Ethanoceras solitudines Kröger & Pohle 2021, gen. et sp. nov.

Ethanoceras solitudines gen. et sp. nov.

urn:lsid:zoobank.org:act: 6627FF29-392E-46AF-89AC-D4528B10385A

Figs 12I, 13A, 14–15, 16A

Diagnosis

Same as for genus, by monotypy.

Etymology

From the Latin ‘ solitudine ’, ‘loneliness’.

Type material

Holotype Specimen FMNH-P30383.

Paratypes Twenty-five specimens (P20265, P30191, P30384 to P30404, P30406, P30433, P30435, P30436; see Supp. file 1 for list of specimens): twenty-two from bed PO 123.3, and three from bed PO 131 Profilbekken section, adjacent to Hinlopenstretet; 120.3 and 128 m above base of Olenidsletta Member, V2 trilobite zone, Blackhillsian, late Floian. Five additional microscopic specimens (FMNH-P30173, P30174, P30192, P30423, P30463) from bed PO 123.3.

Type locality and horizon

Profilstranda section, adjacent to Hinlopenstretet, bed PO 131, 128 m above base of Olenidsletta Member, V 2 trilobite zone, Blackhillsian, late Floian.

Description

The holotype is a 40 mm long orthoconic fragment of a phragmocone with a diameter of 3.4–6.1 mm; it has a circular conch cross section and an apical angle of ca 4°. This is similar in the entire sample. The total sample has a circular conch cross section and a mean angle of expansion of 4° (1 st –3 rd quantile: 3°–5°; n = 10) (Fig. 14). The angle of expansion continuously decreases with growth and with maximum values in those fragments with the smallest diameters (Fig. 14).

The conch surface is ornamented with very fine but distinctive, narrowly and irregularly spaced striae (Fig. 13A). In the holotype ca 10–12 striae occur per one millimeter; these are directly transverse.

The relative chamber length (rCL) varies between 0.3 and 0.6 in the macroscopic specimens and is ca 0.55 in the holotype (Fig. 14). The microscopic fragments have relative chamber lengths of 0.6–0.8, indicating a decrease in rCL during ontogeny. The siphuncle is positioned between the conch center and conch margin with a rSP of 0.08–0.12, except in the fragments with the smallest diameter preserved, where the maximum rSP is 0.18 at a conch diameter of 2.2 mm (specimen FMNH-P30463). The measurements indicate a slight shift of the siphuncle from more detached positions in early growth stages toward more marginal positions in later growth stages. The relative diameter of the septal perforation (rSD) varies between 0.16 and 0.21; it is ca 0.16 in the holotype (Fig. 14).

The septal necks are uniquely shaped and relatively long: in specimens FMNH-P30395 and FMNH-P30394, where the conch diameter is ca 5 mm, the septal perforation is 0.8 mm, and the chamber length is ca 1.8 mm, the length of the septal neck is 0. 4–0.5 mm (0.2–0.3 of corresponding chamber length). The necks are s-shaped at the dorsal (toward the conch center) side of the connecting ring and have a loxochoanitic distal tip (Figs 15, 16A). At the ventral (toward the conch margin) and lateral sides the necks are loxochoanitic.

The connecting ring is relatively thick. Where the siphuncle is 0.8 mm in diameter, the thickness of the connecting ring is ca 0.13 mm and it is slightly s-shaped, with the maximum diameter of the connecting ring segment within the adapical third of the chamber.

The protoconch is preserved in specimen FMNH-P30173, a slightly curved fragment with a length of 8 mm which shows traces of a very fine transverse striation at its adoral end. The protoconch is spherical with a diameter of 0.9 mm and distinguished from the shaft by a distinctive constriction with a diameter of 0.8 mm. The maximum diameter of the fragment is ca 1.4 mm. The chamber lengths are difficult to evaluate, but apparently the first two or three chambers have a length of 0.5–0.6 mm, the fifth and sixth chambers are shortened, with a distance of 0.4–0.5 mm only, and the subsequent chambers have a length of ca 0.7 mm.

Remarks

The single apical fragment with a protoconch preserved can be assigned to Ethanoceras solitudines gen. et. sp. nov. because several fragments of this species exist which preserve intermediate growth stages from different individuals. These pieces can be assembled together, helping to reconstruct the complete early growth of this species. The early growth stages are very similar to those of Bactrites boliviensis in general shape and dimensions, and differ mainly in having a higher rate of expansion at sections just adoral to the protoconch (the shaft), where B. boliviensis is almost tubular and E. solitudines gen. et sp. nov. grows with an angle of ca 4–7°.

Stratigraphic and geographic range

V 2 trilobite zone, Olenidsletta Member, Blackhillsian, Floian, Early Ordovician.