Striatocycloceras hosholmense, Kröger, 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 : 25-28

Striatocycloceras hosholmense sp. nov.

urn:lsid:zoobank.org:act:0530A9F0-1CC9-47F1-97C4-F5AB2352EE38

Figs 9C , 11E, G, 13B, D

Diagnosis

Annulated orthocones with circular conch cross section and nearly tubular conch at conch diameters> 20 mm; ornamented with three to four annulations in a distance equal to the corresponding conch diameter; annulations straight transverse, regularly spaced and with sharp narrow crests; ornamented with ca 10–12 fine transverse growth lines or striae per cycle of annulation; suture lines in the troughs of the annulations and equally spaced to annulations; parallel sutures and annulations; RCL ca 0.25–0.3; siphuncle central, tubular to slightly expanded; septal foramen approximately one tenth of the diameter, septal necks orthochoanitic; shallow endosiphuncular deposits at position of septal necks; cameral deposits not known.

Etymology

Refers to the type locality, Hosholm shore, Vormsi Island.

Type material

Holotype

ESTONIA • Vormsi Island , Hosholm shore; Adila Formation, Pirgu Regional Stage; GIT 878-297 .

Paratypes

ESTONIA • 5 specs; same collection data as for holotype; GIT 878-288 , GIT 878-290 , GIT 378-302 , GIT 878-304 , GIT 878-312 • 1 spec; same collection data as for holotype; TUG 1819-102 .

Description

The holotype is an 81 mm long fragment of a phragmocone with a circular conch cross section, and a conch-diameter of 24–25 mm ( Fig. 13B ). Three to four annulations occur in a distance equal to the corresponding diameter. The entire fragment has 11 equally spaced, directly transverse annulations. The annulations have deep rounded troughs (1.5 mm deep) and sharp, and narrow crests. The conch surface is ornamented with ca 12 transverse growth-lines per annulation ( Fig. 9C ). The sutures are positioned in the troughs of the annulations. The curvature of the septa is ca 3 mm deep with a slightly eccentric septal foramen, ca 2.5 mm in diameter. The siphuncle is nearly tubular, centrally positioned, slightly constricted at the position of the septal foramen. The septal necks are orthochoanitic, ca one millimeter long. At the position of the septal necks thin endosiphuncular annuli are developed (Fig. 11E). Cameral deposits are not developed.

A second specimen, GIT 878-312, is a fragment of a phragmocone with internal characters well-preserved. The fragment has a length of 68 mm, a diameter of 22–23 mm, and nine annulations and chambers ( Fig. 13D ). The siphuncle is nearly tubular with a diameter of ca 2.5 mm. The septal necks are orthochoanitic with a length of ca one millimeter, and thin endosiphuncular deposits are developed at the position of the septal necks. The siphuncle is nearly central. The angles of expansion of all specimens assigned to this species are less than 5° (Fig. 11G).

Remarks

The shell surface is poorly preserved in all specimens which are available for this study. The outer shell is commonly firmly attached to the matrix, covering the surface of the conch, obscuring fine detail of the ornamentation. Where the outer shell is visible, no longitudinal ornamentation is visible. However, it cannot be excluded that in this species very fine, sub-millimetre, longitudinal lirae are present on the shell surface. Therefore, S. hosholmense sp. nov. should also be compared with species of Anaspyroceras Shimizu & Obata, 1935 (see: Flower 1943b: 115, and Sweet 1964c: K230, for a genus diagnosis of Anaspyroceras ). Anaspyroceras is an annulated orthocerid, which is internally similar to Striatocycloceras but has a fine longitudinal ornament.

Comparison

The holotype of this species shows great similarity to the specimen figured in Dzik (1984: pl. 35 fig. 4a–c); subsequently designated as the holotype of Orthoceras clathrato - annulatum Roemer, 1861 therein. However, inconsistencies between Roemer’s (1861) figures and descriptions, and differences between Roemer’s (1861) and Dzik’s (1984) specimens indicate that Roemer’s (1861) type material for O. clathrato - annulatum consisted of multiple species. The specimen figured in Dzik (1984) has no outer shell preserved and the internal characters remain unknown. Further study of Roemer’s (1861) type material is needed to solve this problem (see also discussion under Gorbyoceras clathratoannulatum ).

Striatocycloceras hosholmense sp. nov. differs from S. undulatostriatum , and S. foerstei ( Teichert, 1930) in having a relatively wide annulation of three to four annuli per distance equal to the corresponding conch cross section, compared to more than four annuli in the latter two species. Additionally, endosiphuncluar annuli are not known from the latter two species. S. romingeri ( Foerste, 1932) has a smaller adult size (only ca 10 mm) than S. hosholmense , and in S. obliquum ( Eichwald, 1860) , the annulation is oblique.

Fig. 11 (preceding page). Median sections of phragmocones of Orthocerida Kuhn, 1940 and Pseudorthocerida Flower & Caster, 1935 of the Vormsi–Pirgu regional stages, Estonia. A. Pleurorthoceras sp. , specimen TUG 1745-31 from Mõnuste quarry, Vormsi Regional Stage. B–C. Gorbyoceras clathratoannulatum ( Roemer, 1861) comb. nov. B. Specimen GIT 878-149, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage. C. Specimen GIT 840-138, from Hosholm shore, Vormsi Island, Vormsi Regional Stage. D. Gorbyoceras textumaraneum ( Roemer, 1861) , specimen GIT 878-299 from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage. E. Striatocycloceras hosholmense sp. nov., holotype GIT 878-297, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. F. Striatocycloceras cf. hosholmense , specimen TUG 939-20, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. G. Striatocycloceras hosholmense , paratype GIT 878-312, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. Scale bars = 5 mm for all figures.

Several species of Anaspyroceras are similar to S. hosholmense sp. nov. but differ in the following aspects: A. anellus ( Conrad, 1843) and A. paquettense ( Foerste, 1932) differ in having narrower annulations (ca 5–6 annulations per distance equal to the corresponding conch cross section) and in being ornamented with fine longitudinal lirae. Anaspyroceras anzaas Teichert & Glenister, 1953 has a low annulation and is ornamented with 26–33 longitudinal lirae. Anaspyroceras cylindricum Foerste, 1932 differs in having obliquely transverse annulations. Anaspyroceras cumberlandense Flower, 1946 has a narrower annulation and an irregularly nodose ornamentation. Anaspyroceras williamsae Flower, 1946 is ornamented with longitudinal lirae. The annulation of A. obscurum ( Barrande, 1868) is less pronounced and narrower (> 10 annulations at distance equal to the corresponding conch cross section).

Figures

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.

Fig. 13. Annulated Orthocerida of the Vormsi–Pirgu regional stages, Estonia. A. Palaeodawsonocerina senckenbergi (Teichert, 1930), specimen TUG 939-81, from Saxby shore (N), Vormsi Island, Vormsi Regional Stage. B, D. Striatocycloceras hosholmense sp. nov., holotype GIT 878-297, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. C. Striatocycloceras cf. hosholmense, specimen TUG 939- 20, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. E. Striatocycloceras sp., specimen TUG 42-414, from Saaremõisa (Lyckholm), Vormsi Regional Stage. Scale bars = 10 mm for all figures, same scale in A, C–E.