Nybyoceras intermedium Teichert, 1930

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 : 17-19

Nybyoceras intermedium Teichert, 1930

Figs 7C , 8C–D, G , 9A

Nybyoceras intermedium Teichert, 1930: 288 , 290, pl. 7 fig. 22.

Nybyoceras intermedium – Foerste & Teichert 1930: 277. ― Teichert 1933: 149. ― Strand 1934: 63. ― Balashov 1953a: 210; 1975: 63–64, pl. 5 fig. 10. ― Stumbur 1956: 176. ― Flower 1957: 24. ― Saladzius 1966: tab. 1. ― Barskov 1972: 81.

Emended diagnosis

Nybyoceras ornamented with fine, wavy longitudinal lirae, with subcentral to eccentric siphuncle position (RSP 0.3–0.5), with moderately expanded siphuncular segments (SCR ca 1.5–2); incipient annuli grow in adoral direction on the adoral surface of the septa; fully developed annuli are asymmetrically developed in longitudinal direction and are heavier on the ventral side. (Compiled from Teichert 1930.)

Material examined

ESTONIA • 1 spec.; Hiiumaa Island; Kõrgessaare quarry , Vormsi Regional Stage; TUG 939-50 • 1 spec.; Paluküla quarry ; Kõrgessaare quarry , Vormsi Regional Stage; GIT 426-573 • 1 spec.; Moe stratotype; Moe Formation , Pirgu Regional Stage; TUG 939-49 • 1 spec.; Pahkla; Moe Formation , Pirgu Regional Stage; TUG 1745-293 . (See also supplementary data 1)

Type locality and horizon

Niibi, Estonia; Adila Formation, Pirgu Regional Stage.

Description

Specimen GIT 426-573 is a fragment of a phragmocone, its conch diameter cannot be measured because of its incomplete preservation. The connecting ring is not preserved. The septal necks are short armenoceratoid, adnate. The septal foramen is 4–5 mm wide where the chamber length is 9.5 mm. Incipient endosiphuncular annuli are developed ventrally along the adoral surface of the septum ( Fig. 8C ).

Specimen TUG 939-49 is a 98 mm long fragment of a phragmocone with a diameter of 26–43 mm (angle of expansion = 10°) which has a well-preserved conch surface. The surface is ornamented with fine wavy longitudinal lirae; ca seven of them occur per millimeter. The lirae produce a fine Zigzag pattern ( Fig. 9A ).

Specimen TUG 1745-293 ( Fig. 8G ) is a fragment of a phragmocone 46 mm long and increasing in conch diameter 21–29 mm (angle of expansion = 10°). In this length, eight chambers occur (RCL: 0.23– 0.24). The conch cross section is circular, and the surface is poorly preserved. The siphuncle is slightly eccentrically positioned (RSP: 0.45–0.46) and broadly expanded; where the conch diameter is 27 mm, one segment is 9.5 mm high and 6.2 mm long (SCR = 1.53) and the septal foramen has a diameter of ca 2 mm. The septal necks are short and adnate. The connecting ring forms wide adnate areas on the adapical surfaces of the septa. On the adoral surface of the septa the adnate area is only developed ventrally. Thick endosiphuncular deposits are present. These are more strongly developed on the adoral side of the septal foramen than the adapical side. Hypo- and episeptal cameral deposits are restricted to the ventral side of the siphuncle.

Specimen TUG 939-50 ( Fig. 8D ) is a fragment of a phragmocone with the internal characters preserved. Where the diameter is 23 mm, the chambers are 6 mm long (RCL = 0.26), the septal foramen is 2.8 mm in diameter, the connecting rings inflate to 9 mm in height (SCR = 1.5), and the septal necks are apparently short adnate. Thick, asymmetrically developed endosiphuncular annuli and thin mural, episeptal and hyposeptal deposits occur.

Remarks

Teichert (1930) distinguished N. intermedium from the two co-occurring species of Nybyoceras in the type locality Niibi by three features: (1) its narrower siphuncle (relative to the corresponding conch diameter), (2) its more centrally positioned siphuncle, and (3) unique growth pattern of the endosiphuncular annuli. The material available herein allows for a revision of Teichertʼs (1930) diagnosis:

(1) The relative siphuncular diameter of the type of N. intermedium is 0.31 compared to 0.55 in the specimen determined as N. bekkeri by Teichert (1930: pl. 7 fig. 19). However, often in actinocerids, complete conch cross sections are not available. Therefore, it is better to use the SCR for comparison, which is 1.6–1.9 in the type of N. intermedium , and 2.1–2.5 in the types of N. bekkeri ( Troedsson 1926: pl. 63 figs 2–3).

