Cichowolski and Rustán—Early Devonian bactritids from Argentina
PULR-I 1-2 are preserved as possible body chambers (internal molds) with the last-formed septum preserved, in which the siphuncle is visible. CEGH-UNC 27091 is an isolated phrag- mocone chamber, thus its measurements should be taken cau- tiously. CEGH-UNC 27092, which would be the regionally oldest bactritid, is a partial phragmocone with the ventral sutures exposed, in which the small ventral lobe can be seen (Fig. 5.14).
Remarks.—We assign these specimens to the genus Bactrites, although it seems inappropriate to assign them to a species because their preservation is rather poor. The slender form of the conchs, the straight suture with a ventral lobe, subcircular to slightly ovate cross section, and the values of the cameral ratio are typical for the genus. We are not sure that all specimens belong to one species due to differences of cross sections and septal curvatures. Among the specimens with clear characters typical of the
Bactritidae, at least six should be considered as Bactrites? (doubtfully) mainly because the siphuncle is not preserved: CEGH-UNC 27093 from Quebrada de la Cortadera, from fossiliferous level 14 (Lochkovian) of Salas (1995), CEGH- UNC 27094–96 from the Loma de los Piojos section, San Juan Province, from a stratigraphic interval 30–40m below Keidel’s bed, fossiliferous level 41 of collector B. Waisfeld (Pragian), CEGH-UNC 27097 from the Loma de los Piojos section, from mudstones above Keidel’s bed (Emsian), and PULR-I 3 from the Sierra de las Minitas, La Rioja Province, Lochkovian lower argillaceous interval. Nevertheless, the general shape of the fragments and their proportions strongly suggest they belong to Bactrites as well. In spite of the open nomenclature, the main interest of these records is the age of the host beds, considered to be Lochkovian to Pragian (except CEGH-UNC 27097). Given that we have no data about the apical part of these specimens, we cannot clarify their relations with the Emsian material. The presence of those traits considered “bactritid-like,” such as the ventral lobe of the suture and the small, ventral siphuncle in “Bactrites” bohemicus Ristedt (in Ristedt, 1981), was considered homeomorphism by Kröger and Mapes (2007) and Klug et al. (2015). However, we believe that the fact that they were found within one stratigraphic section suggests these pre-Emsian specimens are most probably bactritids as well.
Genus Devonobactrites Shimansky, 1962
Type species.—Orthoceratites obliquiseptatum Sandberger and Sandberger, 1852, from the Wissenbach Slate, Eifelian of Wissenbach, Germany.
Devonobactrites? sp. Figure 5.22–5.24
Materials.—Two specimens from the Loma de los Piojos section, San Juan Province. CEGH-UNC 27098, and CEGH- UNC27099, greenish mudstones nearly 5mabove Keidel’s bed (Emsian), collectors: R.T. Becker, C. Klug and B. Kröger.
Description.—Specimen CEGH-UNC 27099 is a 3.4mm long and ~3mm wide fragment of an incomplete phragmocone con- sisting of a few small chambers. It was deformed by compaction in a nearly lateral sense (Fig. 5.23, 5.24). This deformation probably accentuated the originally slightly compressed cross section. The septa appear to be somewhat oblique, but due to deformation of the specimen, it is not clear whether this repre- sents their original orientation. The siphuncle is small, with a width of 11% conch diameter, and it is not fully marginal. The cameral length is ~2mm. The shell wall is not preserved. CEGH-UNC 27098 is a longer fragment of a broken and deformed conch. It is impossible to determine whether parts of the body chamber are present (Fig. 5.22). Its deformation is in the dorsoventral plane, resulting in an artificially depressed section. The siphuncle diameter is ~14% of the conch diameter. The measured apical angle is ~4.4º. The shell wall is poorly preserved and the ornamentation appears to be composed of faint, oblique undulations. The sutures are hardly distinguish- able, although on the venter, they appear to be slightly inclined, as seen in the most adapical septum.
Remarks.—The incomplete and poor preservation of these specimens precludes measurement or evaluation of character- istics typical of the genus or individual species (e.g., close septal spacing or the shape of the embryonic conch). The oblique septa and the sub-marginal siphuncle suggest the specimens could be included in the genus Devonobactrites. Better-preserved mate- rial is needed to evaluate the affinities of this form. In compar- ison with records lower in the same formation, they are conspicuously small. The bigger bactritids collected from beds below Keidel’s level are more robust.
Discussion
Paleobiogeography.—Devonian records of Malvinokaffric end- emic marine faunas correspond to southwestern Gondwanan basins (Boucot and Racheboeuf, 1993), including those from South America, excepting Venezuela and Colombia (Morzadec et al., 2015), Antarctica, Ghana, and South Africa (Fig. 6).
Figure 4. (1–16) Bactrites gracilis (Blumenbach, 1803), from latest Pragian?–earliest Emsian levels of the Talacasto Formation at the Quebrada de Talacasto section, San Juan Province, Argentina. All from calcareous sandy nodules from Keidel’sbed,except(6) from nearly 5m below it. (1–4) CEGH-UNC 27103, ventral view of nearly complete specimen, detail of the sinuous growth lines defining a ventral sinus, detail of the orthochoanitic septal necks, and ventrolateral view of three chambers adjacent to the apical part, without shell wall. The arrow points to the ventral lobe of the suture; (5) latex cast of CEGH-UNC 27084 (on top) and CEGH- UNC 27083 (at bottom), lateral views showing the sinuous growth lines; (6) latex cast of CEGH-UNC 27100 (on top), CEGH-UNC 27101 (in the middle) and CEGH-UNC 27102 (at bottom), note the sinuous growth lines and bryozoan epizoans; (7) ventral view of CEGH-UNC 27084, recrystallized and incomplete phragmocone showing the ventral lobe; (8) posterolateral view of CEGH-UNC 27086, fragmentary specimen with partially preserved shell wall; (9) CEGH-UNC 27088 (on top) and CEGH-UNC 27087 (at bottom) in posterior view, incomplete phragmocones showing the spherical septa and the siphuncle; (10)the same specimens in posterolateral view, note the straight sutures; (11–16) CEGH-UNC 27083, incomplete phragmocone in ventral view showing details of the sutures and the siphuncle, detail of the sutures in lateral view (note the adapical inclined suture), anterior view (note the compressed cross section), longitudinal polished section showing detail of the orthochoanitic septal necks, general view of the longitudinal polished section showing the relatively long chambers and the ventral and small siphuncle, and posterior view with details of the compressed cross section. Scale bars = 1mm (4), or 2.5mm (14), or 5mm (1–3, 5–13, 15, 16).
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