eastward from the mid-latitude (~30–40°) Andean region into Avalonia and Baltica, and simultaneously moved into the low-latitude Cuyania taerrane. It should be noted that Gutiérrez-Marco and Villas (2007, fig. 8) adopted the paleogeographic reconstruction of Aceñolaza et al. (2002) and Finney (2007) and placed Cuyania into the gap delimited by southern South America, South Africa, and Antarctica. Because neither tectonostratigraphic (Astini and Rapalini, 2003; Ramos, 2004; Thomas et al., 2004) nor paleontological evidence (Ben- edetto, 2004) supports such a para-autochthonous Gondwanan origin, Cuyania is located here fairly closer to, and at approxi- mately the same paleolongitude as the Famatina-Puna volcanic arc (Fig. 5). Perhaps the major weakness—but not the only one—of the Finney (2007) reconstruction is the complete absence in the Cambrian carbonate rocks of the Precordillera of Redlichiid-realm trilobites, which, as it is known, are distinctive of Australasia and Antarctica. Instead, the Cambrian- Tremadocian trilobites from Cuyania (Astini et al., 1995;


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