418


Journal of Paleontology 91(3):417–433


confirmed from South Africa, Bolivia, Brazil, the Malvinas Islands, and even from Argentina (see Table 1). In turn, it has been widely accepted that the imprint of the


Malvinokaffric Realm disappeared by the Late Devonian, the earlier endemic faunas being replaced by cosmopolitan ele- ments, due to extrinsic factors such as climatic changes or oceanographic events (Isaacson, 2007). Recent studies from Brazil have proposed the “collapse” of this major paleobio- geographic realm by the Middle Devonian, taking the presence of cephalopods as “immigrant or alien elements” as support of this proposal (Bosetti et al., 2010b, p.62 and references therein). The relevance of our new records lies mainly in two


aspects: the geographic and age distribution of bactritids. The Argentine bactritid records (along with those of other cephalo- pod groups) provide new insight into their diversity and dis- tribution during the Early Devonian, with implications for some assumptions concerning recognition of the Malvinokaffric Realm as a major paleobiogeographic unit. In addition, since our oldest bactritids come from Lochkovian beds, discussions on their relationship to the earliest records of bactritids are provided.


Stratigraphy and geological setting


The studied specimens come from the Early Devonian Talacasto Formation in the Argentine Precordillera Basin in west-central Argentina. The Talacasto Formation (Padula et al., 1967) is widely exposed in San Juan Province (Fig. 1.1), but is also known from an isolated outcrop at Sierra de Las Minitas, in La Rioja Province (Fig. 1.2), in the northernmost part of the basin (Rustán et al., 2011a). Based on records from San Juan Province, the Talacasto


Formation is a marine succession of intensely bioturbated greenish-gray mudstones with intercalated beds of sandstone. It typically contains dark argillaceous levels (black to greenish mudstones and shales) basally, passing upwards into sandy levels, with fossiliferous concretions (Figs. 2, 3). According to Astini (1991), this unit corresponds to a muddy shelf deposi- tional system developed during a high stand. It overlies the mainly Silurian shelf deposits of the Los Espejos Formation, and underlies turbiditic deposits of the (mainly) Middle Devonian Punta Negra Formation (Bracaccini, 1950; Bustos and Astini, 1997). In San Juan Province, the unit increases in thickness from


the south (in the type locality at Quebrada de Talacasto), where it is 300mthick, to the north (at the Loma de los Piojos section, near Río Jáchal, Fig. 1), where it is more than 1000mthick. The Talacasto Formation has yielded the majority of the Devonian fossils described from Argentina, including brachiopods (Herrera, 1995a, b; Herrera et al., 1998), trilobites (Vaccari et al., 1994; Rustán and Vaccari, 2010, 2012; Rustán et al., 2011a, b), ostracodes (Salas et al., 2013), corals (García-López and Fernández-Martínez, 1995; Carrera et al., 2013), bivalves (Sánchez et al., 1995), gastropods, and even scarce sponges (Carrera and Rustán, 2016). As in other closely related Lower–Middle Devonian


Malvinokaffric basins, conodonts and graptolites are absent and goniatites are extremely rare in the Precordillera Basin, so that biostratigraphic calibration is based on other fossil groups.


Table 1. Previous reports of Lower Devonian cephalopods from the Malvinokaffric Realm. Region


Age and localities Bolivia Brazil South Africa


Emsian–?Eifelian, Upper Icla Fm. at Icla


Pragian–Emsian, Ponta Grossa, Paraná Basin


Pragian?–Emsian, Gydo Fm., Prince Albert area


Reported taxa


“Michelinoceras ulrichi” (Kozłowski)


“Orthoceras sp.” “Spyroceras zoilus” (Clarke)


Argentine Precordillera NW


Malvinas Islands Pragian, Fox Bay Fm., Pebble Island


Argentina


Lower Devonian, Talacasto Fm., Loma de los Piojos and Zonda localities


Lower Devonian, Lipeón Fm., Sierra de Zapla, Jujuy


“Orthoceras bokkeveldense” “Orthoceras constrictum” “Orthoceras gamkaense” “Orthoceras rex” “Orthoceras” spp. ?Bactrites sp. “Cyrtobactrites” sp. “Orthoceras” sp.


Michelinoceras? cf. M.? gamkaense (Reed)


Eobactrites? sp. “Orthoceras” sp.


Remarks


This species is not valid at present. Illustrations in the paper are not clear enough to determine the taxonomic affinities of the specimens (there are two). Cephalopods are much more diverse in the Middle Devonian of Bolivia.


Specimens are very scarce (personal communication, E.P. Bosetti, 2014) and available illustrations are insufficient to confirm their taxonomic affinities. In turn, Bosetti et al. (2010a, b) reported ?Ctenoceras sp. from the early Givetian of the overlying São Domingos Fm. at Tibagi, but the specimens seem to be crinoid columnal segments. These taxa are only reported, not described or illustrated.


References Braniša (1965); Grahn (2002)


Clarke (1913); Bosetti et al. (2010a, b; 2012); Grahn et al. (2013)


Oosthuizen (1984); Carvalho (2006)


Only reported, not described or illustrated.


Only reported, not described or illustrated. Actually Eobactrites is considered an orthocerid and those beds are of Silurian age (Rickards et al., 2002). Only reported, not described or illustrated.


Thomas (1905); Keidel (1921); García (1945); Leidhold and Wetten (1947)


Cecioni (1953); Feruglio (1930)


Clarke (1913); Baker (1923); Carvalho (2006)


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