Orthoceratidae M'Coy, 1844

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 59

Family Orthoceratidae M'Coy, 1844

Diagnosis.—Ortho− to slightly cyrtoceraconic Orthoceratacea with a shell that is smooth or elaborately ornamented and generally subcircular in cross section. Endosiphuncular deposits are absent or occur only at the most apical parts of the shell.

Remarks.— Dzik (1984) included the Michelinoceratidae Flower, 1945 , the Geisonoceratidae Zhuravleva, 1959 , and the Folioceratidae Collins, 1969 within the Orthoceratidae M’Coy, 1844 . He further subdivided the family level only at the generic level. Because of the pure extent and great differences within the species that are included in the family Orthoceratidae it seems necessary to divide the family into subfamilies and to define the Geisonoceratidae as a separate family. The practice of Sweet (1964) and e.g., Zhuravleva (1978), who subdivided the Orthoceratidae into the subfamilies will therefore be followed here. Sweet (1964) and Zhuravleva (1978), however, defined the Orthoceratidae by the absence of endosiphuncular deposits. As can be shown the genus Orthoceras developed endosiphuncular deposits in the most apicad parts (see Fig. 2A, B ). Consequently, by definition of Sweet (1964) the genus Orthoceras is not a part of the Orthoceratidae or the entire family is obsolete. The dilemma is most easily solved by expanding the scope of the Orthoceratidae . Although no clear phylogenetic relationships are visible yet, this expansion should also be a consequence of the general observation of the evolutionary trend (or polarity) of the early Orthocerida in direction of the suppression of endosiphuncular deposits on the one side and of increased differentiation of the endosiphuncular deposits on the other side ( Geisonoceratidae , Pseudorthoceratidae ).

Subfamilies included.— Orthoceratinae M’Coy, 1844 , emend. Sweet (1964), Michelinoceratinae Flower, 1945 , emend. Ristedt (1968).

Figures

Fig. 2. Longitudinal sections of the apical parts of the phragmocone of Orthoceras. A. Orthoceras sp. from Czerwińsk near Kwidzyn, N Poland, NMB C. 5381, Lasnamägian (apical diameter 3.3 mm). B. Orthoceras regulare from Bydgoszcz, Poland, NMB C. 5382(apical diameter 4.5 mm). Endosiphuncular and cameral deposits dotted.