Trilacinoceras discors ( Holm, 1891 )

Trilacinoceras discors ( Holm, 1891)

Fig. 66A–B View Fig , 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 View Fig ) 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 View Fig ) 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).