Calygirtyoceras darkaouaense Korn, Klug, and Mapes 1999
Creators: name: Klug, Christian
name: Baets, Kenneth De
name: Korn, Dieter
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Ammonoidea
Girtyoceratidae
Calygirtyoceras
Calygirtyoceras darkaouaense

description: Calygirtyoceras darkaouaense Korn, Klug, and Mapes, 1999
Fig. 5.
Holotype: GPIT 1851-87; Korn et al. 1999: pl. 1: 7.
Type locality: 12 km southeast of Dar Kaoua, southeast of Erfoud, eastern Anti-Atlas, Morocco.
Type horizon: Early late Viséan (Entogonites assemblage).
Material.— 6 specimens: three juvenile conchs in PIMUZ 31509, PIMUZ 31516, PIMUZ 31519; two additional juveniles in PIMUZ 31508; an incomplete adult specimen with septal crowding in PIMUZ 31513.
Emended diagnosis (modified after Korn et al. 1999).— Calygirtyoceras with a juvenile cadiconic conch (dm 5–10 mm; ww/dm 0.60–0.71), a transitional discoidal, platyconic preadult conch shape and a large, thinly discoidal, oxyconic adult conch (dm 100–150 cm; ww/dm 0.30–0.40). Umbilicus very wide in juvenile growth stages (uw/dm 0.08; dm 5–10 mm) and very narrow near adulthood (uw/ dm 0.10–0.15; dm> 50 mm). Whorl expansion rate very low in juvenile whorls (dm <10 mm) and increasing to almost 3 at> 50 mm diameter. Adult ornamentation with fine, crenulated growth lines with biconvex and slightly rursiradiate course. Suture line with low median saddle (median saddle width/height ratio 0.30), broadly rounded ventrolateral saddle, and narrow external lobe (external lobe width/height ratio 0.55).
Description.—PIMUZ 31516 (Fig. 5B) is a cadiconic juvenile with a thinly to thickly globular, very evolute conch. The whorl cross section is strongly depressed with a hardly vaulted venter; at the thickest part of the flanks, 12–15 small nodes are developed at a diameter below 10 mm per half whorl. In the subsequent ontogenetic stage, the venter becomes more and more vaulted and rounded. At a whorl height of about 30 mm (PIMUZ 31513), the cross section becomes subtriangular with gently rounded flanks. PIMUZ 31513 is a fragment of an adult conch (septal crowding at dm ca. 110 mm). At this stage, the conch is extremely discoidal and involute (Table 3). In this specimen, the external lobe is incompletely visible but V-shaped with a high ventrolateral saddle as present in the type-material. The V-shaped adventive lobe is inclined dorsally with a convex ventral flank.
Remarks.—In the original species diagnosis by Korn et al. (1999), the ontogenetic changes were described in a somewhat misleading way: in the same article, we (Korn et al. 1999: p. 354) had stated that the “Umbilicus [is] very narrow in early growth stages (uw/dm = 0.08 at 20–30 mm dm) and slightly opening during ontogeny (stages (uw/dm = 0.10 to 0.12 at 40–60 mm dm).” When measuring the umbilical width throughout the entire post-hatching ontogeny, a rather constant reduction of the uw/dm ratio from 0.62 to 0.14 was measured. Also, the latest growth stage was unknown previously.
Stratigraphic and geographic range.—Early late Viséan, so far only known from the Tafilalt (Morocco).
description: Published as part of Klug, Christian, Baets, Kenneth De & Korn, Dieter, 2016, Exploring the limits of morphospace: Ontogeny and ecology of late Viséan ammonoids from the Tafilalt, Morocco, pp. 1-14 in Acta Palaeontologica Polonica 61 (1) on page 6, DOI: 10.4202/app.00220.2015, http://zenodo.org/record/10626349