Bactroceras sandbergeri ( Barrande, 1867 )

Aubrechtová, Martina, Turek, Vojtěch & Zicha, Ondřej, 2020, Early ontogenetic growth stages of Middle Ordovician orthoceratoid cephalopods from Bohemia, Acta Palaeontologica Polonica 65 (3), pp. 575-588 : 577-582

Bactroceras sandbergeri ( Barrande, 1867)

Figs. 2–5 , 6A .

Lectotype: NM L 6584, figured by Barrande (1867, pl. 245, fig. 18), designated and re-figured by Aubrechtová (2015, p. 202, fig. 2A).

Type locality: Osek u Rokycan, central Bohemia, Czech Republic Type horizon: Middle Ordovician, lower–middle Darriwilian Stage, Šárka Formation, Corymbograptus retroflexus Biozone .

Material.— Thirteen internal moulds and counterparts of fragments of juvenile shell parts from the localities Mýto (Svatoštěpánský rybník-Teletník, 49.7863217N, 13.7601539E; MWB S 06761), Mýto (Svatoštěpánský rybník-Kařízek, 49.7959761N 13.7655044E; MWB S 06762), Praha-Hanspaulka (50.1055886 N, 14.3824408 E; NM L 59577), Rokycany-Díly (49.7569444 N, 13.6013889 E; MWB S 06759, 06763), Těškov (49.7945714 N, 13.6958797 E; MWB S 06760, 06764) and Volduchy (49.77091 N, 13.626269 E; MWB S 06823–06828); Šárka Formation , lower and middle Darriwilian Stage, Middle Ordovician; Czech Republic .

Description.—MWB S 06759 ( Fig. 2A ) is a part and counterpart of an embryonic portion of the shell with the initial chamber. The total length of the specimen is 2.7 mm. The subspherical initial chamber (= first phragmocone chamber) is bean-shaped in median section, 2.5 mm in diameter and 0.9 mm in length (shape ratio 0.36). The second phragmocone chamber is 2.1 mm in basal diameter and 1.8 mm in length. The diameter of the initial chamber exceeds that of the second phragmocone chamber (constriction ratio 0.84). The phragmocone is circular in cross-section. The marginal (ventral) siphuncle is weathered out and a cavity is left. The cavity appears at about the mid-length of the initial chamber. The caecum is cylindrical, 0.3 mm in diameter; its apical end clearly bends towards the inside of the initial chamber at about its mid-length. The external surface as shown by the latex cast of the counterpart is smooth. The specimen is accompanied by another orthoceratoid shell ornamented with transverse lirae, an indeterminate hyolith, the trilobite Ormathops and the brachiopod Euorthisina sp.

MWB S 06760 ( Fig. 2C ) is a counterpart of a shell and the third septum of the phragmocone showing the position of the siphuncle. The specimen shows an imprint of the initial chamber. The total length of the specimen is 5.2 mm. The initial chamber (= first phragmocone chamber) was bean-shaped in median section, 2.5 mm in diameter and 1.2 mm in length (shape ratio 0.48). The diameter of the initial chamber exceeds that of the second phragmocone chamber (basal diameter of the second phragmocone chamber was 2.2 mm, constriction ratio 0.88). The phragmocone is circular in cross-section. The specimen is accompanied by the hyolith Bactrotheca teres and other indeterminate invertebrate fragments.

MWB S 06761 ( Fig. 3A ) is a fragment of a counterpart of a juvenile shell. The total length of the specimen is 13.1 mm. The initial chamber (= first phragmocone chamber) is bean-shaped in median section, 2.2 mm in diameter and 1.1 mm in length (shape ratio 0.5). The diameter of the initial chamber exceeds that of the second phragmocone chamber (basal diameter of the second phragmocone chamber was 2 mm, constriction ratio 0.91). The phragmocone has a maximum diameter of 2.5 mm and expands very slowly (angle of expansion 2°). The phragmocone is circular in cross-section. The external surface is smooth. The specimen is accompanied by a hyolith and other indeterminate invertebrate fragments.

MWB S 06762 ( Fig. 3B ) is a fragment of a counterpart of a juvenile shell. The total length of the specimen is 20 mm. The initial chamber is bean-shaped in median section, 2.5 mm in diameter and 1.6 mm in length (shape ratio 0.64). The diameter of the initial chamber exceeds that of the second phragmocone chamber (basal diameter of the second phragmocone chamber was 2.3 mm, constriction ratio 0.92). The phragmocone expands very slowly. The phragmocone is circular in cross-section. The external surface is smooth.

