The diachronism of the genus Ffynnonia
- An outer-ramp brachiopod assemblage from the uppermost San Juan Formation (middle Arenig), northern Argentine Precordillera: paleoecologic and biogeographic implications
The diachronism of the genus Ffynnonia
The genus Ffynnonia Neuman and Bates is a distinctive brachiopod of the Celtic Province defined by Neuman (1976), which developed during the Arenigearly Llanvirn along the periphery of Gondwana, at mid- to high-latitudes. Celtic brachiopods have been recorded typically in high-latitude siliciclastic or volcaniclastic rocks deposited in peri-insular or continental margin settings (Neuman and Harper, 1992), and less frequently in warmer water carbonate platforms surrounding isolated terranes (e.g. Precordillera terrane; Benedetto, 1998 and references therein). They include a high proportion of endemic taxa, most of them confined to a single or a few localities. Ffynnonia was first recorded in Wales, from the upper Arenig Treiorwerth Formation (Bates, 1968). It was subsequently discovered in upper Arenig-lower Llanvirn strata of the San Juan Formation, in the Argentine Precordillera (Benedetto, 2001a; Benedetto, 2003b) and in the Suri Formation of the Famatina Range, western Argentina (Benedetto, 2003a). According to the well-constrained mid Arenig age of the Suri Formation ( O . evae Zone, Albanesi and Astini, 2000), the Famatina basin contains the earliest known representatives of Ffynnonia . This supports the contention that volcanic islands and volcanic chains peripheral to the Iapetus Ocean may have acted as ?centers of origin' of numerous Ordovician taxa (Neuman, 1976; Fortey, 1984) and may have played a key role in the early Ordovician radiation of brachiopods and bivalves (Benedetto and Sánchez, 2003; see also Harper and McNiocaill, 2002). In the Famatina Range Ffynnonia suddenly appears in the Suri Formation; no brachiopods have been found so far in the underlying laminated siltstones and black shales of lower Arenig age. The ancestor of Ffynnonia , as well as that of the closely related Platystrophia (Zyukov, 2001), remain uncertain. We believe that the best candidate, at the present state of knowledge, is the alimbellid Astraborthis quebradensis Benedetto, from upper Tremadoc beds of the Central Andean basin of northwestern Argentina and Bolivia, which possesses a number of features suggesting a link with the platystrophiids and, in particular, with Ffynnonia (Benedetto and Carrasco, 2002). If so, the latter originated in the Famatina basin and then migrated to other Celtic sites (e.g. Wales, Precordillera terrane).
It is noticeable that the age of the outer-shelf, Ffynnonia -bearing beds in the northern Precordillera is intermediate between those of Famatina Range and central-southern Precordillera basin. In the context of the hypothesis that the Precordillera is an allochthonous terrane -recently reassessed by one of us (Benedetto, 2004)-, the existence of a remnant basin ca. 300 km wide separating this microcontinent from the Gondwana margin has been inferred (figure 4). By the early Llanvirn the first stages of collision initiated and this seaway became much narrower. In turn, the back-arc basin developed behind the Famatina basin underwent a regressive event and finally emerged, as lithofacial analysis and increasing vulcano-tectonic activity indicates (Astini and Benedetto, 1996; Astini, 1999). As result of these phenomena, the high-gradient narrow shelves adjacent to the volcanic edifices became gradually unstable promoting the migration of a number of brachiopod taxa towards the open waters lying to the west (in present coordinates). Two main migration waves can be discerned. The first included some taxa recorded in the
Suri Formation, such as Ffynnonia , Skenidioides and Hesperonomiella of which only the former is present in the intermediate-age, deep-water deposits of the northern Precordillera. It seems, therefore, that Ffynnonia migrated from the Famatina back-arc basin to the open platform waters of the northern Precordillera basin and then to shallower settings of the central and southern parts of the carbonate ramp. The second migratory event involved a larger number of taxa present in the overlying Molles Formation of the Famatina Range (upper O. evae-B . navis Zones), including Skenidioides , Monorthis , Hesperonomia , Productorthis , Ahtiella , Rugostrophia and Camerella . All of them have been recorded in upper Arenig and/or lower Llanvirn carbonate beds of the San Juan Formation in inner or, more frequently, in mid-ramp settings. These genera appear to have an earlier history of expansion in volcanosedimentary rocks that continues on the carbonate ramp as they adapt to a new set of physical and biotic conditions. It should be noted, however, that the majority of these taxa become abundant only in the uppermost part of the San Juan Formation (lower Llanvirn, L. variabilis Zone), which deposited in deeper waters than the underlying mid-ramp Arenig carbonates (Cañas, 1999).
Up to now, Taffia ( Chaloupskia ) and Leptella ( Petroria ) have not been reported from the Famatina basin. Both taxa are known from younger (lower Llanvirn) beds of the Precordillera carbonate platform, in settings ranging from subtidal, demosponge-dominated biofacies (e.g. Talacasto-Villicum area) to outer-shelf brachiopod-dominated biofacies (topmost levels of the Cerro Viejo area) (Sánchez et al ., 1996). The occurrence of T . ( Chaloupskia ) minima in the outer-ramp deposits of the Guandacol area slightly predates that of the type species T. ( Chaloupskia ) scabrella Neuman in the Hølonda Limestone of Norway (upper Arenig-lower Llanvirn; Neuman and Bruton, 1989). The lower Llanvirn Precordilleran species T . ( Chaloupskia ) anomala Benedetto and Herrera, from Cerro Viejo, and T . ( Chaloupskia ) sp. nov., from Talacasto (not yet described) are thought to be direct descendants from the outer-shelf species T. ( C .) minima . It should be noted that the two Llanvirn species are larger and thick-shelled compared to the deep water Arenig species, and comae are always strongly developed. All these features are interpreted as being advantageous for inhabiting shallower waters.
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