REVISION OF SOME COMMON CARBONIFEROUS GENERA OF NORTH AMERICAN ORTHOCERID NAUTILOIDS

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

INTRODUCTION

READING THE voluminous lists of synonymy of the taxa described in this report, it may seem useless at first glance to add yet more descriptions and potentially continue an already confused situation. However, as taxonomists know, long lists of synonymy may indicate that certain taxa serve as a "garbage-can" for specimens which lack distinctive morphological features. An example of a "garbage-can taxon" is the pseudorthoceratid genus Mooreoceras Miller, Dunbar, and Condra, 1933, which originally was very broadly defined and therefore comprised about 40 species.

Unfortunately, once the type of such a taxon with a very simple or "basic" morphology is widely known, repeatedly examples of poorly preserved specimens may be referred to it, thereby expanding it unnaturally, exemplifying the "garbage-can taxon" effect. In cephalopods, the orthocerid Michelinoceras Foerste, 1932 provides a good example. The type of the genus has been well known since the works of Ristedt ( 1968) and Serpagli and Gnoli (1977). Now the genus includes more than 180 species, which are, for the most part, established on very poorly preserved and very poorly known specimens. Thus, finding order in a "garbage-can taxon" may resemble the unending task of Sisyphus.

Several species discussed in this paper have either been assigned to such broadly defined taxa or may even define the type of such a taxon. These species belong to the most common and widespread cephalopods in the Carboniferous of North America. They represent the characteristic Carboniferous orthocerids. Finally, one genus described herein, Pseudorthoceras Girty, 1911, is the type for the fairly well-known high-rank taxon Pseudorthocerataceae (see Sweet, 1964, p. K242). Consequently, a more realistic taxonomic placement of these taxa is of central interest not only for improved future work on orthocerids, but also for the understanding of the evolution of Carboniferous marine faunas in general.

In order to develop a more realistic placement, we identified and considered new or previously unknown features in the problematic genera Mooreoceras, Pseudorthoceras, and Mitorthoceras Gordon, 1960, especially features in the early ontogenetic stages, and we investigated intraspecific variation of crucial characters.

MATERIAL

More than 1,100 specimens were used for the current investigation. They were collected during the last three decades by one of us (RHM) at numerous localities within the central and southwestern United States. The collection sites are given in the Appendix. The studied material comprise a range of very different preservational states, including testate, calcified external molds and pyritized and goethite/limonite steinkerns. In most cases the shell, when present, is recrystallized to sparitic calcite. The material is reposited in the Ohio University Zoological Collection, Athens (OUZC). For comparison, we examined type materials at the Department of Geological Sciences, University of Iowa, Iowa City (SUI), and at the Smithsonian Institution, Washington, DC (USNM).

METHODS

This investigation is based on measurements on more than 1,000 specimens to determine the intraspecific variation of the discussed species. In all specimens we measured the dorsoventral diameter of the shell (d^sub dv^), the lateral diameter of the shell (d^sub 1^), the lateral diameter of the shell at the distance (1) in direction of growth relative to d^sub 1^, which is d^sub 2^, the septal spacing at d^sub 1^, which is l^sub ss^, and the distance of the siphuncle from the venter at d^sub 1^, which is h^sub s^ (see Fig. 1). From these six values we calculated four basic parameters of the shell: the shape of the cross section, cs, which is d^sub dv^/d^sub 1^; the expansion rate, cot (a) which is the difference between d^sub 1^ and d^sub 2^ divided by 1; the septal spacing, ss, which is d^sub dv^/l^sub ss^; and the position of the siphuncle, ps, which is h^sub s^/d^sub dv^. Using all four parameters, cs, α, ss, and ps, we calculated the Eigenshape values and several simple statistical characters with the program PAST (Hammer et al., 2001).

SYSTEMATIC PALEONTOLOGY

Order ORTHOCERIDA Kuhn, 1940

Superfamily PSEUDORTHOCERATACEAE Flower and Caster, 1935 (nom. transi. Sweet, 1964 ex PSEUDORTHOCERATIDAE Flower and Caster, 1935)

Family PSEUDORTHOCERATIDAE Flower and Caster, 1935 emend. Flower, 1939

Genus PSEUDORTHOCERAS GIRTY, 1911 emend. Kröger and Mapes, 2004

PSEUDORTHOCERAS KNOXENSE (McChesney, 1859)

Figure 2.1, 2.2

Orthoceras knoxensis MCCHESNEY, 1859, p. 69.

Pseudonhoceras knoxense (McCHESNEY). GIRTY, 1911, p. 143, 144; GIRTY, 1915, p. 227-234, pi. 27, figs. 1-6; KRÖGER AND MAPES, 2004, p. 562, 563, figs. 1.7, 4.7, 4.15, 5.9 (see for prior synonymy).

Diagnosis.-Pseudorthoceratidae with smooth shell; circular or slightly depressed cross section; siphuncle central to subcentral; blunt embryonic shell slightly cyrtoconic, faint longitudinal lirae; first segment of siphuncle bulged outward into first chamber; septal neck of first septum short and orthochoanitic; faint nepionic constriction at approximately 3 mm adorai of apex.