Orthoceras regulare Schlotheim, 1820

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 59-60

Orthoceras regulare Schlotheim, 1820

Fig. 2B .

Orthoceras regulare Schlotheim ; Troedsson 1931: 13–16, 24–29 [cum syn.]

Material.— Forty specimens housed in the NRM, NMB, MMW, and GIH (in a large part the material of Troedsson 1931 a). Many of these specimens preserve the adult chambers of the phragmocone and the living chamber; sometimes the recrystallized shell is preserved.

Description.— Orthoceras regulare was described in great detail by Troedsson (1931). The current description is therefore mainly an abstract of Troedsson’s work supplemented by some new measurements and observations.

Conch form: The conch shows a circular cross section. With a maximum diameter: 40 mm, mean diameter of last chamber of the adult phragmocone: 25 mm (max: 29 mm, min: 19 mm) it is the largest Orthoceras . The conch shows an apical angle of 1.7–2.9° (mean e: 2.3°).

Ornamentation: The shell displays fine, narrowly spaced transverse striae; the elevated space between the striae appear imbricated and flat in nearly mature and mature specimens, and is sharp and acute in the juvenile parts of the shell. In addition to the transverse ornamentation, very minute longitudinal striae form a net−like ornamentation only detectable in well preserved specimens. The transverse banding forms an adapical lobe on the ventral side. The inner part of the shell is smooth, sometimes irregularly punctured. The cast shows a normal line at the ventral side of the phragmocone.

Phragmocone: Chambers 0.5 (mean) of the respective diameter in high. Sutures straight and septa concave. Cast of the mural areas displays fine longitudinal lines. Septal necks orthochoanitic. The central to subcentral tubular siphuncle shows a diameter of 0.15 (0.14–0.17) of the respective diameter of the conch. Endosiphuncular lining has been observed in the more apicad parts of the phragmocone (see Fig. 2B ). The deposits are more strongly developed in the ventral part of the shell.

Living chamber: The long adult living chamber is conical in shape. Three equiangularly arranged longitudinal impressions are found at about one third of the length of the adult living chamber. The impressions vary slightly in general shape but strongly in length (15–50 mm). They form 1–7 mm deep elongated notches in the shell. The conch at the position of the impressions is slightly constricted. The aperture widens rapidly orad of the constriction.

Distribution.— Orthoceras regulare occurs in the Seby and Folkeslunda Limestone (Lasnamägian) and in the Persnäs Limestone (Uhakuan) of Öland. It is also found in the Upper Grey Limestone (Lasnamägian) of the erratics of northern Germany and Poland.

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.