REVISION OF SOME COMMON CARBONIFEROUS GENERA OF NORTH AMERICAN ORTHOCERID NAUTILOIDS

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

According to Miller et al. (1933, p. 88), the apices of Mooreoceras are round as in Pseudorthoceras, but lack the curvature of the latter. This statement within the diagnosis of Mooreoceras cannot be verified, because no apical part of either the type or any other Mooreoceras was described by Miller et al. (1933) or later by Miller and Owen (1934) or Miller and Moore (1938). Gordon (1965) and Ristedt (1971) figured apices of Mooreoceras but could not assign the specimens to any certain species. Gordon (1965, p. 112) included the phrase "apex rather blunt, bullet shaped: tip with slightly cyrtoconic exogastric curvature" within the emended diagnosis of the genus, but the investigated material, or the evidence for this statement, is not specified. Later, Kröger and Mapes (2004) figured apices assigned to two species, Mooreoceras imoense Kröger and Mapes and Mooreoceras striatulum Kröger and Mapes. The extensive collection (n > 150) in the repository at Ohio University supports a detailed investigation of the apical characters of specimens assigned to P. knoxense or M. normale. We attempted to distinguish M. normale from P. knoxense by the shape of the cross section. We found that the shape and dimensions of the apex are essentially the same in specimens assigned either to P. knoxense or to M. normale, namely as described by various authors for the first (Girty, 1915; Gordon, 1965; Ristedt, 1971; Blind, 1988). Instead, the size and morphology of the putative Mooreoceras normale and Pseudorthoceras knoxense vary extremely in the investigated sample (Fig. 2.1, 2.2). More importantly, this demonstrates that there are no discernible differences between the two species in discussion (see Table 2, Figs. 5, 6). Additionally, we checked the entire sample for differences in the Morrowan, Desmoinesian, and Virgilian time intervals. Again, no significant morphological differences in the subsamples are discernible (Table 3).

Thus, our investigations show that Mooreoceras normale cannot be distinguished from Pseudorthoceras knoxense. Mooreoceras normale is therefore a junior synonym of Pseudorthoceras knoxense. Because M. normale is the type of the genus Mooreoceras, the entire genus is considered to be invalid and should be abandoned. Mooreoceras, prior to our evaluation, comprised more than 40 species. The generic identity of each of these species must be checked in the future.

Genus MITORTHOCERAS Gordon, 1960 emended

Type species.-Mitorthoceras perfilosum Gordon, 1960.

Emended diagnosis.-Small orthoconic pseudorthoceratids, ornamented with transverse lirae and striae; lirae gently sinuous; apex blunt, or with a faint acute tip, straight, first six chambers inflated; septa saucer-shaped, at right angles to the growth axis; septal necks cyrtochoanitic; siphuncle subcentral; connecting rings subcylindrical, slightly constricted at septal necks; annulosiphuncular deposits; strong lamellar episeptal and weaker hyposeptal deposits.

Discussion.-The genus was originally based on two characters: the transverse liration and the "dolorthoceroid" siphuncle Gordon (1960, p. 135). But, as shown by Kröger and Mapes (2004) and by this study, these characters occur in orthocerids of different affinities (e.g., Ristedtoceras Kröger and Mapes, 2004, Hebetorthoceras n. gen.) at very different growth stages. As a consequence of the different apex characters explored in putative Mitorthocems, the genus must be restricted to those species with an apex that resembles the apex of the type species. Therefore the Mississippian species "M." girtyi Gordon, 1965, which differs strongly in the shape of the embryonic shell, must be excluded from the genus. Also excluded are the Devonian species "M." instabile Zhuravleva, 1978 and "M." aktjubense Zhuravleva, 1978, both of which differ significantly in septal neck shape and the shape of the lirae. Therefore, at this time, the genus Mitorthoceras appears to be an exclusively Mississippian pseudorthoceratid restricted to North America.