Rhynchorthoceras angelini
Creators: name: Aubrechtová, Martina
name: Korn, Dieter
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Tarphycerida
Lituitidae
Rhynchorthoceras
Rhynchorthoceras angelini

description: Rhynchorthoceras angelini (Boll, 1857)
Figs 9–12, Tables 2–3
Lituites Angelini Boll, 1857: 89.
Rhynchorthoceras Oelandicum Remelé, 1882: 134–135, pl. 5 fig. 5.
Rhynchorthoceras tenuistriatum Remelé, 1882: 137–138.
Rhynchorthoceras Angelini var. lineata Remelé, 1882: 136.
Rhynchorthoceras Angelini var. virgata Remelé, 1882: 136.
Orthoceras Angelini – Boll 1857: pl. 4 fig. 11.
Ancistroceras Angelini – Remelé 1881: 195.
Rhynchorthoceras Angelini – Remelé 1882: 135; 1890: 19. — Rüdiger 1889: 37.
Rhynchorthoceras Oelandicum – Rüdiger 1889: 40.
Rhynchorthoceras tenuistriatum – Remelé 1890: 74, pl. 5 fig. 7.
Rhynchorthoceras Angelini var. lineata – Remelé 1890: pl. 5 fig. 6, pl. 6 fig. 6.
Rhynchorthoceras Angelini var. virgata – Remelé 1890: pl. 3 fig. 5, pl. 5 figs 3–5.
Rhynchorthoceras angelini var. lineata – Neben & Krueger 1971: pl. 21 figs 10–12.
? Rhynchorthoceras angelini – Sweet 1958: 140.
Diagnosis
Species of the genus Rhynchorthoceras with expansion angles between 8° and 13°; whorl profile circular, in the apical part of the conch sometimes slightly compressed and flattened on the dorsum. Shell surface consists of lirae with oblique and straight course across the flank, not oblique and straight ventrally, a low dorsal projection. Transverse profile of lirae either crested or flat, commonly asymmetric. Shell wall sometimes faintly undulated. Distance of lirae regular or slightly fluctuating, gradually decreases during ontogeny. Siphuncle 0.12–0.19 of whorl height, central or almost central in position.
Type material
Not available for study; Boll (1857) did not specify a type but he had at least three specimens available, of which one was figured (Boll 1857: pl. 4 fig. 11).
Material examined
GERMANY • 1 spec.; Berlin-Schmargendorf; Ordovician, Upper Red Orthoceratite Limestone; Schmidt Coll.; MB.C.11727 • 12 specs; Brandenburg, Heegermühle; Ordovician, Upper Red Orthoceratite Limestone; Kgl. Forstakademie Eberswalde Coll.; MB.C.11724, MB.C.11726.1 to MB.C.11726.7, MB.C.11729.2 to MB.C.11729.4, MB.C.11732.2 • 8 specs; Brandenburg, Heegermühle; Ordovician, Upper Red Orthoceratite Limestone; Remelé Coll.; MB.C.11725.1, MB.C.11725.2, MB.C.11731.1 to MB.C.11731.6 • 6 specs; Brandenburg, Heegermühle; Ordovician, Upper Red Orthoceratite Limestone; MB.C.22577, MB.C.30494, MB.C.30495.1, MB.C.30495.2, MB.C.30496, MB.C.30497 • 1 spec.; Brandenburg, Hohensaaten; Ordovician, Upper Red Orthoceratite Limestone; Neben and Krueger Coll.; MB.C.30498 • 1 spec.; Brandenburg, Joachimsthal (Uckermark); Ordovician, Upper Grey Orthoceratite Limestone; Hentig 1880 Coll.; MB.C.11716 • 1 spec.; Mecklenburg-Vorpommern, Neu-Ruthenbeck (Parchim); Ordovician, Upper Grey Orthoceratite Limestone; Helms 1974 Coll.; MB.C.11717.
SWEDEN • 1 spec.; Dalarna, Skattungbyn; Ordovician, Upper Red Orthoceratite Limestone; Dames 1890 Coll.; MB.C.11720.1 • 1 spec.; Island of Öland; Ordovician; MB.C.11723 • 2 specs; Island of Öland, Skärlöv; Ordovician, Segerstad Limestone (upper part of the Upper Red Orthoceratite Limestone); Bottke Coll.; MB.C.29656, MB.C.29653.
