Lispoceras orbis, Korn & Miao & Bockwinkel, 2022

Korn, Dieter, Miao, Luyi & Bockwinkel, Jürgen, 2022, The nautiloids from the Early Carboniferous Dalle à Merocanites of Timimoun, western Algeria, European Journal of Taxonomy 789, pp. 104-129 : 111-115

Lispoceras orbis sp. nov.

urn:lsid:zoobank.org:act:06A766FF-8347-4085-ACB2-39F48B4AD4AE

Figs 4–6 ; Table 2

Diagnosis

Species of Lispoceras reaching about 90 mm conch diameter. Conch with nearly circular whorl profile (ww/wh = 1.00–1.10). The outer whorl just touches the preceding ones. Ornament in the juvenile stage with spiral lines on the flank and the venter, in the adult stage without spiral lines. Growth lines with weakly biconvex course with a shallow lateral sinus, a moderately high ventrolateral projection and a deep, semicircular ventral sinus.

Etymology

From the Latin nomen ‘ orbis ’, meaning ‘circle’ and referring to the whorl profile.

Type material

Holotype ALGERIA • Gourara , Sebkha de Timimoun 14.5 km west-southwest of Timimoun; “ Dalle à Merocanites ” (Tournaisian-Viséan boundary interval); illustrated in Fig. 4A ; MB.C.30379.1 .

Paratypes ALGERIA • 14 specimens; Gourara , Sebkha de Timimoun 14.5 km west-southwest of Timimoun; “Dalle à Merocanites ” (Tournaisian-Viséan boundary interval); MB.C.30379.2 – MB.C.30379.15 .

Description

Holotype MB.C.30379.1 consists of just over two and a half volutions, in which it reaches a conch diameter of 84 mm ( Fig. 4A ). Its initial stage is loosely coiled with an 8 mm wide umbilical window; the poorly preserved protoconch has a circular whorl profile of about 2.5 mm diameter. The umbilical window is closed at 26 mm diameter of the conch. The proportions of the conch change only slightly ontogenetically; the whorl profile is circular throughout ontogeny. At a diameter of 84 mm, the conch is thinly discoidal and subevolute (ww/dm = 0.42; uw/dm = 0.39), with a high coiling rate of the conch (WER = 2.66). Near the maximum diameter of the specimen, the whorl width increases (ww/wh = 1.10), which may indicate adulthood of the specimen.

The ornament shows a succession of four developmental stages in the present specimens:

(1) The first volution, up to a conch diameter of 18 mm, possesses a combination of coarse spiral lines (about 50 from middorsum to midventer) and growth lines of the same strength. Together, they

produce a coarse granulation, in which the elements are much wider than their interstices. The growth lines extend backwards on the flank.

(2) In the three quarters of a whorl between 18 and 36 mm diameter, the ornament is reticulate with a continuous weakening of both the spiral lines and the granulation. At a diameter of 20 mm, the spiral lines are as wide as their interspaces, but at 36 mm diameter, the interspaces are already five times as wide as the spiral lines. The growth line course is rectiradiate and weakly biconvex.

(3) In a quarter of a volution (between 36 and 44 mm dm), the spiral lines are very weak; the growth lines are also weaker when compared with the preceding stage.

(4) The last stage above 44 mm conch diameter is then characterised by the loss of the spiral lines. Very fine growth lines extend across the flank in a weakly biconvex course, forming a deep, semicircular ventral sinus.

Intraspecific variation is limited in the material ( Fig. 6B–E ), but the cardinal conch parameters show some ontogenetic changes in the growth interval between 20 and 85 mm conch diameter: (1) the ww/ dm ratio increases slowly from an average of 0.38 to 0.42 ( Fig. 6B ), (2) the uw/dm ratio decreases from nearly 0.50 to 0.38 ( Fig. 6C ), (3) the ww/wh ratio decreases from 1.20 to 1.00 ( Fig. 6D ) and (4) the whorl expansion rate shows a more prominent increase from 2.20 to 2.80 ( Fig. 6E ).

Variation in the ornament is also limited. The change from the third to the fourth ornament stage, for instance, takes place between 37 and 42 mm conch diameter in specimens MB.C.30379.4 ( Fig. 5B ), MB.C.30379.2 ( Fig. 4B ) and MB.C.30379.3 ( Fig. 5A ).

Remarks Lispoceras orbis sp. nov. has a circular to very weakly depressed whorl profile (ww/wh = 1.00–1.10) and

thus differs from L. trivolve with a slightly more depressed whorl profile (ww/wh = 1.25). In addition, L. trivolve has much coarser spiral lines ( Kummel 1964). L. proconsul also has more depressed whorl profile (ww/wh = 1.05–1.25) and whorls that detach from the previous whorl at about 30 mm diameter of the conch ( Shimansky 1967).

Figures

Fig. 4. Lispoceras orbis sp. nov. A. Holotype MB.C.30379.1. B. Paratype MB.C.30379.2. Scale bar units = 1 mm.

Fig. 5. Lispoceras orbis sp. nov. A. Paratype MB.C.30379.3. B. Paratype MB.C.30379.4. Scale bar units = 1 mm.

Fig. 6. Lispoceras orbis sp. nov. A. Reconstructed cross section of holotype MB.C.30379.1. B–E. Ontogenetic trajectories of the cardinal conch parameters of selected specimens. Scale bar units = 1 mm.