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

publication ID

https://doi.org/ 10.4202/app.00721.2020

persistent identifier

https://treatment.plazi.org/id/0391878B-7A7A-FF87-A019-FC38F690EDF7

treatment provided by

Felipe

scientific name

Orthoceratida
status

 

Orthoceratida View in CoL indet. sp. 2

Figs. 5 View Fig , 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 GoogleMaps .

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 View Fig , 7C 1 View Fig ). In outline, these bilaterally symmetrical deposits resemble a figure “8”; they are more massive ventrally than dorsally ( Figs. 5 View Fig , 7C 1 View Fig ). 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 View Fig ). 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.

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