MWB S 06763 ( Fig. 3C ) is a fragment of a counterpart of a juvenile shell with one phragmocone chamber preserved adorally. The total length of the specimen is 10.4 mm. The initial chamber is bean-shaped in median section, 2 mm in dorso-ventral diameter and 1.3 mm in length (shape ratio 0.65). The diameter of the initial chamber exceeds that of the second phragmocone chamber (basal diameter of the second phragmocone chamber is 1.8 mm, constriction ratio 0.9). The phragmocone has a maximum dorso-ventral diameter of 2.3 mm and expands very slowly. The phragmocone is circular in cross-section. The septum of the adoralmost phragmocone chamber displays the submarginal (subventral) siphuncle, which has a circular cross-section and a diameter of 0.25 mm. The external shell surface is smooth. The specimen is accompanied by the bellerophontid Tropidodiscus pusillus , the trilobite Pricyclopyge cf. binodosa and an indeterminate bivalve.

MWB S 06764 ( Fig. 2B ) is a fragment of a counterpart of a juvenile shell with one phragmocone chamber preserved adorally. The total length of the specimen is 3.5 mm. The initial chamber is bean-shaped in median section, very short, 2.6 mm in dorso-ventral diameter and 1.4 mm in length (shape ratio 0.54). The diameter of the initial chamber exceeds that of the second phragmocone chamber (basal diameter of the second phragmocone chamber was 2.2 mm, constriction ratio 0.85). The phragmocone has a maximum dorso-ventral diameter of 2.2 mm and expands very slowly; it is circular in cross-section. The length of the adoralmost phragmocone chamber is 1.2 mm. The siphuncle is narrow, submarginal (ventral) with a circular cross-section. The external shell surface is smooth. The specimen is accompanied by the bellerophontid Tropidodiscus pusillus .

NM L 59577 ( Fig. 2D ) is a counterpart of an embryonic shell with the initial chamber. The adoral part of the shell is very slightly bent. The total length of the specimen 4.5 mm. The initial chamber (= first phragmocone chamber) is bean-shaped in median section, 2.0 mm in diameter and 1.2 mm in length (shape ratio 0.6). The diameter of the initial chamber exceeds that of the second phragmocone chamber (basal diameter of the second phragmocone chamber is 1.7 mm, constriction ratio 0.85). The phragmocone is circular in cross-section. No shell ornamentation is discernible. The nodule containing this specimen also yielded the rare graptolite Dichograptus ? leptotheca (Petr Kraft, personal communication 2019).

MWB S 06823 ( Fig. 6A ) represents the third phragmocone chamber (estimated based on the shell diameter and length of this chamber) and the adapical part of the living chamber. The phragmocone has a minimum dorso-ventral diameter of 1.8 mm, a minimum lateral diameter of 1.6 mm and expands very slowly; it is slightly compressed in cross-section. The length of the third phragmocone chamber is 2.7 mm. The siphuncle is narrow, submarginal (ventral) with a circular cross-section; its diameter at the foramen of the adapical septum is 0.25 mm. The specimen is accompanied by undetermined hyoliths and the bellerophontid Tropidodiscus pusillus .

The three remaining specimens MWB S 06824–06826 from the locality Volduchy are juvenile shell fragments that morphologically correspond to the above described specimens. However, their preservation hinders precise measurements.

Remarks. —MWB S 06759, 06763, and 06764 are shell fragments of larger individuals as indicated by the absence of living chambers. By contrast, specimen MWB S 06823 probably represents the shell of an early juvenile individual. The rest of the specimens (counterparts) do not show any internal structures and therefore can represent both complete shells of freshly hatched individuals, or adapical parts of later growth stages.

The specimens unequivocally belong to Bactroceras sandbergeri ( Barrande, 1867) because they exhibit short, bean-shaped in median section, rather large and constricted initial chambers. These characters correspond in all aspects to the only apex previously known from the species ( Barrande 1868: pl. 247: 26–28; Barrande 1874: 801; see also Dzik 1981 and Aubrechtová 2015). The only difference concerns the caecum, which is slightly more curved in MWB S 06759 than in Barrande’s specimen mentioned above.

In Bactroceras the apicalmost chamber is known also in B. angustisiphonatum ( Rüdiger, 1889) (see Evans 2005), where it is, however, much smaller in size (1.3 mm in height, 1 mm in diameter, shape ratio 1.3), less constricted (constriction ratio 0.95) and more ovoid in shape ( Table 2 and Figs. 4, 5 ).

