REVISION OF SOME COMMON CARBONIFEROUS GENERA OF NORTH AMERICAN ORTHOCERID NAUTILOIDS

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

MITORTHOCERAS CREBRILIRATUM (Girty, 1909) emended

Figures 7.7-7.9, 10.2

Orthocems crebriliratum GIRTY, 1909, p. 46, pi. 6, figs. 9a, 10; GORDON, 1960, p. 136.

non Mooreoceras crebriliratum (URTY). MILLER AND YOUNGQUIST, 1948, p. 653, pi. 94, figs. 4, 5.

non Mooreoceras crebriliratum (GiRTY). MILLER, YOUNGQUIST, AND NIELSEN, 1952, p. 152, pi. 25, figs. 10-16.

Dolorthoceras aff. D. crebriliratum (GIRTY). CORDON, 1957, p. 24, pl. 2, figs. 8-10.

Mitorthoceras crebriliratum (GiRTY). CORDON, 1965, p. 122, 123, pi. 7, figs. 19, 20, 28; SHIMANSKY, 1968, p. 81; ZHURAVLEVA, 1978, p. 122; NIKO, 1996, p. 358.

Mitorthoceras aff. M. crebriliratum (GiRTY). GORDON, 1965, p. 123.

Dolorthoceras cerebriliratum (GIRTY). HISTON, 1998, p. 42.

Emended diagnosis.-Mitorthoceras with nearly circular cross section; ornamented with narrow, shallow, asymmetric angular lirae, with steep slope toward the aperture, lirae straight; expansion of the conch less than 5°; apex blunt, with first six chambers inflated, cicatrix slightly dorsal, distance of septa less than diameter of cross section; siphuncle ventrally eccentric, one-seventh of diameter, cyrtochoanitic septal necks, annulosiphonate deposits; hypo- and episeptal deposits.

Description.-The characteristics of parts of nearly mature shells were well described by Girty (1909), Gordon (1965), and Davis and Campbell (1966). New collections at Ohio University provide the basis for a description of the mature body chamber.

Five adult body chambers were examined, comprising two size classes. Body chamber of specimens OUZC 5124 and OUZC 5125, with maximum diameters of 30 mm and 35 mm, respectively, diameter at last septum 23 mm and 25 mm, respectively, and length of 65 mm and 80 mm, respectively. Both specimens with straight aperture and apertural shell thickening, which causes 5 mm wide constriction on mold of body chamber. Both specimens display straight, transverse wrinkle layer with fingerprint pattern, approximately 10 wrinkles per millimeter (Fig. 7.8). Wrinkle layer expands backward 20 mm (OUZC 5126), 22 mm (OUZC 5127), respectively, from aperture. Body chamber with length of chamber 20 mm (OUZC 5126), 27 mm (OUZC 2127), with maximum diameter of 11 mm (OUZC 5126), 13 mm (OUZC 5127) (Fig. 7.9), and diameter at last septum 9 mm (OUZC 5126), 11 mm (OUZC 5127). Specimens OUZC 5124, 5125 with wrinkle layer similar, as in specimens OUCZ 5124, 5125, expands 12 mm, 15 mm backward from aperture. Apex of specimen OUZC 5120 (Fig. 7.7) blunt, slightly compressed cross section; first 5 mm inflated, with diameter about 1.4 mm at distance of 1.5 mm from tip, faint constriction of the shell at distance of about 5 mm from tip, with diameter of 1.3 mm; ornamentation initially very gentle, with irregularly spaced growth lines, successive development of more accentuate lirae at about 10 mm from apex; siphuncle slightly ventral, nearly central, tubular.

Material examined.-One apex (OUZC 5120), five adult body chambers (OUZC 5121-5125), and more than 20 fragments of individuals at different growth stages in the repository at Ohio University.