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. 4

Fig. 6B .

Material. — Two specimens (MWB S 06827, 06828), counterparts of juvenile shell parts from the locality Volduchy ; Šárka Formation , lower and middle Darriwilian Stage, Middle Ordovician; Czech Republic .

Description. —MWB S 06827 is a counterpart of the embryonic shell including the initial chamber. The length of the specimen is 3.2 mm. The initial chamber (= first phragmocone chamber) is bean-shaped in median section, 1.8 mm in diameter. The phragmocone is constricted at a distance of 1.3 mm from the tip of the initial chamber and at 1.5 mm of shell diameter. This constriction is assumed to correspond to the position of the first phragmocone septum; the shape ratio of the initial chamber is then 0.72 and constriction ratio 0.83. The phragmocone is circular in cross-section. Longitudinal, faintly sinuous shell ornamentation; it disappears at the adoral part of the specimen where it is replaced by slightly oblique transverse sculpture.

Figures

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.