Ephippiorthoceras vormsiense, 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 : 34-38

Ephippiorthoceras vormsiense sp. nov.

urn:lsid:zoobank.org:act:9A7BEADF-FA56-4756-9CAE-D36E2CDCF182

Figs 15A–C , 16A–B, 17

Diagnosis

Faintly exogastrically curved longicones to orthocones with an angle of expansion of up to 9°; lateral sutural lobes shifted slightly adorally at venter; mature conch height ca 38–40 mm, with maximum height reached near the base of the mature body chamber or at adoral-most part of the mature phragmocone; mature body chamber ca 90 mm long and constricted; siphuncular position subcentral in early growths stages, eccentric in latest growth stages; siphuncle fusiform, expanded in early growth stages and nearly tubular in latest growth stages with septal necks changing from cyrtochoanitic to suborthochoantic during ontogeny.

Etymology

Refers to the type locality, Vormsi Island.

Type material

Holotype

ESTONIA • Vormsi Island , Hosholm shore (tower locality), Adila Formation, Pirgu Regional Stage; GIT 878-192 .

Paratypes

ESTONIA • 2 specs; same collection data as for holotype; GIT 878-195 , GIT 878-228 • 2 specs; Vohilaid Island , Vohilaid shore (E), Adila Formation, Pirgu Regional Stage; GIT 878-193 , GIT 878-194 • 2 specs; same data as for preceding; TUG 1745-28 , TUG 1743-75 .

Description

The holotype (GIT 878-192) ( Fig. 15A ) is the largest specimen, it reaches a conch height of 40 mm ca 25 mm adapically of the last septum on phragmocone. There, the conch width is 33 mm (CHI = 1.21). The phragmocone decreases in height and width toward 39 mm, 32 mm, respectively at the base of the body chamber. Only the basal ca 60 mm of the body chamber are preserved, they indicate a decreasing height toward the aperture. The sutures form a marked lateral lobe and have a distance of 7 mm (the adoralmost chamber) to 9 mm (RCL up to 0.23). The septal foramen is subcentral with a RSP of 0.39 at a conch height of 39 mm.

The mature body chamber is preserved in three specimens (GIT 878-192, GIT 878-194, GIT 878-228). Specimen GIT 878-228 ( Fig. 15B ) is a fragment of a phragmocone and a nearly complete mature body chamber, which is slightly deformed. At the base of the body chamber, the height and width are 35 mm, 37 mm respectively (CHI 0.95). The body chamber has a length of 90 mm and continuously decreases in diameter toward the aperture, where the width is 33 mm. The adapical ca 5 mm are constricted and apparently a ca 15 mm deep hyponomic sinus occurs on the prosiphuncular side. The adoralmost chamber of the phragmocone has a length of 4 mm, further adapicallly the sutures are 7 mm distant (RCL = 0.19). The sutures form a deep lateral lobe and are slightly more adorally located on the prosiphuncular side (the side with the hyponomic sinus) than on the opposite side. The preserved part of the phragmocone has an angle of expansion of 3° at a width of 35–37 mm. The septal foramen is nearly centrally positioned.

Specimen GIT 878-194 is a mold of a mature body chamber with a conch height and width at its base of 36 mm, 28 mm respectively (CHI = 1.29) and a preserved length of 70 mm. The height and width decrease adorally until at a point ca 45 mm from the base, the height is 31 mm. Further toward the aperture the height increases slightly, resulting in a body chamber outline with concave margins. The septal foramen at the last septum is subcentral (RSP = 0.33).

Specimen TUG 1743-75 is a slightly curved fragment of a phragmocone with heights of 23–31 mm and an angle of expansion of 9°, The sutures form distinct lateral lobes and are 5 mm apart at a conch height of 23 mm and 7 mm at a conch height of 30 mm (RCL: 0.22–0.23).

