Orthoceratida, Kuhn, 1940

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 : 582-584

Orthoceratida indet. sp. 1

Fig. 7A.

Material.— One specimen (NM L 46566), phragmocone, from the locality Praha-Šárka cihelna (brickyard); Šárka Formation , lower and middle Darriwilian Stage, Middle Ordovician; Czech Republic .

Description. —The specimen is a part and a counterpart of a phragmocone with the initial chamber. Adapical third of the phragmocone is longitudinally broken off and in-filled with a brownish carbonate mineral; the broken portion was reconstructed using a latex cast prepared from the counterpart. The total length of the specimen is 46.8 mm. A weakly sinuous line along the contact of the fossil and the surrounding rock indicates the presence of a low annulation on the surface of the shell. The surface is, however, slightly corroded and does not show any other ornamentation. The initial chamber is egg-shaped, very slightly asymmetric from one side ( Figs. 5 , 7A), 3 mm in diameter and 3.1 mm in

C. Orthoceratida indet. sp. 2, MWB S 06765, Mýto-Svatoštěpánský rybník, detail of adoralmost septum with hyposeptal cameral deposits around the subcentral siphuncle (C 1), the second counterpart (C 2), note the absence of cameral deposits at septum situated more apically than in C 1, latex cast of the previous associated with bellerophontid Tropidodiscus pusillus (Barrande in Perner, 1903) (C 3).

length (shape ratio c. 1). The diameter of the initial chamber exceeds that of the adjacent phragmocone chamber (basal diameter of the second phragmocone chamber is 2.6 mm, constriction ratio 0.87). The first, straight and directly transverse, suture is faintly indicated. The phragmocone is circular in cross-section with a maximum diameter of c. 7 mm and expands with an angle of 6°.

Remarks. —The initial chamber of the specimen stands out among other apical shell parts known from Ordovician orthoceratids in its large size reaching 3 mm in diameter and 3.1 mm in length. It may resemble the initial chamber described by Evans (2007, Arionoceras ?), which is ovoid in shape; the latter is, however, smaller in dimensions, more distinctly constricted (see Table 2 and Figs. 4, 5 ) and has a greater angle of expansion (13°).

The Bohemian specimen more likely belongs to one of the Middle Ordovician orthoceratid species described by Joachim Barrande, in which early ontogenetic stages are not yet known. Unfortunately, the collection of Joachim Barrande contains either fragmentary material, or specimens representing only late ontogenetic stages. Moreover, the preservation of the specimen studied here does not show the character of the siphuncle and ornamentation is only slightly indicated, which prevents the specimen from being more precisely identified.

Figures

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).