Imitoceras initium sp. nov.

urn:lsid:zoobank.org:act: F4100C55-64E5-48AF-8EDC-87CDA35580F1

Fig. 65; Table 63

Diagnosis

Species of Imitoceras with a conch reaching 60 mm diameter. Conch at 50 mm dm discoidal, involute (ww/dm ~0.45; uw/dm ~0.05); whorl profile at 30 mm dm weakly compressed (ww/wh ~0.75); coiling rate very high (WER ~2.30). Venter rounded, umbilical margin rounded, flanks strongly convergent. Growth lines very fine, narrow-standing, with slightly biconvex course. Weak constrictions on the shell surface of the inner flank. Suture line with a weakly pouched external lobe and a V-shaped adventive lobe.

Etymology

After Latin ‘ initium ’ = ‘the beginning’; because of the position of the species at the base of a long-ranging evolutionary lineage.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen, west of railway cutting; Hangenberg Limestone, loose material; Korn 1977 Coll.; illustrated in Fig. 65; MB.C.31144.

Description

Holotype MB.C.31144 is well preserved and has a conch diameter of 52 mm (Fig. 65A). It is thinly discoid with a slightly opened umbilicus (ww/dm = 0.43; uw/dm = 0.05) and has a compressed whorl profile that is widest at the rounded umbilical margin; it has fairly strongly converging flanks and a rounded venter. The coiling rate is very high (WER = 2.30). The shell bears barely visible, extremely faint growth lines with a biconvex course and also a shell constriction limited to the inner flank. The suture line has a weakly pouched external lobe that is about two-thirds the depth of the nearly symmetric V-shaped adventive lobe (Fig. 65B).

Remarks

The occurrence of the genus Imitoceras in the Gattendorfia Limestone near Oberrödinghausen was previously unknown, but is actually not a great surprise. Representatives of Imitoceras have already been found together with early Tournaisian ammonoids in Guizhou (Sun & Shen 1965; Ruan 1981), originally described as “ Imitoceras planolobatum ” and “ Imitoceras (Imitoceras) crassum ”, and the Anti-Atlas (Bockwinkel & Ebbighausen 2006).

The only specimen available is unique in the ammonoid assemblage from Oberrödinghausen because of its high coiling rate of 2.30. Such a value is rarely reached by other members of the prionoceratid ammonoids. In the subfamily Acutimitoceratinae, the whorl expansion rate is normally between 1.70 and 1.90 in Stockumites and between 1.90 and 2.05 in Nicimitoceras .

Of these genera, only Nicimitoceras has a short external lobe like in Imitoceras initium sp. nov. However, I. initium is stouter than the size-equivalent holotypes of other Nicimitoceras species from Oberrödinghausen: its ww/dm ratio is 0.43 at 52 mm dm in I. initium, while the value in N. heterolobatum, for example, is 0.36 at 54 mm dm. In addition, the whorl profile shows rapidly converging flanks in I. initium, which tend to be subparallel in species of Nicimitoceras .

A superficially similar species to Imitoceras initium sp. nov. is Acutimitoceras procedens from the Stockum limestone equivalent of the Müssenberg (Korn 1981). This also shows a high coiling rate, rapidly convergent flanks, and a shell ornament with weak growth lines and constrictions. The holotype of this species is much stouter (ww/dm = 0.54 at 33 mm dm) than holotype MB.C.31144 of A. initium (ww/dm = 0.43 at 52 mm dm). This difference cannot be explained by the advanced ontogenetic evolution towards a more slender conch in specimen MB.C.31144, as this has half a whorl in front of the largest diameter, a ww/dm ratio of only 0.47 at 36 mm conch diameter.