COUNTRY UNKNOWN • 1 spec.; Ordovician; Otto Coll.; MB.C.11729.1.
Description
All available specimens are fragments of faintly curved or orthoconic phragmocones (Figs 9–10) and represent various growth stages; the smallest preserved whorl height is 3 mm and the largest is 40 mm. Initial parts of the phragmocones and also mature body chambers are not preserved in any of the specimens. The expansion angles vary between 8° and 13° (median = 11°, n = 13; Fig. 13A). The relative chamber length is variable with a CLR between 0.25 and 0.50 (Fig. 11). The siphuncle has a central or almost central position, with diameters between 0.12 and 0.19 (median = 0.14, n = 13; Fig. 13B) of the whorl height.
The shell ornament consists of lirae, which are up to 1.3 mm apart in the largest specimens. The distance of lirae is more or less regular, fluctuates only slightly and gradually decreases during ontogeny (e.g., MB.C.11724, relative distance from ca 0.04 at wh 19 mm to ca 0.02 at wh 39 mm, Fig. 12B–C). The lirae extend straight across the venter; they are oblique and straight on the flanks and form a low projection on the dorsum (Fig. 12A, MB.C.11725.1). Their profile is crested and symmetric (MB.C.30495.1, MB.C.11724, MB.C.29656), crested and asymmetric (MB.C.11725.1) or flat and weakly asymmetric or symmetric (MB.C.11727, MB.C.22577).
Remarks
Rhynchorthoceras angelini differs from R. tenuistriatum (from Middle Ordovician erratics of northern Germany) in the slightly lower expansion angle (8–12° vs 13°) and from R. oelandicum (from Middle Ordovician strata of Öland and erratics of northern Germany) in the central (vs slightly eccentric) position of the siphuncle in large growth stages. In all other characters, including the shell ornament, R. angelini is indistinguishable from R. tenuistriatum and R. oelandicum. Consequently, both R. tenuistriatum and R. oelandicum are synonymised with R. angelini herein.
The intraspecific variation of R. angelini also concerns the shell ornament. Remelé (1882, 1890) recognised two varieties within the species: R. angelini var. virgata with more prominent lirae with acute, symmetric or asymmetric crests, while the lirae in R. angelini var. lineata are flat and only very slightly asymmetric. Remelé considered them as varieties of a single species because of common transitions between the two types of ornament. Furthermore, the specimens do not differ in any other morphological characters. Based on the study of original specimens of Remelé (1882, 1890) and subsequently collected material, the varieties are not distinguished here and only the name R. angelini is used.
Rhynchorthoceras angelini is, together with R. conicum, by far the most abundant representative of Rhynchorthoceras in the Aserian and Lasnamägian (Middle Ordovician) strata of Baltoscandia and erratics in Germany and Poland. Most of the reported occurrences come from the Upper Red Orthoceratite Limestone, where R. angelini co-occurs with Trilacinoceras discors (Holm, 1891) and possibly other lituitids (e.g., Jaanusson 1960). The species is palaeogeographically widespread and stratigraphically long-ranging compared to other species of the genus; it probably occurs already in the latest Dapingian (early Middle Ordovician) rocks of central Bohemia (Marek 1999; Manda 2008; Aubrechtová & Turek 2018).
Geographic and stratigraphic occurrence
Norway (?), Sweden (in situ) and northern Germany (in erratics within Pleistocene gravels), central Bohemia (?); middle Volkhov (?), Kunda to Lasnamägi regional stages (latest Dapingian (?) to late Darriwilian, Middle Ordovician).
description: Published as part of Aubrechtová, Martina & Korn, Dieter, 2022, Taxonomy and ontogeny of the Lituitida (Cephalopoda) from Orthoceratite Limestone erratics (Middle Ordovician), pp. 1-108 in European Journal of Taxonomy 799 (1) on pages 13-19, DOI: 10.5852/ejt.2022.799.1681, http://zenodo.org/record/6341270