Pleurorthoceras sp.

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 : 23-24

Pleurorthoceras sp.

Fig. 10 , 11A, 12B

Material examined

ESTONIA • 1 spec.; Vormsi Island, Saxby shore; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-154 • 2 specs; same data as for preceding; TUG 1745-220 , TUG 1837-81 • 1 spec.; Vormsi Island, Saxby shore (N); Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-18 • 1 spec.; Hiiumaa Island, Kõrgessaare quarry; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-1110 • 1 spec.; same data as for preceding; TUG 1745-30 • 1 spec.; Paope quarry; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-557 • 1 spec.; Saaremõisa (Lyckholm); Kõrgessaare Formation , Vormsi Regional Stage; TUG 1745-225 • 2 specs; Mõnuste quarry; Kõrgessaare Formation, Vormsi Regional Stage ; TUG 939-14 , TUG 1745-31 • 1 spec.; Kersleti quarry; Kõrgessaare Formation, Vormsi Regional Stage ; TUG 44-42 . (See also supplementary data 1.)

Description

The material comprises fragments of orthocones with an apparent or preserved smooth conch surface and a central to subcentral tubular siphuncle with orthochoanitic to suborthochoanitic septal necks and without endosiphuncular or cameral deposits. The material comprises two separate size classes of fragments (compare Figs 11A, 12B):

In the smaller fragments (conch diameters <15 mm; specimens GIT 878-154, GIT 878-18, TUG 1745- 220), the conch surface is smooth, the septal distance is relatively wide (RCL: 0.3–0.5; Fig. 10B ); the septal curvature is 4 mm deep where the conch diameter is 12 mm (GIT 878-18); the sutures are directly transverse and straight; the siphuncle is nearly tubular (GIT 878-154: siphuncular height is 1.7 mm at mid-length between septa where the conch diameter is 13 mm, RSH = 0.13); and the septal necks are suborthochoanitic and ca 0.3 mm long (GIT 878-154).

In the larger fragments, the conch surface is not preserved but was apparently smooth.All larger fragments are deformed internal molds of phragmocones or fragmentary preserved in conch cross section. The largest specimen (GIT 426-557) has a maximum diameter of ca 66 mm with an RCL of ca 0.24. The RCL is in all specimens of the larger size class (> 30 mm) between 0.23–0.26 ( Fig. 10B ). The exact position of the siphuncle cannot be determined in these specimens because of poor preservation; it was apparently at a subcentral to central position. In specimen TUG 1745-31 (Fig. 11A) and TUG 939-14, the internal characters are preserved. The siphuncle is apparently tubular or nearly so and the septal necks are orthochoanitic to suborthochoanitic, 1 mm long where the septal distance is 13 mm, the conch diameter ca 50 mm, and the septal foramen is 5 mm in diameter.

Remarks

The two size classes of specimens with diameters <15 mm and> 30 mm agree in apparent smooth conch surface, siphuncular position and siphuncular shape. However, no intermediate growth stages are known. Hence, the possibility exists that these represent two different species. However, despite the gap in size the resulting distribution ( Fig. 10B ) equally well could be interpreted as showing the ontogenetic trend in cameral depth seen in many other orthocerids and pseudorthocerids (i.e., rapid rise in early stages then decreasing more or less rapidly in later growth).

Because of the uncertainties in adult siphuncle position, ornamentation, and ontogentic trends of the angle of expansion, and other characters, the specimens are left in open nomenclature. The fragments differ from P. osmundsbergense Kröger, 2013 , in having a lower angle of expansion (angle of expansion in P. osmundsbergense is ca 7°) and the conch is slightly curved. The American species of Pleurorthoceras differ in having a wider siphuncle (e.g., Frey 1995; Fig. 9B ).

Figures

Fig. 10. Diagrams of morphological variability of specimens of Pleurorthoceras sp., Pirgu–Vormsi regional stages, Estonia. A. Angle of expansion. B. Relative siphuncle position (RSP). Note the decreasing RCL with increasing conch height.

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.