Palaeodawsonocerina senckenbergi

Palaeodawsonocerina senckenbergi (Teichert, 1930) Figs 9B, 13A, 14 Spyroceras senckenbergi Teichert, 1930: 280, pl. 5 figs 3–4. Spyroceras saxbyense Stumbur, 1956: 180, pl. 1 fig. 1. Spyroceras sp. A – Strand 1934: 21, pl. 3 fig. 7. Gorbyoceras duncanae – Balashov 1975: 68, pl. 1 figs 3–6. Spyroceras senckenbergi – Dzik 1984: 122, 125, pl. 35 fig. 5, text-figs 48a, 49.20. Gorbyoceras? senckenbergi – Kiselev 1991: 92, pl. 1 fig. 2. Palaeodawsonocerina senckenbergi – Kröger & Isakar 2006: 154–156, figs 8a, 10i, 12d–e. ― Kröger 2013: 34–36, figs 12a, 13ca. ― Pohle et al. 2022: fig. 2. Diagnosis Slightly compressed orthocones with very low apical angle of approximately 1°–2°; adult dorsoventral diameter approximately 25 mm; ornamented with five to six annulations in a distance that equates to the corresponding shell diameter in adult specimens; annulations straight or very slightly oblique to the normal of the conch axis and regularly spaced; very fine growth lines, about 50 per cycle of annulations; very fine longitudinal striae or raised lines, about five to seven per mm; suture line at the ridges of the annulations; parallel sutures and annulations; siphuncle central, slightly expanded within the chamber; septal foramen about 0.07 of the shell diameter. (From Kröger & Isakar 2006.) Material examined ESTONIA • 1 spec.; Aulepa quarry; Kõrgessaare Formation, Vormsi Regional Stage; TUG 895-150 • 3 specs; Harilaid; Vormsi–Pirgu regional stages; TUG 1745-241 to TUG 1745-243 • 1 spec.; Hosholm shore; Adila Formation, Pirgu Regional Stage; TUG 939-78 • 1 spec.; Kersleti quarry; Kõrgessaare Formation, Vormsi Regional Stage; TUG 1745-17 • 2 specs; Kõrgessaare quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 426-30, GIT 426-49 • 5 specs; Moe trench; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-103, GIT 878-105, GIT 878-110, GIT 878-111, GIT 878-112 • 1 spec.; Mõnuste quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-8 • 4 specs; Paluküla quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 426-62, GIT 426-65, GIT 426-87, GIT 426-88 • 2 specs; same data as for preceding; TUG 1745-20, TUG 1745-21 • 3 specs; Paope quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 426-29, GIT 426-563, GIT 426-564 • 1 spec.; same data as for preceding; TUG 1745-18 • 1 spec.; Saaremõisa (Lyckholm); Kõrgessaare Formation, Vormsi Regional Stage; TUG 1745-283 • 4 specs; same data as for preceding; GIT 878-119 to GIT 878-121, GIT 878- 124 • 2 specs; Saxby old quarry; Moe Formation, Pirgu Regional Stage; TUG 1745-218, TUG 939-28 • 30 specs; Saxby shore; Kõrgessaare Formation, Vormsi Regional Stage; TUG 939-28, TUG 1745-264, TUG 1666-12, TUG 1745-226, TUG 1745-251, TUG 1745-255, TUG 1745-256, TUG 1745-261 to TUG 1745-272, TUG 1745-321, TUG 1837-82, TUG 38-815, TUG 39-777, TUG 39-792, TUG 939- 17, TUG 939-19, TUG 939-27, TUG 939-73, TUG 939-81, TUG 939-82 • 4 specs; same data as for preceding; GIT 426-96 to GIT 426-99 • 16 specs; Saxby shore (N); Kõrgessaare Formation, Vormsi Regional Stage; GIT 426-1125, GIT 426-1126, GIT 426-1131, GIT 426-24, GIT 426-42, GIT 426- 43, GIT 878-32 to GIT 878-35, GIT 878-37, GIT 878-51, GIT 878-53, GIT 878-54, GIT 878-72, GIT 878-81 • 2 specs; Sutlema quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-134, GIT 878-162 • 1 spec.; Urge quarry; Kõrgessaare Formation, Vormsi Regional Stage; TUG 939-18 • 1 spec.; Vormsi Island; Kõrgessaare Formation, Vormsi Regional Stage; TUG 42-283. Type locality and horizon Saaremõisa (Lyckholm), N of Haapsalu; Kõrgessaare Formation, Vormsi Regional Stage. Remarks Previous descriptions of this common species in the Vormsi Regional Stage strata of Estonia focused on well-preserved features of mature specimens, including the siphuncle, septal necks (Kröger & Isakar 2006), and ornamentation (Kröger 2013). Here, new data about the variability of the angle of expansion, and ornamentation (Figs 9B, 13A) complement this knowledge, permitting the synonymy of Spyroceras sp. A of Strand (1934) with P. senckenbergi, because of its general similarity in conch shape and ornamentation. Also, based on the new data, and in contrast to previous lists of synonymies, Spyroceras senckenbergi in Dzik (1984) is synonymised with P. senckenbergi. The data show that the angle of expansion tends to decrease with increasing conch size, being almost tubular in adult stages and up to 4–5° in juvenile growth stages (Fig. 13A). The largest specimen in the collection, specimen GIT 878-112, is a short fragment of a body chamber with a diameter of 30 mm. Only three specimens (GIT 878-53, GIT 878-112, TUG 1745-263) of the total collection (n= 89) have a diameter of more than 26 mm. This suggests an approximate adult size of the species of ca 25–30 mm. It has been observed that the relative spacing of the annulation in P. senckenbergi decreases with conch size although it was not possible to substantiate quantitatively (Kröger & Isakar 2006). Here, based on measurements from 22 specimens, it can be shown that there is no ontogenetic trend in relative annulation distance in P. senckenbergi (Fig. 14B–C). The spacing of the annulation varies between 1–6 mm in all specimens, it is widest in fragments of mature specimens (Fig. 14B).