Ancistroceras barrandei Dewitz 1880

Ancistroceras barrandei Dewitz, 1880 Figs 20–22, 23A–C, Tables 4–6 Ancistroceras Barrandei Dewitz, 1880: 389, pl. 17 fig. 6–6a. Strombolituites Barrandei – Remelé 1881: 192. Ancistroceras Barrandei – Noetling 1884: 128. — Patrunky 1926: 120. Ancistroceras undulatum – Noetling 1884: pl. 18 fig. 6–6a. — Rüdiger 1889: 44. — Qi 1980: 254, pl. 2 fig. 6, text-fig. 5. — Neben & Krueger 1971: pl. 17 fig. 1 Endoceras barrandei – Foerste 1930: 300. Ancistroceras barrandei – Neben & Krueger 1971: pl. 21 fig. 9. Diagnosis Species of the genus Ancistroceras with strongly curved end of the coiled part of the conch, expansion angle of straight part 17–21°. Tubular connecting rings. Dorsal projection of ornament elements continuously rounded, without dorsolateral sinuses (after Aubrechtová & Meidla 2020, emended). Type material Not available for study; Dewitz (1880) mentioned three specimens, one of them was figured (Dewitz 1880: pl. 17 fig. 6), from Mayakovskoye (former Nemmersdorf; Kaliningrad Region, Russia), Middle Ordovician erratics from Pleistocene gravel. Material examined GERMANY • 1 spec.; Brandenburg, Heegermühle; Ordovician; MB.C.11700 • 1 spec.; Brandenburg, Niederfinow; Ordovician, Upper Grey Orthoceratite Limestone; Neben and Krueger Coll.; MB.C.30515 • 1 spec.; Mecklenburg-Vorpommern, Gross Zicker (Island of Rügen); Ordovician, Upper Grey Orthoceratite Limestone; Helms 1966 Coll.; MB.C.11991a • 2 specs; Mecklenburg-Vorpommern, Rerik (Ostsee); Ordovician, Upper Grey Orthoceratite Limestone; MB.C.30516, MB.C.30517 • 1 spec.; Mecklenburg-Vorpommern, Rerik (Ostsee); Ordovician, Upper Grey Orthoceratite Limestone; Neben and Krueger 1971 Coll.; MB.C.30518. Description Specimen MB.C.30518 (Figs 21A, 23A) consists of the coiled part (dm = 12 mm; inner whorls not preserved, last quarter of the last whorl detached from preceding whorl) and a part of the straight uncoiled part of the conch (length = 140 mm; wh = 45 mm; EA ~ 18°). The shell ornament in the coiled and beginning of the uncoiled part consists of annuli (1 mm apart) that bear raised pairs of lirae (0.3–0.5 mm apart) on their crests. In the remaining part of the uncoiled conch, the annuli are wide and flat and the lirae are standing up to 1.3 mm apart. The dorsal projection of ornament elements is low and continuously rounded, i.e., not flanked by dorsolateral sinuses. The length of the phragmocone chambers can be measured only in the middle part of the straight part (CLR = 0.34 at wh = 40.5 mm). Specimen MB.C.11991 (Figs 21C, 23C) consists of the almost complete coiled part (dm = 15 mm; coiling tight, only the last part of the last volution is detached; WER = 4.20) and a part of the weakly sigmoid uncoiled part (length = 90 mm; wh = 27 mm; EA ~ 17°). The shell ornament consists of narrow annuli (up to 1.4 mm in distance) and lirae (up to 0.3 mm in distance) in the coiled part and beginning of the uncoiled part; the annuli become wider and applanate in the remaining uncoiled part; the distance of lirae increases up to 1 mm. The ventral sinus is deep but becomes shallower during ontogeny. On the flanks, the ornament elements extend transversely with a lateral sinus that develops at a whorl height of ca 17 mm. The dorsal projection is low, continuously rounded and is not flanked by any dorsolateral sinuses. The chamber length increases during ontogeny (CLR = 0.32–0.42). Specimen MB.C.30516 (Figs 21B, 23B) consists of the almost complete coiled part (inner whorls are tightly coiled and the last whorl widely detaches from preceding whorls; dm = 10 mm) and a part of the straight uncoiled part (length = 115 mm; wh = 40 mm; EA ~ 20°) of the conch. The shell ornament in the coiled part of the specimen and after uncoiling consists of indistinct annuli and raised pairs of lirae (up to 0.5 mm apart). In the larger half of the specimen, the annuli are very wide and flat, and the shell surface is ornamented by slightly irregularly developed lirae (up to 0.7 mm apart). The ornament elements form deep sinuses on the venter and flanks of the conch; the dorsal projection is low and continuously rounded. The chamber length is variable (CLR = 0.16–0.33) and decreases towards the aperture (Fig. 22). Remarks Ancistroceras barrandei is very similar to A. undulatum (described above), from which it differs in the stronger curvature at the end of the coiled part of the conch (Aubrechtová & Meidla 2020). The specimens studied here also show that the dorsal projection of the ornament is continuously rounded without dorsolateral sinuses in A. barrandei, while the dorsal projection is roof-shaped with shallow adjacent dorsolateral sinuses in A. undulatum. In A. barrandei, the coiled conch is so far known only from a single specimen (Noetling 1884: pl. 18 fig. 6) (Fig. 20). Here, we describe additional specimens with preserved coiled parts. Their morphology is variable in the degree to which the last whorl loses contact with the preceding whorl (Fig. 23). In other characters, the specimens are similar to each other. Specimen MB.C.11991 has a weakly sigmoid uncoiled part of the conch and a rather low expansion angle (17°), which might suggest it should be assigned to Holmiceras. However, the specimen has a tightly, not loosely coiled apical part of the conch and at the conch length of 90 mm, it does not reach the mature growth stage. Geographic and stratigraphic occurrence Estonia (in situ) and northern Germany, northern Poland and the Kaliningrad Region (Russia) (in erratics within Pleistocene gravels); Aseri to Uhaku regional stages, Middle Ordovician. According to Dewitz (1880), the type material of A. barrandei comes from the “lower Silurian” of Königsberg (= Kaliningrad), where it was found in co-occurrence with Ancistroceras undulatum. The co-occurrence of the two species suggests a late Darriwilian age.

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