Cichowolski and Rustán—Early Devonian bactritids from Argentina
A number of Lower and Middle Devonian cephalopod records have been known from these basins since the early twentieth century, yet their paleobiogeographic affinities are still poorly known, pending a more exhaustive taxonomic revision. In Argentina, with the exception of the putative records of
Devonian cephalopods from theNWarea (proven to be Silurian by Rickards et al., 2002), the wastebasket genus Orthoceras constitutes the main taxon cited from the Lower Devonian of the Argentine Precordillera (see summary in Castellaro, 1966). Otherwise, the rare ammonoid Tornoceras baldisi Leanza, 1968, from the Middle Devonian Chigua Formation is the only other Devonian cephalopod reported from Argentina (Leanza, 1968). In addition, Clarke (1913) and Baker (1923, cited in Castellaro, 1966) reported Michelinoceras? from the Lower Devonian of the Malvinas Islands. Michelinoceras was reported from the Early Devonian Icla
Formation at Sucre in Bolivia, but the higher diversity and abundance here corresponds to the Middle Devonian (Braniša, 1965; Troth et al., 2011). “Orthoceras” has also been reported from the Emsian of
South Africa (Oosthuizen, 1984), along with ?Bactrites and “Cyrtobactrites.” Two wastebasket taxa were reported from the Emsian of the Paraná Basin in Brazil: Orthoceras and Spyroceras (Bosetti et al., 2010b, 2012). Table 1 summarizes the non-ammonoid cephalopods
reported from this region. The large number of published specimens contradicts the hypothesis (mainly stated by Boucot and Racheboeuf, 1993) that nautiloid cephalopods were virtually absent in Early and Middle Devonian Malvinokaffric associations. In fact, based on this misconception and local evidence from the Paraná Basin, Bosetti et al. (2010b) argued that the presence of cephalopods in early Givetian beds indicates that the Malvinokaffric Realm collapsed during the Middle Devonian due to the rising marine temperature. However, through palynological analysis in Bolivia, Troth et al. (2011) demonstrated that the climate continued to be cold through the Middle and Late Devonian, and that immigration of warmer water Malvinokaffric taxa (e.g., the brachiopod Tropidoleptus) was due to discrete transgressive events in South America rather than a permanent immigration process. In spite of this, the cephalopods were interpreted as immigrants or alien taxa and used as evidence for a great paleobiogeographic change during
Figure 5.
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the Eifelian–Givetian transition, overlooking the previous records of cephalopods from Lower Devonian strata in south- western Gondwana. In addition, fossils assigned to ?Ctenoceras are almost certainly crinoid stems. Indeed, they exhibit a homogeneous diameter, homogeneous distance between sup- posed “sutures,” and no evidence of an aperture, chambers, or embryonic conch, but exhibit the constrictions and annulated, ridged ornamentation that characterize crinoid columnals (Bosetti et al., 2010a, figs. 7e, 7f; Bosetti et al., 2010b, figs. 6I–6L; Horodyski et al., 2013, figs. 6b, 6f). Ctenoceras, in turn, is an Ordovician genus (Kröger, 2004). On the other hand, some of these fossils have been interpreted as “roll marks” by the same authors, including some specimens which exhibit the typical “zig-zag” articulations usually visible in crinoid stems (Bosetti et al., 2010b, fig. 6L; Horodyski et al., 2013, fig. 6f). Thus, there is no reliable evidence of cephalopods in the units studied and the hypothesis of a Middle Devonian collapse of the Malvinokaffric Realm, as proposed by those authors, should be revised. The available evidence from southwestern Gondwana
contrasts markedly with the hypothesis of a virtual absence of cephalopods in the Malvinokaffric Realm. It is worth mention- ing that the bactritids described herein are associated with orthoceratoids (orthocerids, pseudorthocerids, lituitids) and oncocerids. In particular, it is remarkable that the lituitid family Lamellorthoceratidae Teichert, 1961 is present, a group which previously has only been reported from warmer regions such as Morocco (Termier and Termier, 1950; Kröger, 2008), France (Teichert, 1961; Babin, 1964), Germany (Beyrich, 1850), Bohemia (Kolebaba, 1999), Russia (Zhuravleva, 1961, 2006), North America (Stanley and Teichert, 1976; Bandel and Stanley, 1989), Turkey, and Japan (Niko, 2001). Recognition herein of the widely distributed bactritid taxa Bactrites gracilis and Devonobactrites agrees with the lamellorthoceratid dis- tribution. Bactrites is a cosmopolitan taxon, and B. gracilis was reported from the late Emsian–Givetian of Morocco, Germany, the United Kingdom, and USA (Kröger, 2008). Devonobactrites is known from the early Emsian–early Eifelian of Germany and Morocco (Kröger, 2008; Klug et al., 2010). Because its faunas were established in isolated tectonic
basins, the Malvinokaffric Realm has been considered as a natural model to evaluate the relationships among the spatial distribution of taxa, the geologic history of a region, and
(1–12) Bactrites gracilis (Blumenbach, 1803) from the Early Devonian Talacasto Formation, San Juan, Argentina: (1–9) specimens from Keidel’s
bed (latest Pragian?–earliest Emsian) at the Talacasto section; (10–12) specimen from grayish sandstones 30m below Keidel’s bed (late Pragian?) at the Talacasto section. (1) CEGH-UNC 27080, incomplete phragmocone without shell wall showing the sutures with their small ventral lobes; (2) CEGH-UNC 27081, incomplete phragmocone with partially disarticulated chambers in ventrolateral view; (3) CEGH-UNC 27080 (on top) and CEGH-UNC 27081 (at bottom) showing part of the body chamber and the incomplete phragmocone; (4–9) CEGH-UNC 27082: disarticulated and incomplete phragmocone, detail of the septum in posterior view, showing the siphuncle, lateral view, showing the suture and the curvature of the septum, ventral view, showing the suture with the ventral lobe and siphuncle, detail of the sutures in ventral view, and general ventral view; (10–12) CEGH-UNC 27089, longitudinal polished section with detail of the septal necks, detail of the chambers and siphuncle, and fragment representing part of the body chamber and incomplete phragmocone, showing the low apical angle and the length of the chambers; (13–15) Bactrites sp., CEGH-UNC 27092, internal mold of part of the phragmocone in a nodule from the lower argillaceous part of the Talacasto Formation (Lochkovian) at Quebrada de las Cortaderas, San Juan: anterior view showing circular cross section, incomplete phragmocone in ventral view showing the sutures, and complete specimen; (16–20) Bactrites sp., PULR-I 1, internal mold of part of the body chamber and incomplete phragmocone preserved in a nodule from the lower argillaceous part of the Talacasto Formation (Lochkovian) at Sierra de las Minitas, La Rioja Province: ventral view, lateral view, detail of the suture and the siphuncle in anterioventral view, detail of the septum and siphuncle in lateral view, and detail of the cross section in posterior view; (21) Bactrites sp., CEGH-UNC 27091 from lower argillaceous interval of the Talacasto Formation (upper Lochkovian– Pragian?) of the Quebrada de las Aguaditas section: ventral view of isolated chamber showing the ventral lobe of the suture and the convexity of the septum; (22–24) Devonobactrites? from greenish mudstones nearly 5m above Keidel’s bed (early Emsian?) at Loma de Los Piojos section: (22) CEGH-UNC 27098, poorly preserved, dorsoventrally flattened specimen; (23–24) CEGH-UNC 27099, right lateral view, note the oblique septum and suture, and compressed section in posterior view. Scale bars = 5mm (1–9, 11–24), or 1mm (10).
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