REVISION OF SOME COMMON CARBONIFEROUS GENERA OF NORTH AMERICAN ORTHOCERID NAUTILOIDS

Journal of Paleontology, Sep 2005 by Kröger, B, Mapes, R H

Occurrence.-Mississippian, Meramecian, and Chesterian of Arkansas and Oklahoma, USA.

Discussion.-Mitorthoceras crebriliratum is distinguished from M. perfilosum mainly by ornament, which in the former is flatter and asymmetrical (Fig. 10). Gordon (1960) distinguished between the two species by the spacing of the lirae, the curvature of the septal neck, and the dimension of the cameral deposits. These differences are sufficient to distinguish between the two species. The two size classes discerned in the adult body chambers probably represent sexual dimorphs.

Genus HEBETORTHOCERAS new genus

Type species.-Orthoceras unicamera Smith, 1938, Pennsylvanian, Desmoinesian; Boggy Formation, Buckhorn Asphalt, Murray County, Oklahoma, USA.

Included species.-H. indianum (Girty, 1909), H. unicamera (Smith, 1938), H. tricamerae (Smith, 1938), H. brokenarrowense n. sp., and possibly H.? caneyanum (Girty, 1909).

Diagnosis.-Smooth or slightly ornamented orthocones with very low apical angle; nearly circular cross section; apex blunt, with very short, cup-shaped first chamber; slightly inflated first three chambers; suture straight; septal necks cyrtochoanitic, siphuncle central; connecting ring cylindrical, slightly constricted at septal necks; siphuncular deposits only very near apex, strong lamellar episeptal and weaker hyposeptal deposits.

Etymology.-From hebetis, hebes (Latin) = blunt, dull.

Discussion.-Hebetorthoceras includes the earliest members of a group of pseudorthocerids that became common in the Permian with forms like Bitaunioceras Shimizu and Obata, 1936 and SAzkanoceras Shimansky, 1954. The genus resembles Bitaunioceras which has, like Hebetorthoceras, a small apex with a very short first chamber, widely spaced septa, a small apical angle, and an empty, tubular siphuncle, but Bitaunioceras differs by its apex undulation, more expanded apical chambers, shell constrictions, and ornament (compare description in Shimansky, 1954). In addition, a close relationship to the Permian genus Aidaroceras Doguzhaeva, 2002 is apparent because of the very short first chamber in both Hebetorthoceras and Aidaroceras and their generally similar apex shape. However, Aidaroceras differs from Hebetorthoceras by its ornament, subcentral siphuncle, and more pointed apex. There are several species from the Desmoinesian Buckhorn Asphalt of Oklahoma which might belong to Hebetorthoceras or which may be closely related to it, namely, Pseudorthoceras sp. of Fischer and Teichert (1969), Orthoceras sp. A, Orthoceras sp. B, and, perhaps, Kwnoceras (?) sp. of Ristedt (1971). Whether any of these are synonyms of one of the species assigned by Smith (1938) to Orthoceras Bruguiére, 1789 remains an open question at this time. A description of this particularly rich pseudorthoceratid fauna, which includes material with well-preserved apex characters, is beyond the scope of this paper, and needs to be examined in detail in a future work.

HEBETORTHOCERAS UNICAMERA (Smith, 1938)

Figure 9.2

Orthoceras unicamera SMITH, 1938, p. 7-8, pi. 1, rigs. 1-3; GORDON, 1965, p. 106; HEWITT, 1984, p. 673; DZIK, 1984, p. 105, text-fig. 39.50; BRAND, 1987, p. 257.