Gorbyoceras textumaraneum ( Roemer, 1861 )

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 : 43-45

Gorbyoceras textumaraneum ( Roemer, 1861)

Figs 11D, 12D–E, L–M, 18

Orthoceras textum-araneum Roemer, 1861: 58–59 , pl. 7 fig. 3a–b.

Orthoceras textum-araneum – Rüdiger 1889: 52–53.

Spyroceras textum-araneum – Teichert 1930: 280, pl. 5 fig. 6.

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

Spyroceras textum-aranaceum [sic] – Balashov 1953a: 212; 1955: 98–99, pl. 45 fig. 3; 1962b: 108, pl. 48 fig. 1.

Gorbyoceras textumaraneum – Dzik 1984: 121, text-fig. 49.15, pl. 35 fig. 6. ― Kröger & Isakar 2006: 158–159, figs 8c, 10c–d, 12a–b. ― Pohle et al. 2022: fig. 2.

Diagnosis

Slightly compressed orthocones with curved mature body chamber; adult conch diameter ca 40 mm; ornamented with three to four annulations in a distance equal to the corresponding conch diameter; annulations straight or very slightly oblique to the normal of the conch axis and slightly irregularly spaced; fine growth lines, about 15 per annulation; additionally ornamented with fine longitudinal lirae; suture lines in the troughs of the annulations and equally spaced with the annulations; parallel sutures and annulations; siphuncle eccentric in juvenile, subcentral in more adult growth stages; septal foramen approximately one tenth of the diameter. (Compiled from Roemer 1861 and Kröger & Isakar 2006.)

Material examined

ESTONIA • 1 spec.; Haapsalu holm; Adila Formation, Pirgu Regional Stage; TUG 1745-238 • 1 spec.; Vormsi Island , Hosholm shore; Adila Formation, Pirgu Regional Stage; TUG 1743-77 238 • 5 specs; Vormsi Island , Hosholm shore (N); Adila Formation, Pirgu Regional Stage; GIT 878-223 , GIT 878- 224 , GIT 878-282 to GIT 878-284 • 1 spec.; Mõnuste quarry; Kõrgessaare Formation, Vormsi Regional Stage; TUG 1745-193 • 1 spec.; Paluküla quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 225-968 • 5 specs; Vohilaid Island , Vohilaid shore (E); Adila Formation, Pirgu Regional Stage; GIT 878- 143 , GIT 878-299 to GIT 878-301 , GIT 878-306 .

Type locality and horizon

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

Description

The largest specimen of this species in the Estonian collections is TUG 1745-193 from Mõnuste quarry, Vormsi Regional Stage (Fig. 12L). The specimen is a nearly complete internal mold of a slightly curved body chamber and one chamber of the phragmocone with maximum conch height of 37 mm and conch width 41 mm at the adoral end of the preserved part of the body chamber. The body chamber is ca 100 mm long with conch height ca 32 mm and conch width ca 35 mm at its base, and with eight directly transverse annulations. The five adapical annulations of this specimen are regularly spaced, 10–12 mm apart, with widely rounded troughs and more sharper ridges. The three adoral annulations are less well pronounced and irregularly spaced with distances between 12–16 mm. On the adoralmost part of the specimen, traces of distinct irregularly spaced longitudinal lirae are preserved, which are ca 3–4 mm apart, and which would amount to ca 40 around the entire circumference. The two septa preserved at the adapical end of the specimen are 11 mm apart and are positioned in the mid-troughs of the respective annulation. Traces of the septal foramen and siphuncle are not preserved.

Seven specimens with adult body chambers are preserved (GIT 878-143, GIT 878-224, GIT 878-223, GIT 878-282, GIT 878-283, TUG 1745-193, TUG 1745-238) with conch heights at their bases ranging from 32–35 mm (mean = 33 mm); all are slightly curved and have in the adoral third, a section with irregularly spaced, less well pronounced, annulations (Fig. 12L–M).

The internal characters are preserved in GIT 878-299, a slightly curved fragment of a phragmocone with a diameter of 29–32 mm. There, the septal foramen has a diameter of ca 2.8 mm, is eccentric on the convex side of the conch curvature 12 mm from the conch margin at its adapical end. The septal necks are poorly preserved, but apparently suborthochoanitic, and the siphuncle is slightly expanded to nearly tubular in shape (Fig. 11D).

Remarks

The information available for this species regarding the number of longitudinal lirae across the circumference is confusing. Roemer (1861: 59) counted 22–24 lirae, but in the original illustration ( Roemer 1861: pl. 7 fig. 3) 17–18 lirae are visible in lateral view, suggesting a total of ca 35 lirae. There is no evidence that the number of lirae changed during ontogeny. Therefore, Kröger & Isakar (2006: 158) included number of “approximately 35 irregularly spaced longitudinal ridges” in their species diagnosis of G. textumaraneum .

In contrast, a photograph of the lectotype of G. textumaraneum in Dzik (1984: pl. 35 fig. 6) supports the 22–24 lirae count of Roemer (1861: pl. 7 fig. 3). The specimens described by Kröger & Isakar (2006) under G. textumaraneum have ca 40 lirae. The only Estonian specimen with surface characters preserved (specimen TUG 1745-193) also has ca 40 longitudinal lirae around the circumference. More well-preserved material is needed to evaluate the diagnostic value of this character (i.e., the number of longitudinal lirae) for G. textumaraneum . Strand (1934: 21) described annulated specimens (with three and a half annulations per length equal to diameter), ornamented with ca 40 longitudinal lirae from the Late Ordovician “Gastropod Limestone” under Spyroceras cf. clathrato-annulatum , which can be synonymized with the material described herein.

Additionally, growth stages of G. textumaraneum with conch diameters <ca 20 mm are poorly known. The species most conspicuous features are confined to the mature body chamber and a smaller species with similar general conch shape and ornamentation exists ( G. stumburi ). Fragments with diameters smaller than the adult diameter of G. stumburi (ca 20 mm) therefore cannot be assigned to either G. stumburi or G. textumaraneum . Five specimens (TUG 1745-275, 1745-300, 1745-301, TUG 1819-85, TUG 1827-61) with an ornamentation of the aspect of G. stumburi and G. textumaraneum and diameters> 20 mm and assigned to Gorbyoceras sp. are included in the measurements to illustrate the morphological variability of the entire cohort ( Fig. 18A–B ) (see measurements in supplementary data 3).

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.