Gorbyoceras clathratoannulatum ( Roemer, 1861 ), 2025

Kröger, Björn, 2025, The Lyckholm acme of cephalopods - Review of the late Katian (Vormsi-Pirgu regional stages) Ordovician cephalopods of Estonia, European Journal of Taxonomy 978, pp. 1-169 : 40-42

Gorbyoceras clathratoannulatum ( Roemer, 1861) comb. nov.

Figs 11B–C, 12C, H, 18

Orthoceras clathrato-annulatum Roemer, 1861: 57–58 , pl. 7 figs 4a–b.

Orthoceras clathrato-annulatum – Roemer 1885: 52, 67, fig. 10a–b, pl. 3 fig. 10. ― Rüdiger 1889: 51–52, 85. ― Sauramo 1929: 8, 10. ― Dzik 1984: pl. 35 fig. 4a–c.

Spyroceras clathrato-annulatum – Teichert 1930: 278, 280, pl. 5 fig. 5. ― Balashov 1953a: 212.

Spyroceras cf. clathrato-annulatum – Strand 1934: 21 (partim).

Diagnosis

Annulated orthocones with circular conch cross section; adult conch diameter ca 20 mm; mature body chamber with decreasing conch diameter toward aperture and slightly bent against adapical conch part; ornamented with three to four annulations in a distance equal to the corresponding conch diameter; annulations straight transverse or very slightly oblique and slightly irregularly spaced; ornamented with ca 30 fine longitudinal and subordinate transverse lirae, about 10–15 transverse lirae per cycle of annulations; suture lines located in the troughs of the annulations and equally spaced as annulations; parallel sutures and annulations; siphuncle central, barrel-shaped, expanded in early growth stages, nearly tubular in late growth stages; septal foramen approximately one tenth of the diameter, septal necks short cyrtochoanitic to suborthochoanitic, endosiphuncular and cameral deposits not known.

Material examined

ESTONIA • 1 spec.; Vormsi Island , Hosholm shore; Adila Formation, Pirgu Regional Stage; GIT 840- 138 • 3 specs; same data as for preceding; TUG 899-80 , TUG 1445-232 , TUG 1745-14 • 7 specs; Vormsi Island , Hosholm shore (tower locality); Adila Formation, Pirgu Regional Stage; GIT 840-265 , GIT 840-266 , GIT 878-285 to GIT 878-287 , GIT 878-298 , GIT 878-305 • 1 spec.; Vohilaid Island ; Vohilaid outcrop 8 (8 after B. Stein), Pirgu Regional Stage; GIT 878-149 .

Type locality and horizon

Zawidowice by Oleśnica, Poland; erratic boulder, Vormsi–Pirgu regional stages (“Lyckholm Stufe” of Teichert 1930).

Description

Nine specimens with mature body chambers are in the Estonian collections (GIT 840-265, -266, GIT 878-149, -285, -286, -287, -298, -305, TUG 899-80, TUG 1445-232, TUG 1745-14) (Fig. 12H). Amongst these specimens’ the conch diameter at the base of the body chamber varies between 18–22 mm (mean = 20.1 mm). At the position of the base of the mature body chamber the conch is slightly curved. In specimen GIT 878-149, the body chamber deviates ca 10° against the adapical part of the conch; it is ca 45 mm long and decreases from 22 mm at its base to a diameter of 19 mm at ca 30 mm from the base, indicating maturity. Adorally, toward the mature aperture, the diameter again increases to 20 mm, forming an irregular constriction with one or two shallower and irregularly spaced annuli. Similar body chamber modifications occur in the other specimens.

The conchs are annulated with ca three to four annulations in a distance equal to the corresponding conch cross section (mean RCL = 0.28, n = 16). There is no ontogenetic trend of decreasing or increasing relative distance of the annulations ( Fig. 18B ). The conch surface is poorly preserved in all specimens, but where visible, ca 28–30 shallow longitudinal lirae occur around the circumference.