Of all known Ordovician orthoceratoids, the shape of the initial chamber of Bactroceras sandbergeri is most sim-

Archigeisonoceras sp. Orthoceras sp. Orthoceratida indet. sp. 1 Arionoceras ? sp.? Isorthoceras sp. Orthoceratida indet sp. Bactroceras angustisiphonatum Orthoceras hudsonicum Orthoceratida indet sp. Bactroceras sandbergeri Orthoceratidae Gen. et. sp. indet Orthoceratida indet sp.

Eriditidae Gen. et sp. indet. Orthoceratidae Gen. et. sp. indet Orthocerida indet.

ilar to that of Orthoceras sp. ( Balashov 1957; Figs. 4, 5 herein). The latter is, however, much smaller in diameter see Table 2), the caecum is much shorter and the siphuncle has a ventro-central position. Compared to B. sandbergeri , the initial chamber of Archigeisonoceras sp. ( Kröger 2006; Figs. 4, 5 herein) is smaller and only very slightly constricted; the caecum is bent inwards and relatively long but expands inside the initial chamber.

Stratigraphic and geographic range. —Šárka Formation, lower and middle Darriwilian, Middle Ordovician of central Bohemia, Czech Republic; middle Darriwilian, Middle Ordovician of Wales, United Kingdom; middle and upper Darriwilian, Middle Ordovician, Oslo Region, Norway; upper Darriwilian, Middle Ordovician, Seby Limestone Formation of Öland, Sweden; Grey Lituites Limestone, Folkeslunda Limestone , upper Darriwilian, Middle Ordovician of Mecklenburg, Germany (Pleistocene glacial erratic boulders).

Figures

Fig. 2. Apical shell parts of the orthoceratoid Bactroceras sandbergeri (Barrande, 1867), Middle Ordovician, lower Darriwilian Stage, Šárka Formation, Czech Republic. A. MWB S 06759, Rokycany-Díly, internal mould in ventral view (A1), with accompanying trilobite Ormatops sp., brachiopod Euorthisina sp., and indeterminate hyolith; detail (A2), note the decrease of apical angle at second phragmocone chamber; latex cast of counterpart in ventral view (A3). B. MWB S 06764, Těškov, internal mould in lateral view, venter on right hand-side (initial chamber bent towards dorsum), initial and second phragmocone chamber preserved as counterpart. C. MWB S 06760, Těškov, latex casts of both sides of counterparts (C1, C2), note suspicious surface structures. D. NM L 59577, Praha-Hanspaulka, latex cast of counterpart.

Fig. 3. Apical shell of the orthoceratoid Bactroceras sandbergeri (Barrande, 1867); Middle Ordovician, lower Darriwilian Stage, Šárka Formation, Czech Republic. A. MWB S 06761, Mýto-Svatoštěpánský rybník, latex cast of one of counterparts in lateral view, note fine, obliquely transverse surface ornamentation. B. MWB S 06762, Mýto-Svatoštěpánský rybník, counterpart (B1) and its latex cast (B2), in lateral view (initial chamber bent towards dorsum). C. MWB S 06763, Rokycany-Díly, internal mould in lateral view, single (adoralmost) phragmocone chamber showing ventrally situated siphuncle (C1), latex cast of the second counterpart (C2).

Fig. 6. Apical and juvenile shell parts of orthoceratid cephalopods; Middle Ordovician, Darriwilian Stage, Šárka and Dobrotivá formations, Czech Republic. A. Bactroceras sandbergeri (Barrande, 1867), MWB S 06823, Volduchy, third phragmocone chamber (estimated based on shell diameter) and adjacent part of body chamber. B. Orthoceratida indet. sp. 4, MWB S 06827, Volduchy, counterpart of initial chamber and adjacent part of phragmocone and one corroded phragmocone septum; note longitudinal striae. C. Orthoceratida indet sp. 5, NM L 59872, Malé Přílepy, counterpart (C1), latext cast C2); note longitudinal striae.

Fig. 5. Drawings of known embryonic shells with initial chambers of Ordovician orthoceratoids (cf. Kröger and Mapes 2007: fig. 4 and Klug et al. 2015: fig. 1.3.). Dotted lines indicate the assumed position of the siphuncle in Arionoceras? sp. and Orthoceratida indet. sp. 2. In Orthoceratida indet. sp. 2, note a sketch of hyposeptal cameral deposits (see the text below and Fig. 7C1). The stratigraphic position of Bactroceras angustisiphonatum and Arionoceras? sp. is either uppermost Lower Ordovician, or lowermost Middle Ordovician (David Evans, personal communication 2019; see also Evans 2005 and 2007). Orthoceratidae gen. et sp. indet B (Kröger 2007) is likely to be conspecific with Transorthoceras osmundsbergense according to Kröger et al. (2011a).