Measurements of the position of the septal foramen indicate a slight marginal shift of the siphuncle during growth with an RSP of 0.43 at a conch height of 23 mm (TUG 1745-286), toward and RSP of 0.38–0.39 at a conch height of 38–39 mm (GIT 878-192, GIT 878-193). The details of the siphuncle and septal neck are preserved in specimens GIT 878-192, GIT 878-193, and TUG 1745-28 (Fig. 16A–B). They reveal that the septal necks are cyrthochoanitic at earlier growth stages (conch heights <33 mm) and suborthochoanitic near maturity (conch height 36–39 mm). The siphuncular segments are expanded fusiform in early growth stages and nearly tubular in later growth stages ( Table 1).

Comparison

Ephippiorthoceras vormsiense sp. nov. has a smooth, or nearly so, conch surface, which differs from species which have a wrinkled or lirate ornamentation such as E. laddi Foerste, 1935b , E. formosum , Ephippiorthoceras plicatuloides Strand, 1934 (see also below), E. plicatulum Foerste in Twenhofel, 1928, E. schucherti Foerste, 1928c , E. tenuistriatum Foerste, 1935b . Ephippiorthoceras altocameratum Foerste, 1928c , and E. sieboldi ( Billings, 1866) reach larger adult sizes (conch heights more than 55 mm

Fig. 16 (next page). Median sections of phragmocone of cephalopods of the Pirgu Regional Stage, Estonia. A–B. Ephippiorthoceras vormsiense sp. nov. A. Holotype (GIT 878-192), from Hosholm shore (tower), Vormsi Island. B. Specimen TUG 1745-28, from Vohilaid shore (E), Vohilaid Island. C–D. Danoceras piersalense ( Teichert, 1930) comb. nov. C. Specimen TUG 80-510, from Piirsalu quarry. D. Specimen TUG 1745-296, from Pirgu River outcrop. E. Dowlingoceras kallholnense Frye, 1987 , from GIT 878- 247, from Hosholm shore, Vormsi Island. F. Danoceras piersalense specimen GIT 840-271, from Hosholm shore, Vormsi Island. G. Beloitoceras cf. sinuoseptatum ( Roemer, 1861) , specimen GIT 426-2 from Hosholm shore, Vormsi Island. Scale bars = 5 mm in all figures, same scale in A–G.

at base of mature body chamber) ( Foerste 1928c). Ephippiorthoceras compressum Foerste, 1928b differs in having strongly obliquely sloping sutures and a very low angle of expansion. Ephippiorthoceras decorum Teichert & Glenister, 1953 differs in having cameral and endosiphuncular deposits and a clearly more eccentrically positioned siphuncle. The siphuncle of E. modestum Troedsson, 1926 is more centrally positioned.

Strand (1934) described two species of Ephippiorthoceras from late Katian strata of Norway ( E. plicatuloides Strand, 1934 , E. frognoyense Strand, 1934 ). Details of the siphuncle and septal necks are unknown for either species. The two species are similar to E. vormsiense sp. nov. in size and suture shape but differ in having more narrowly spaced chambers (RCL = 0.13–0.14 in E. frognoyense ) and have conspicuously wrinkled conchs ( E. plicatuloides ). Both species are known from one or two specimens only, making comparisons with the Estonian material difficult. The narrow chamber spacing could be interpreted as an effect of maturity, but the wrinkled surface is known from other species of the genus (see above). A synonymy of E. vormsiense with one of the Norwegian species is therefore unlikely.

Figures

Fig. 15. Pseudorthoceratidae Flower & Caster, 1935 of the Vormsi–Pirgu regional stages, Estonia. A–C. Ephippiorthoceras vormsiense sp. nov. from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. A. Specimen GIT 878-192, lateral view. B–C. Specimen GIT 878-228. B. Lateral view. C. Ventral view. D. Isorthoceras saaremense (Balashov, 1959) comb. nov., specimen GIT 878-41, from Saxby shore (N), Vormsi Island, Vormsi Regional Stage. Scale bar = 10 mm in all figures, same scale in A–C.