Bactroceras fluvii Kröger & Pohle 2021, sp. nov.

Bactroceras fluvii sp. nov.

urn:lsid:zoobank.org:act: A0C49BC7-0551-4BD0-ABC5-77A446BBF5D1

Figs 12D, 29A, C–D, 30

Diagnosis

Bactroceras with smooth shell or very faint transverse ornamentation; angle of expansion relatively low, ca 5°; siphuncle slightly expanded within chambers, marginal; diameter of siphuncle ca ¹/5 of the corresponding conch diameter during early growth stages, increasing during ontogeny.

Etymology

From the Latin ‘ fluvius ’, ‘river’, referring to the type locality of this species.

Type material

Holotype Specimen FMNH-P30177.

Paratypes One additional specimen from type locality and horizon and 18 specimens from Profilbekken river basin, locality PR-phosphatic, adjacent to Hinlopenstretet, Spitsbergen, uppermost Olenidsletta Member, V3 trilobite zone, Dapingian.

Type locality and horizon

From Profilstranda section, adjacent to Hinlopenstretet, Spitsbergen, topmost exposed layer ca 153 meters above the base of the Olenidsletta Member, V 3 trilobite zone, Dapingian.

Description

The holotype is an apical part of a phragmocone with a length of 32 mm, with a maximum measurable diameter of 3.3 mm, and a subspherical protoconch with a diameter 1.1 mm (Fig. 29C–D). The conch surface is apparently smooth and the conch cross section circular. The protonconch is clearly distinguished from the subsequent shaft by a constriction with a diameter of 0.9 mm at a distance of ca 1 mm from the apical end of the shell. The conch is weakly curved with a concave curvature at the ventral (prosiphuncular) side and grows within the first 2 mm only to a conch diameter of 1.1 mm. A diameter of 1.3 mm is reached at a distance 5 mm from the apical end of the shell. The siphuncle is marginal throughout the entire length of the holotype, it is tubular to slightly expanded within the chambers and has a diameter of 0.6 mm at the adoral end of the specimen (rSD = 0.18). The septal necks are short orthochoanitic with a length of ca ⅓ of the length of the siphuncular segment. The chamber spacing varies greatly depending on the growth stage. The first chamber has a length of 0.6 mm, the next three chambers, which are roughly at the position of the constriction and initial shaft, are very short, with lengths of 0.25–0.35 mm, and the subsequent chambers have a length of 0.5–0.7 mm. At a conch diameter of 3.3 mm the chamber length is 1.2 mm (rCL = 0.37) and the septum curvature is 0.7 mm deep. The caecum is slightly elongated and reaches through the adoral half of the initial chamber with a length of 0.35 mm and a maximum diameter of 0.3 mm. The septal perforation of the first septum has a diameter of 0.2 mm.

In specimens FMNH-P30168 and FMNH-P30177 (holotype) the sutures of the initial growth stages are well preserved. They run directly transverse and form a shallow ventral lobe in the vicinity of the ventral siphuncle (Fig. 12D).

Only two specimens with diameters >> 3 mm are available. Specimen FMNH-P30200 is a natural sagittal section through a phragmocone with a diameter of 2.2–7.5 mm and a length of 34 mm (angle of expansion 9°), a siphuncular diameter of 1.5 mm (rSD = 0.21) and a chamber height of 3.8 mm (rCL = 0.51) at the adoral end of the fragment.

The largest specimen (FMNH-P30167) is a straight, ca 97 mm long fragment of a phragmocone with partly imploded septa with circular conch cross section, a smooth conch surface and a diameter of 12–19 mm (angle of expansion 4.4°) (Fig. 29A). The specimen has a marginal siphuncle with a 4.5 mm wide siphuncle at a corresponding conch cross section of 14.5 mm (rSD = 0.31) and a septal distance of 7.5 mm at the same position on the conch (rCL = 0.51). The septal necks are 1.7 mm long where the siphuncular segment is 7.5 mm long, and the septa have a curvature of 5 mm where the diameter of the septa is 14 mm.

Remarks

The sample of specimens of Bactroceras fluvii sp. nov. consists of 18 microscopic specimens with diameters <3 mm or slightly larger, one specimen with a maximum diameter of 7.5 mm and a single specimen with conch diameters of 12–19 mm. The large specimen differs from the microscopic specimens in its large relative siphuncle size (rSD = 0.31) and in its deep septal curvature (Fig. 30). Because no transitional growth stages are known, the combination of the two size classes and the assumption of large ontogenetic changes is somewhat speculative. However, because a similar increasing rSD is known from Bactroceras mourgesi Kröger & Evans, 2011, which also has a slightly expanded siphuncle, the assumption of a similar change in the phragmocone dimensions in B. fluvii gen. et sp. nov. is not unrealistic.

Comparison

This new species of Bactroceras is similar to B. mourgesi, from the Tremadocian of the Montagne Noire, in having a slightly expanded siphuncle with a diameter of ca 0.2 of the conch diameter and in having slightly curved early growth stages. Bactroceras fluvii gen. et sp. nov. differs from B. mourgesi in its concave curvature on the prosiphuncluar side in early growth stages and in having a lower angle of expansion. Bactroceras boliviensis differs in its distinctively striated conch and in being nearly tubular and straight immediately adoral of the protoconch. Additionally, the siphuncle of B. boliviensis is nearly tubular. Other species of Bactroceras have a narrower siphuncle (compare Aubrechtová 2015).