Trilacinoceras cf. hunanense Lai & Qi, 1977

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 : 95

Trilacinoceras cf. hunanense Lai & Qi, 1977

Figs 69B , 71 , Tables 12–13

Material examined

CHINA • 1 spec.; Guizhou, Songtao; Ordovician; Bottke Coll.; MB.C.29655 .

Description

Specimen MB.C.29655 ( Fig. 69B ) is a 326 mm long conch, which is partially embedded in the surrounding dark-brown limestone. The coiled part has a diameter of 36 mm and consists of at least 2.25 tightly coiled volutions (initial chamber is not preserved); the whorl expansion rate is 2.10 ( Fig. 71 ). The uncoiled part is relatively strongly curved and expands with an angle of 4°. Ornament is preserved along the whole length of the specimen and is slightly irregular in distance. In the coiled part, the ornament consists of growth lines and raised lirae; in inner whorls, the ornament elements are straight, later, a shallow sinus is developed ventrolaterally. In the rest of the conch, narrow annuli (up to 2.7 mm in distance) and lirae (up to 0.3 mm apart) are present; the distance and thickness of the ornament elements is irregular; the conch is slightly constricted, where the ornament elements are the finest. The ornament elements in the backcoiled part have a rectiradiate direction but towards the straight part, they become rursiradiate; a marked dorsal projection and a sinus between the mid-flank and the ventrolateral projection is developed. The adapertural part of the specimen might represent the body chamber but terminal aperture is not preserved and suture lines are not visible.

Remarks

Trilacinoceras cf. hunanense differs from T. hunanense in a smaller diameter of the coiled conch (36 mm vs 40 mm) and a more strongly curved (vs straight) uncoiled part (Xiang Fang, pers. com.). Trilacinoceras knoefleri sp. nov. is distinguished by its smaller expansion angle and coarser, rectiradiate ornament of the backcoiled part. The rursiradiate direction of ornament elements in the straight conch part of Trilacinoceras cf. hunanense distinguishes the Chinese specimen from all specimens in the collection that come from European regions.

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. 71. Trilacinoceras cf. hunanense Lai & Qi, 1977. A. Coiling outline of specimen MB.C.29655. B–E. Conch proportions and coiling rates of specimen MB.C.29655. Scale bar units = 1 mm.