Rhynchorthoceras conicum

Rhynchorthoceras conicum (Hisinger, 1837)

Fig. 14A–C, Tables 2–3

Orthoceratites conicus Hisinger, 1837: 29, pl. 9 fig. 5.

Orthoceras conicum – Boll 1857: 72, pl. 4 fig. 12. — Angelin 1880: 5, pl. 9 figs 2–3. — Rüdiger 1889: 18, pl. 1 fig. 4.

? Orthoceras conicum – Angelin 1880: pl. 9 fig. 1

Conorthoceras conicus – Troedsson 1931: 24

Rhynchorthoceras angelini – Hardt 1953: 44, text-fig. 9.

Rhynchorthoceras conicum – Dzik 1984: 133, pl. 38 figs 3–4, text-fig. 50d–e. — Kröger 2004b: 65. — Kröger et al. 2007: 1280.

Sinoceratidae, gen et sp. nov. cf. “ Conorthoceras ” conicum – King 1999: fig. 6a–b.

non Orthoceras conicum – Woodward 1856: 379, pl. 6 fig. 2.

non Nevadaceras conicum – Flower 1968: 43.

Diagnosis

Species of the genus Rhynchorthoceras with expansion angle of 9–13°; whorl profile circular. Shell surface smooth or decorated with extremely fine transverse and/or longitudinal lines. Siphuncle diameter 0.11–0.19 of whorl height, central or nearly central in position.

Type material

Not available for study; Hisinger (1837: pl. 9 fig. 5) did not specify a type but he figured one specimen that was re-figured by Boll (1857: pl. 4 fig. 12).

Material examined

GERMANY • 1 spec.; Brandenburg, Gellmersdorf (Angermünde); Ordovician; Schröter 1979 Coll.; MB.C.11715 • 2 specs; Brandenburg, Heegermühle; Ordovician, Upper Red Orthoceratite Limestone; MB.C.30499, MB.C.30500 • 3 specs; Brandenburg, Niederfinow; Ordovician, Upper Red Orthoceratite Limestone; Neben and Krueger Coll.; MB.C.30501 to MB.C.30503 • 1 spec.; Brandenburg, Niederfinow; Ordovician, Upper Red Orthoceratite Limestone; Neben and Krueger Coll.; MB.C.30504 • 1 spec.; Brandenburg, Niederlehme; Ordovician; Siegert 1914 Coll.; MB.C.11703 • 2 specs; Brandenburg, Oderberg; Ordovician, Upper Red Orthoceratite Limestone; Neben and Krueger Coll.; MB.C.30505, MB.C.30506 • 1 spec.; Brandenburg, Oderberg; Ordovician, Upper Red Orthoceratite Limestone; Remelé Coll.; MB.C.30507.

POLAND • 1 spec.; West Pomerania, Dewitz Castle, Dobra (former Dewitzburg bei Daber); Ordovician, Upper Red Orthoceratite Limestone; MB.C.30508 • 1 spec.; West Pomerania, Skowarcz (former Schönwarling), Gdańsk County; Ordovician, Upper Red Orthoceratite Limestone; Hoyers 1912 Coll.; MB.C.30509.

SWEDEN • 1 spec.; Dalarna; Ordovician; Coll.; MB.C.11721 • 2 specs; Island of Öland; Ordovician; Coll.; MB.C.11702, MB.C.11723 • 1 spec.; Island of Öland, Triberga; Ordovician, Upper Red Orthoceratite Limestone; Dames 1881 Coll.; MB.C.11722.

COUNTRY UNKNOWN • 2 specs; Ordovician; Otto Coll.; MB.C.11719.1, MB.C.11719.2.

Description

All specimens are fragments of faintly curved or orthoconic phragmocones (Fig. 14A–C) and represent various growth stages of the conch, with the smallest preserved whorl height of 3 mm and the largest of 40 mm. None of the specimens has initial parts of the phragmocone or a mature body chamber preserved. The expansion angles vary between 9° and 13° (median = 11°, n = 15; Fig. 13A). The relative chamber length is variable, the CLR is between 0.18 and 0.32. The siphuncle is central or almost central in position and its diameter is between 0.11 and 0.19 (median = 0.14, n = 9; Fig. 13B) of the whorl height. The shell surface is nearly smooth; only some specimens (MB.C. 11723a, MB.C.11721, MB.C.30501) possess delicate longitudinal and/or transverse lines or impressions.

Remarks

Troedsson (1931) stated that Orthoceratites conicus is the type species of a new genus Conorthoceras. However, Troedsson did not formally describe the genus, nor did he provide any diagnosis, descriptions or figures. The validity of the name Conorthoceras was disputed by Jaanusson & Mutvei (1953) and Teichert (1964), but Sweet (1958) considered the name as valid. The systematic position of O. conicus remained unclear, until Dzik (1984) assigned the species to Rhynchorthoceras, thereby acknowledging its lituitid affinities. King (1990, 1999) agreed that O. conicus represented a lituitid but suggested that the smooth shell surface would justify a separate genus. Later authors (e.g., Kröger 2004b; Kröger et al. 2007) retained the species in Rhynchorthoceras, which is also the approach followed herein.

Rhynchorthoceras conicum must not be confused with Nevadaceras conicum Flower, 1968. The latter is known from a single, fragmentary specimen, which has oblique sutures, compressed whorl profile and a wider siphuncle (0.25 of whorl height) located between the centre and venter. Thus, N. conicum can neither be synonymised with R. conicum, nor assigned to the order Lituitida.

Rhynchorthoceras conicum is almost identical, in conch proportions and internal conch morphology, to R. angelini, R. kranepuhlense sp. nov. and R. rugium sp. nov. (all described herein), but it can be readily distinguished by the smooth surface of its shell.

Geographic and stratigraphic occurrence

Sweden (in situ) and northern Germany and Poland (in erratics within Pleistocene gravels); Aseri to Lasnamägi regional stages (middle to late Darriwilian, Middle Ordovician).