Trilacinoceras discors ( Holm, 1891 )

Aubrechtová, Martina & Korn, Dieter, 2022, Taxonomy and ontogeny of the Lituitida (Cephalopoda) from Orthoceratite Limestone erratics (Middle Ordovician), European Journal of Taxonomy 799 (1), pp. 1-108 : 87-89

Trilacinoceras discors ( Holm, 1891)

Fig. 66A–B , Tables 12–13

Lituites discors Holm, 1891: 26 , pl. 1 figs 5–7, pl. 3 figs 3–4.

Ancistroceras discors – Hyatt 1894: 508.

Trilacinoceras discors – Sweet 1958: 147, pl. 13 figs 1, 4, 6, pl. 15 fig. 5, pl. 16 figs 1, 3, 7–8, 10. — Balashov 1962: pl. 7 fig. 6. — Dzik 1984: 138, text-fig. 55.12.

Lituites discors – Dzik 1984: 137, text-fig. 53b, e, pl. 40 figs 4–10.

Diagnosis

Species of the genus Trilacinoceras with coiled conch 18–33 mm in diameter; 2–3 coiling open or whorls contiguous. Whorl profile slightly compressed through ontogeny, commonly ventrally flattened; whorl expansion rate ca 2.70. Expansion angle of uncoiled part 3–6°; whorl height at maturity about 23 mm. Shell surface with annuli and/or lirae, typically fine in the coiled part (less than 1 mm apart) and coarser in the uncoiled part (up to ca 3 mm apart) of the conch. Mature aperture with very shallow and broad lateral sinuses and a short and wide ventral sinus (after Holm 1891 and Sweet 1958; modified).

Type material

Not available for study; Holm (1891) did not specify a type but he illustrated several specimens ( Holm 1891: pl. 1 figs 5–7, pl. 3 figs 3–4); from Lerkaka (Island of Öland, Sweden), uppermost Red Lituites Limestone (Lasnamägi Regional Stage, late Darriwilian, Middle Ordovician).

Material examined

GERMANY • 1 spec.; Brandenburg, Niederfinow; Ordovician, Upper Red Orthoceratite Limestone; Neben and Krueger Coll.; MB.C.30552 • 1 spec.; Brandenburg, Schwedt; Ordovician, Upper Red Orthoceratite Limestone; MB.C.30553 .

POLAND • 1 spec.; West Pomerania, Skowarcz (former Schönwarling), Gdańsk County; Ordovician, Upper Red Orthoceratite Limestone; MB.C.30554 • 2 specs; West Pomerania, Ustronie Morskie (former Henkengahen); Ordovician, Upper Red Orthoceratite Limestone; Krause Coll.; MB.C.11632.1 , MB.C.11632.3 .

COUNTRY UNKNOWN • 1 spec.; Ordovician, Upper Red Orthoceratite Limestone; MB.C.30555 .

Description

Specimen MB.C.30552 ( Fig. 66A ) is a fragment of a coiled part (dm = 30 mm; 1.3 openly coiled whorls but the whorls get in contact just before uncoiling) and the almost straight backcoiled part (length = 60 mm; wh = 14 mm; EA ~ 4°) of a conch. The whorl profile is compressed (WWI increases from 0.83–0.90 during the backcoiled stage). The shell ornament of the coiled part consists of rhythmically strengthened growth lines (0.10–0.15 mm apart), which ontogenetically transform into coarser lirae (up to 0.5 mm in distance); narrow annuli (ca 2.5 mm apart) appear after uncoiling. The ornament elements form a deep ventral sinus, a shallow to very shallow lateral sinus and a low dorsal projection; the ventral sinus is delineated by raised longitudinal lines in the coiled part, where the venter of the conch is slightly flattened. The ornament elements extend with a concavo-convex course; they are rectiradiate in the coiled part and prorsiradiate in the backcoiled part.

Specimen MB.C.30553 ( Fig. 66B ) is a fragment of a coiled conch, 25 mm in diameter. The shell ornament consists of growth lines, which ontogenetically transform into transverse, slightly asymmetric lirae (0.8 mm distance) with fine growth lines in between them. The ornament elements form a very shallow lateral sinus and a deep ventral sinus; a low subangular ridge is present ventrally.

Remarks

Trilacinoceras discors is most similar to Lituites toernquisti (described above), but differs in the possession of a three-lappeted aperture (instead of five-lappeted) and in being usually larger in conch dimensions ( Holm 1891; Sweet 1958).

Geographic and stratigraphic occurrence

Baltoscandia (in situ) and northern Germany and Poland (in erratics within Pleistocene gravels); late Aseri to Lasnamägi regional stages (late Darriwilian Middle Ordovician), possibly also early Kukruse Regional Stage (early Sandbian, early Late Ordovician).

Trilacinoceras discors has a relatively wide stratigraphic and palaeogeographic range of occurrence. It is characteristic for the early Lasnamägian Lituites discors Zone of Baltoscandia ( Jaanusson & Mutvei 1953; Jaanusson 1960; Evans et al. 2014) but occurs also in the late Lasnamägian rocks of Germany (Pleistocene erratics) and possibly in the even younger, Uhakuan, strata of peri-Gondwana ( Sá & Gutiérrez-Marco 2009; Aubrechtová & Turek 2018).

Figures

Fig. 66. Species of Trilacinoceras Sweet, 1958 from the Upper Grey Orthoceratite Limestone. A. Trilacinoceras discors (Holm, 1891), specimen MB.C.30552 (Neben & Krueger Coll.) from Niederfinow(Brandenburg); previouslyfiguredby Neben&Krueger(1971:pl. 21fig.1). B.Trilacinoceras discors (Holm, 1891), specimen MB.C.30553 from Schwedt (Brandenburg). C. Trilacinoceras knoefleri sp. nov., holotype MB.C.11658 (Knöfler Coll.) from Teschendorf near Löwenberg (Brandenburg). D. Trilacinoceras knoefleri sp. nov., paratype MB.C.30559 (Krueger Coll.) from Mukran (Sassnitz, Island of Rügen) (Mecklenburg-Vorpommern). Scale bar units = 1 mm.