Isorthoceras luhai ( Stumbur, 1956 ), 2025

Isorthoceras luhai ( Stumbur, 1956) comb. nov.

Figs 19A–C, F View Fig , 20–21

Orthoceras luhai Stumbur, 1956: 179–180, text-fig. 1, pl. 1 figs 2–3, pl. 2 fig. 4, pl. 3 fig. 1. Michelinoceras dnestrovense Balashov, 1975: pl. 2 figs 7–8.

Isorthoceras dalecarlense Kröger et al., 2011: 45, fig. 9c, g.

Isorthoceras dalecarlense – Kröger 2013: 58–59 View Cited Treatment , figs 4b, 26.

Diagnosis

Smooth, nearly straight Isorthoceras with circular cross section and comparatively low angle of expansion of 6°, chamber distance 0.3 of corresponding cross section, depth of septal curvature ca 0.26 of corresponding cross section; adult size> 35 mm in cross section; siphuncle eccentric in early, subcentral in later growth stages, siphuncular segments slightly expanded within chambers with diameter ca 0.1 of conch cross section; septal necks short, bordering between cyrtochoanitic and suborthochoanitic; in apical parts of siphuncle thin parietal deposits, that ventrally fuse forming irregular endosiphuncular lining in extreme apical part of conch; cameral deposits not known. (From Kröger et al. 2011.)

Material examined

ESTONIA • 1 spec.; Haapsalu holm ; Adila Formation , Pirgu Regional Stage; TUG 2-359 • 1 spec.; Vormsi Island, Hosholm shore (tower outcrop); Adila Formation , Pirgu Regional Stage; GIT 878-291 View Materials • 1 spec.; Jootma ditch ; Moe Formation , Pirgu Regional Stage; GIT 426-577 View Materials • 1 spec.; Kohila ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 939-1 • 1 spec.; Paluküla quarry ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-1108 View Materials • 1 spec.; Saksi manor ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 1745-215 • 2 specs; Vormsi Island, Saxby shore ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 80-474 , TUG 895-24 • 4 specs; Vormsi Island, Saxby shore (N); Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-1124 View Materials , GIT 878-30 View Materials , GIT 878-45 View Materials , GIT 878-60 View Materials • 1 spec.; same data as for preceding; TUG 1745-222 .

Type locality and horizon

Kohila, Estonia; Vormsi Regional Stage.

Description

This species was described in detail as I. dalecarlense in Kröger et al. (2011) and Kröger (2013). Here, new data regarding the variability of the angle of expansion, the relative chamber length and the relative siphuncle position can be added – especially for specimens with diameters larger than 20 mm ( Figs 20– 21 View Fig View Fig ). The data show that the apical angle decreases from a maximum of 8° at conch diameters 16–20 mm (specimen TUG 80-474) to a minimum of 3° at conch diameters 29–32 mm (specimens GIT 426-1108, GIT 878-30) ( Fig. 20A View Fig ). Similarly, the relative chamber length decreases in growth stages greater than 16 mm in diameter, reaching maximum values at ca 16 mm with an RCL of 0.32 (specimen TUG 895- 24) and minimum values of an RCL of less than 0.2 at conch diameters larger than 24 mm (specimens GIT 426-1108, GIT 878-60, GIT 878-291) ( Fig. 20B View Fig ). The position of the siphuncle changes during ontogeny from RSP 0.32 (specimen TUG 2-359) at a conch diameter of 12 mm toward a central position in specimens with diameters larger than 30 mm (specimen GIT 878-30) ( Fig. 20C View Fig ).

The septal necks are short suborthochoanitic with a siphuncle that appears to be more expanded, barrel-shaped in earlier growth stages and more tubular in later growth stages ( Fig. 19A–C, F View Fig ). Endosiphuncular and cameral deposits are absent in the specimens which were cut and polished at median position. These have diameters larger than ca 15 mm.

Remarks

The species was established based on a single, poorly preserved specimen (holotype: GIT 939-1, Fig. 19C View Fig ). With the nine specimens, described herein, the range of variation of this species becomes apparent (measurements are available in supplementary data 3). The new data reveal that the type-material of I. dalecarlense Kröger et. al., 2011 is, at comparable growth stages, well within the known range of variation of the shell features of I. luhai ( Fig. 20 View Fig ). Therefore, I. dalecarlense must be interpreted as subjective junior synonym of I. luhai . The same can be said for Michelinoceras dnestrovense Balashov, 1975 from late Katian strata of Podolia, Ukraine (see Fig. 20 View Fig ).

The Estonian fragments are on average larger in diameter than the types of I. dalecarlense from the Glisstjärn Formation, Hirnantian, Sweden ( Kröger et al. 2011). Together with the Swedish material described as I. dalacarlense from the Boda Limestone, Sweden, this now permits an evaluation of the ontogenetic changes of the angle of expansion, of the RCL, and the RSP from juvenile stages to near maturity ( Figs 20–21 View Fig View Fig ).

As a result, divergent trajectories in early growth stages for the angle of expansion between the samples of I. dalecarlense from the Glisstjärn, and from that of the slightly older Boda Formation, Sweden (see Kröger 2013: fig. 26) are evident. The specimens assigned to D. dalecarlense from the Boda Formation initially have a low angle of expansion, which increases toward a diameter of ca 15 mm. The Glisstjärn material, in contrast, shows a continuously decreasing angle of expansion throughout ontogeny ( Fig. 21 View Fig ) (Note: in Kröger 2013: fig. 26, correctly the grey dots represent the type-material and the dots with black circles the Boda material, compare Kröger et al. 2011: fig. 10). This difference could be interpreted as an intraspecific variability within I. luhai or, alternatively as evidence for the presence of two different species, one in the Boda Formation and another in the Glisstjärn Formation. Additional specimens of the smaller size fraction from Estonia are needed to test if the Estonian I. luhai follows the early ontogeny trajectory of the I. dalecarlense - type material from the Glisstjärn Formation or that of the Boda Formation. The latter case would open the possibility to distinguish between I. luhai with a low juvenile angle of expansion and I. dalecalense with a high juvenile angle of expansion. Until more material is known, the most parsimonious solution to the problem is followed, which is to synonymize the three samples under I. luhai , which has priority.

Comparison

Isorthoceras luhai differs from the co-occurring I. saaremense ( Balashov, 1959) in having a smaller angle of expansion at comparable growth stages ( Fig. 20A View Fig ) and in having a less eccentric siphuncle position (mean RSP is 0.4, compared with 0.3 in in I. saaremense , Fig. 20C View Fig ).