Maccoyoceras concavum, Korn & Bockwinkel, 2022

Korn, Dieter & Bockwinkel, Jürgen, 2022, Early Carboniferous nautiloids from the Central Sahara, southern Algeria, European Journal of Taxonomy 831, pp. 67-108 : 102-103

publication ID	https://doi.org/ 10.5852/ejt.2022.831.1871
publication LSID	lsid:zoobank.org:pub:EAA2EAE2-DA8B-4516-B332-CB8423B8EEAA
DOI	https://doi.org/10.5281/zenodo.6902642
persistent identifier	https://treatment.plazi.org/id/CC00C3E7-5DF4-4FA9-844C-AAA60F1F5016
taxon LSID	lsid:zoobank.org:act:CC00C3E7-5DF4-4FA9-844C-AAA60F1F5016
treatment provided by	Felipe
scientific name	Maccoyoceras concavum
status	sp. nov.

Treatment

Maccoyoceras concavum sp. nov.

urn:lsid:zoobank.org:act:CC00C3E7-5DF4-4FA9-844C-AAA60F1F5016

Fig. 29 View Fig , Table 17 View Table 17

Diagnosis

Species of Maccoyoceras reaching about 100 mm conch diameter. Conch with weakly compressed whorl profile (ww/wh ~ 0.90); venter weakly concave, ventrolateral shoulder narrowly subangular. Whorls weakly embracing the preceding. Ornament in the juvenile stage with few coarse, granulated spiral lines on the flank, in the adult stage without spiral lines. Fine, sharp growth lines on the flank, with weakly biconvex course with a shallow lateral sinus and a moderately high ventrolateral projection. Venter with delicate growth lines with deep sinus.

Etymology

After Latin ‘ concavum ’, referring to the shape of the venter.

Type material

Holotype ALGERIA • Ahnet , west-southwest of Gara Azzel Matti; ‘ Dalle des Iridet’ (Ammonellipsites-Merocanites Assemblage; Tournaisian–Viséan boundary interval); Wendt and Kaufmann 1995 Coll.; illustrated in Fig. 29 View Fig ; MB.C.30486 .

Description

Holotype MB.C.30486 is a rather well-preserved, almost completely chambered specimen with 76 mm conch diameter, from which the last, poorly preserved segment was removed before photography ( Fig. 29 View Fig ). The specimen allows the study from both sides. The conch is extremely discoidal (ww/ dm =0.32) and subevolute (uw/dm =0.41) with a very high coiling rate (WER =2.35). The whorl profile is compressed hexagonal (ww/wh =0.93) and widest at the rounded umbilical margin. The flanks stand almost parallel and converge barely towards the angular ventrolateral shoulder that separates the flanks from the weakly concave venter. The umbilical wall is convexly rounded.

Two ontogenetic stages of the ornament development can be separated: up to 17 mm conch diameter, six coarse, crenulated spiral lines on the flank are the dominant elements in the juvenile stage. Thereafter, fine but sharp narrow-standing growth lines with weakly biconvex course form the ornament on the flanks, but the venter bears very delicate growth lines with a deep ventral sinus. The suture line shows a shallow ventral lobe and a shallow lateral lobe. The siphuncle is almost central.

Remarks

Maccoyoceras concavum sp. nov. has conch proportions very similar to those of M. pentagonum from the contemporaneous ‘Dalle à Merocanites ’ of Timimoun. However, the new species differs from this and from the other species of the genus by the concave venter.