Trilacinoceras filix, Aubrechtová & Korn, 2022

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 : 92-94

Trilacinoceras filix sp. nov.

urn:lsid:zoobank.org:act:76E53596-0229-4CB3-A62F-E12F3A296FEB

Figs 69A , 70 , Tables 12–13

Lituites cf. perfectus – Neben & Krueger 1971: pl. 31 fig. 4.

Lituites sp. – Neben & Krueger 1971: pl. 31 fig. 6.

Diagnosis

Species of Trilacinoceras with an openly coiled conch, 28 mm in diameter. Whorl profile slightly compressed throughout ontogeny. Backcoiled part of the conch moderately curved. Juvenile expansion angle <1°, later in ontogeny up to 5°. Shell ornament with narrow annuli (up to 4 mm in distance) and fine lirae in between; rectiradiate direction with low dorsal projection, deep ventral sinus and shallow lateral sinus.

Etymology

From the Latin ‘ filix ’ = ‘fern’, referring to the shape of the holotype, which resembles the convoluted young leaves of some ferns.

Type material

Holotype GERMANY • Mecklenburg-Vorpommern, Rerik (Ostsee); Ordovician (Lasnamägi Regional Stage, late Darriwilian), Upper Grey Orthoceratite Limestone; Neben and Krueger 1961 Coll.; illustrated by Neben & Krueger (1971: pl. 31 fig. 4) and re-illustrated here in Fig. 69A ; MB.C.30557 .

Paratype GERMANY • Brandenburg, Oderberg; Ordovician, Upper Grey Orthoceratite Limestone; Neben Coll.; MB.C.30556 .

Description

Holotype MB.C.30557 ( Fig. 69A ) consists of the coiled part (dm = 28 mm; 1.25 openly coiled whorls; WER dm = 2.50, WER ah = 2.00) and a long portion of the almost straight backcoiled part and straight part (length = 250 mm; EA increases from ~ 1° to 4.6°). The whorl profile is compressed (WWI increases from 0.86–0.92). It can be estimated that about half of the original body chamber is preserved. The CLR is 0.40–0.50, without any apparent ontogenetic trend ( Fig. 70 ). The shell ornament consists of annuli (1 to 3 mm in distance); fine lirae are locally preserved. The ornament elements form a deep ventral sinus and a low dorsal projection; on the flank, the elements are either straight or form a very shallow sinus.

Paratype MB.C.30556 is an incomplete, very slightly curved specimen of a body chamber with one chamber of the phragmocone preserved. The conch has similar proportions like the holotype but with better preserved shell ornament. The ornament consists of irregularly spaced annuli and lirae. The annuli are more strongly approximated behind the position of the last preserved septum (ca 1 mm apart) and have widest distances in the mid-length of the specimen (4 mm apart). The ornament elements form a deep ventral sinus, shallow lateral sinuses and a low dorsal projection. The single preserved phragmocone chamber has CLR of 0.32.

Remarks

The new species is assigned to the genus Trilacinoceras because the conch has a conspicuously low expansion angle and the whorl profile remains compressed even in the apertural part of the conch (max. WWI 0.92 in the holotype and 0.94 in the paratype). The species is most similar to the type species T. discors , in which the conch has a higher expansion angle (3–6°), particularly after uncoiling. Furthermore, the ornament is different in both species. In T. discors , narrow but prominent annuli and lirae are developed in the coiled part and wider and flatter annuli in the uncoiled part. By contrast, the holotype of T. filix sp. nov. is decorated by rather widely spaced and strong annuli (1 mm apart) in the coiled part and comparatively narrower and widely spaced annuli (up to 4 mm apart) towards the apertural part.

Trilacinoceras filix sp. nov. is also rather similar to Lituites nebeni sp. nov. but the latter has a circular whorl profile, a more prominent shell ornament and a stronger curvature of the backcoiled part.

Geographic and stratigraphic occurrence

Northern Germany (in erratics within Pleistocene gravels); late Lasnamägi Regional Stage, late Darriwilian, Middle Ordovician.

Figures

Fig. 69. Species of Trilacinoceras Sweet, 1958. A. Trilacinoceras filix sp. nov., holotype MB.C.30557 (Neben & Krueger 1961 Coll.) from the Upper Grey Orthoceratite Limestone of Rerik (Ostsee) (Mecklenburg-Vorpommern); previously figured by Neben & Krueger (1971: pl. 31 fig. 4). B. Trilacinoceras cf. hunanense Lai & Qi, 1977, specimen MB.C.29655 (Bottke Coll.) from the Datianba Formation of Songtao County (Guizhou, South China). Scale bar units = 1 mm.

Fig. 70. Trilacinoceras filix sp. nov., chamber length/whorl height of holotype MB.C.30557 (Neben & Krueger Coll.) from Rerik (Ostsee) (Mecklenburg-Vorpommern). A. Empirical data. B. Three data point averaged.