The maximum apical angle measured in a juvenile specimen is ca 8° (specimen GIT 840-138). The angle of expansion decreases with increasing conch diameter ( Fig. 18A ). The septa are equally spaced as the annulations, are invariably positioned within the troughs of the annulations, and like the annulations, are directly transverse. The position of the siphuncle is central or nearly so. In specimen GIT 840-138, the septal foramen is ca 1 mm in diameter where the conch diameter is 10 mm and in specimen GIT 878-149, the septal foramen is 1.5 mm where the conch diameter is 22 mm. The septal necks are short cyrtochoanitic in early growth stages (specimen GIT 840-138, Fig. 11C) and suborthochoanitic in late growth stages (specimen GIT 878-149, Fig. 11B). The shape of the siphuncle accordingly varies from an expanded barrel shape in early growth stages to subtubular in late growth stages.

Remarks

Roemerʼs (1861) original description of O. clathrato-annulatum is based on material, which very likely represents multiple taxa of annulated orthocones. The original description emphasized the reticulate pattern (with finer transverse than longitudinal lirae), and the decreasing conch diameter of the mature body chamber as diagnostic characters of this species ( Roemer 1861: 58). No explicit information regarding the number of longitudinal lirae was provided by Roemer (1861). The only illustrated specimen has 22 longitudinal lirae in lateral view (suggesting a total of ca 40–45 lirae), and ca three to four annulations in a distance similar to the conch diameter (Roemer 186: pl. 7 fig. 4).

Roemer (1861: 58) distinguished O. clathrato-annulatum from the similar O. textum-araneum by the position of the siphuncle (central in the former), the adult size (much larger in the latter), and the shape of the mature body chamber (curved in the latter).

However, Dzik (1984: pl. 35 fig. 4a–c) figured a fragment of a phragmocone as holotype of O. clathrato-annulatum with no traces of the fine reticulate pattern described in the original description, leaving some uncertainty about the ornamentation of this species. Moreover, this specimen has great similarities to specimens described herein under Striatocycloceras hosholmensis sp. nov. The internal characters and details of ornamentation of this specimen, however, remain unknown.

Teichert (1930), followed Roemerʼs (1861) concept of O. clathrato-annulatum (i.e., reticulate ornamentation, relatively small mature body chamber with decreasing conch diameter), and distinguished two, roughly similar species in the strata of Lyckholm, Estonia: a large-sized Spyroceras textumaraneum and a smaller Spyroceras clathrato-annulatum . A reinvestigation of Roemerʼs (1861) type material is needed to clarify if Teichertʼs (1930) interpretation of O. clathrato-annulatum is consistent with the types or if this species needs a radical reinterpretation (i.e., if S. hosholmensis sp. nov. is a subjective junior synonym of O. clathrato-annulatum and if the species described herein under G. clathratoannulatum represents a new species).

For the time being, the diagnosis, given herein, follows Teichertʼs (1930) interpretation. Details of the ornamentation of O. clathrato-annulatum , and of the siphuncle and septal necks, which are missing in Roemer (1861) and Teichert (1930), are appended to the diagnosis, based on the new Estonian material.

Strand (1934) described several specimens from the Late Ordovician “Gastropod Limestone” of Norway under Spyroceras cf. clathrato - annulatum . The Norwegian specimens, ornamented with ca 40 longitudinal lirae, and described under this taxon, probably belong to G. textumaraneum or Gorbyoceras? stumburi ( Kröger 2013) (see below). The specimen described by Strand (1934: 21), ornamented with ca 60 longitudinal lirae and an apical angle of 7–8° at conch diameters up to 21 mm, is certainly not a G. clathratoannulatum , because at this size, this species has a nearly tubular conch or is decreasing in diameter, as well as possessing a smaller number of longitudinal lirae. It can probably be synonymized with specimen TUG 939-78, described herein as Palaeodawsonoceras? sp. A.

Figures

Fig. 18. Diagrams of morphological variability of specimens of Gorbyoceras Shimizu & Obata, 1935 from Vormsi–Pirgu regional stages, Estonia. A. Angle of expansion. Note the low values (nearly mature specimens of G. sp. and G. stumburi Kröger 2013). B. Distance of annuli relative to corresponding conch height. A species distinction of specimens smaller ca 15–20 mm is not possible based on angle of expansion and relative chamber distance. Circles = G. textumaraneum (Roemer, 1861); filled circles = G. clathratoannulatum (Roemer, 1861) comb. nov.; stars = G. sp., and G. stumburi.