(2) The figured specimens of the two species in Teichert (1930) differ only slightly in RSP with 0.30 in N. intermedium , and 0.26 in N. bekkeri . Two specimens, assigned here to N. intermedium , have subcentral siphuncle positions (TUG 939-49, 1745-293), where the conch diameter is between 20– 29 mm. In the type specimen, the more eccentric siphuncular position occurs at diameters> 60 mm. This is interpreted here as an ontogenetic change in the siphuncular position, and it is included in the diagnosis of N. intermedium .

(3) Teichert (1930) distinguished N. intermedium from N. bekkeri based in the growth pattern of the endosiphuncular annuli. In N. intermedium , he found annuli, which initially grew toward the adoral side of the septal neck and septum. Little is known about the variability in the development of the annuli. When fully developed, the annuli of N. bekkeri and N. intermedium are virtually indistinguishable in dorsoventral median section as both exhibit stronger development the ventral adoral quadrant (compare Troedsson 1926: pl. 63 fig. 3). Additionally, the high potential for misleading interpretations of 3D-structures of endosiphuncular annuli have been described by Pohle et al. (2024) and Turek & Aubrechtová (2024).

In conclusion, the relative expansion of the siphuncle seems to be the most reliable character to distinguish between N. bekkeri and N. intermedium . Also included in the diagnosis is the ornamentation, which is now known from this species from specimen TUG 939-49.

Figures

Fig. 7. Actinocerida Teichert, 1933 of the Vormsi–Pirgu regional stages, Estonia. A. Nybyoceras bekkeri Troedsson, 1926, specimen GIT 426-1120 from Kasari, Vormsi Regional Stage, dorsal view. B. Gorbormoceras vohilaidense gen. et sp. nov., holotype GIT 878-235, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view. C. Nybyoceras intermedium Teichert, 1930, specimen TUG 939-49, from Moe stratotype outcrop, Pirgu Regional Stage, lateral view. Scale bar = 10 mm for all figures, same scale in A–C.

Fig. 8(preceding page). Median sections of phragmocones of ActinoceridaTeichert, 1933 of the Vormsi– Pirgu regional stages, Estonia. A–B. Nybyoceras bekkeri Troedsson, 1926. A. Specimen GIT 426-1107 from Paluküla quarry, Vormsi Regional Stage. B. Specimen GIT 426-1120 from Kasari, Vormsi Regional Stage. C–D, G. Nybyoceras intermedium Teichert, 1930. C. Specimen GIT 426-573 from Paluküla quarry, Vormsi Regional Stage. D. Specimen TUG 939-50 from Kõrgessaare quarry, Hiiumaa Island, Vormsi Regional Stage. E. Ormoceras heckeri Teichert, 1930, specimen TUG 1680-19 from Saxby shore, Vormsi Island, Vormsi Regional Stage. F. Gorbormoceras vohilaidense gen. et sp. nov., holotype GIT 878-235, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage. G. Specimen TUG 1745- 293 from Pahkla, Pirgu Regional Stage. H. Ormoceras sp., specimen TUG 1745-239 from Salu, Pirgu Regional Stage. Scale bars = 10 mm for all figures, same scale in A–C, and in D–G.

Fig. 9. Details of the conch surface of orthoconic cephalopods from the Vormsi–Pirgu regional stages, Estonia. A. Nybyoceras intermedium Teichert, 1930, specimen TUG 939-49, from Moe stratotype outcrop, Pirgu Regional Stage. B. Palaeodawsonocerina senckenbergi (Teichert, 1930), specimen GIT 878-53, from Saxby shore (N), Vormsi Island, Vormsi Regional Stage.C. Striatocycloceras hosholmense sp. nov., holotype GIT 878-297, from Hosholm shore, Vormsi Island, Vormsi Regional Stage, view of the inner surface of the outer shell. Scale bars = 5 mm for all figures. All figures with apertural side of specimens directed upward.