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. 3

Fig. 7B.

Material.— One specimen (NM L 46565), juvenile shell, from the locality Praha-Šárka ; Šárka Formation , lower and middle Darriwilian Stage, Middle Ordovician; Czech Republic .

Description. —The specimen is a counterpart of a juvenile shell with only the adoralmost septum preserved; the initial chamber is broken off. Total length is 25 mm. Minimum preserved diameter of the phragmocone is 1.7 mm, the maximum diameter is 4.5 mm; the phragmocone is circular in cross-section. The siphuncle is narrow, 0.7 mm in diameter (at maximum shell diameter) and central in position. The phragmocone expands with an angle of 7°. The external shell surface shows irregular, fine and straight transverse ornamentation; in the adapical and middle parts of the specimen the ornamentation is accentuated and regularly developed.

Remarks. —NM L 46565 has an angle of expansion comparable to Orthoceratida indet. sp. 1 but the former must have had a much smaller initial chamber. In its small size, NM L 46565 resembles Orthoceratida indet. sp. 2 but the latter has a lower angle of expansion. Based on the comparisons to Orthoceratida indet. sp. 2, the initial chamber in NM L 46565 probably did not exceed 1.9 mm in diameter.