Trilacinoceras Sweet, 1958

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 : 86-87

Genus Trilacinoceras Sweet, 1958

Type species

Lituites discors Holm, 1891 ; designated by Sweet (1958).

Species included

Trilacinoceras aqiaense Lai & Wang in Wang, 1981; Trilacinoceras costatum Flower, 1975 ; Lituites discors Holm, 1891 ; Trilacinoceras filix sp. nov., Trilacinoceras hunanense Lai & Qi, 1977 ; Trilacinoceras kalpinense Lai & Wang in Wang, 1981; Trilacinoceras knoefleri sp. nov., Trilacinoceras norvegicum Sweet, 1958 ; Trilacinoceras reubeni Flower, 1975 ; Trilacinoceras undulatum Flower, 1975 ; Trilacinoceras xinjiangense Lai & Wang in Wang, 1981.

Diagnosis

Genus of the family Lituitidae with coiled part 18–40 mm in diameter; two to three open or tightly coiled volutions; whorl expansion rate 2.00–3.00. Whorl profile compressed throughout ontogeny. Uncoiled part straight to moderately curved, compressed, expansion angle up to 7°. Shell surface with narrow or prominent annulations and lirae, with conspicuously deep ventral sinus and five projections adapically, but only three projections (ventrolateral and dorsal) throughout most of the uncoiled part. Aperture with deep ventral sinus and three lappets: two incurving, tongue-shaped ventrolateral lappets and broad, low dorsal lappet; secondary dorsolateral lappet pair present in some species but this is not heralded in either the growth-lines, or annulations at the extreme adoral end of the body chamber. Siphuncle dorsad of centre throughout the length of phragmocone (after Sweet 1958 and Aubrechtová & Turek 2018).

Remarks

Flower (1975) attributed three newly described species to the genus Trilacinoceras , which are supposed to differ from all other species in the combination of a slightly depressed cross section (WWI = 1.10–1.25) and suture lines with broad and shallow ventral lobe. These characters would require an emendation of the genus diagnosis provided by Sweet (1958). However, the type specimens described by Flower (1975) are rather incomplete and the preservation raises doubts about the primary origin of the peculiarities of their conch morphology. Therefore, any emendation of the original diagnosis of the genus is avoided here.

Geographic and stratigraphic occurrence

Nevada, Baltoscandia, northern Germany and Poland, Iberian Peninsula (?), central Bohemia and China; Middle to Late Ordovician.