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 : 584

Orthoceratida indet. sp. 2

Figs. 5 , 7C.

Material. — One specimen (MWB S 06765), phragmocone, from the locality Mýto-Svatoštěpánský rybník-Teletník (49.7863217 N, 13.7601539 E); Šárka Formation , lower and middle Darriwilian Stage, Middle Ordovician; Czech Republic .

Description. —The specimen consists of two counterparts of phragmocone composed of two dislocated fragments. In each of the two counterparts there is one septum preserved. The rest of the shell was reconstructed from a latex cast. Total combined length is 15.3 mm (apical fragment 9.1 mm and adoral fragment 6.2 mm). The initial chamber is egg-shaped, 2 mm in dorso-ventral diameter and 2 mm in length shape ratio 1). Its diameter very slightly exceeds that of the adjacent phragmocone chamber (basal diameter of the second phragmocone chamber is 1.9 mm, constriction 0.95). The phragmocone is circular in cross-section. The siphuncle is 0.25 mm in diameter (corresponding shell diameter 23 mm) and sub-central in position (shifted slightly towards the venter, situated 10 mm from the ventral side of the shell wall, 13 mm from the dorsal). The phragmocone has a maximum diameter of 25 mm and expands slowly with an angle of 3°. The convex side of the adoralmost preserved septum shows massive hyposeptal deposits surrounding the siphuncle (see Figs. 5 , 7C 1 ). In outline, these bilaterally symmetrical deposits resemble a figure “8”; they are more massive ventrally than dorsally ( Figs. 5 , 7C 1 ). The external surface of the shell is smooth. The adoral phragmocone fragment shows two straight and slightly oblique suture lines spaced 1.8 mm apart (at a corresponding shell diameter 25 mm). The specimen is accompanied by a small lingulate brachiopod and the bellerophontid Tropidodiscus pusillus .

Remarks. —The initial chamber of MWB S 06765 classified here as Orthoceratida indet. sp. 2 differs from Orthoceratida indet. sp. 1 in being smaller in dimensions, less ovoid and less constricted ( Figs. 4, 5 ). Hyposeptal cameral deposits are present. The only known cephalopod from the Ordovician of Bohemia comparable in shape and position of deposits is Orthoceras bonum Barrande,1867 .However, this species was previously speculated to represent an actinoceratid ( Marek 1999; Manda 2008; Evans et al. 2013) or alternatively a pseudorthoceratid (unpublished personal observation of MA and VT), none of which exhibit the initial chamber morphology described above in MWB S 06765.

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).

Fig. 1. Position of Ordovician rocks of the Prague Basin within the Bohemian Massif and the localities from which the herein studied specimens originate see the text below). 1, Praha-Šárka cihelna (brickyard); 2, Praha-Šárka; 3, Praha-Hanspaulka; 4, Malé Přílepy; 5, Lhotka u Berouna; 6, Těškov; 7, Mýto- Svatoštěpánský rybník; 8, Rokycany-Díly; 9, Volduchy. Adapted after Manda (2008).