Pharciceras galeatum Wedekind, 1918

Korn, Dieter & Bockwinkel, Jürgen, 2021, The pharciceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea), European Journal of Taxonomy 771, pp. 1-79 : 33-35

Pharciceras galeatum Wedekind, 1918

Figs 25–27 ; Table 14

Pharciceras galeatum Wedekind, 1918: 128 , 168, pl. 20 fig.1, text-fig. 37b.

Pharciceras galeatum – Matern 1931: 92. — House in House & Ziegler 1977: 79, pl. 5 figs 29–31.

Diagnosis

Pharciceras with extremely discoidal, subevolute conch at 80 mm dm (ww / dm ~ 0.27; uw / dm ~ 0.35). Whorl profile weakly compressed at 80 mm dm (ww / wh ~0.65); whorl expansion moderate (WER ~ 1.95). Venter narrowly rounded in the preadult stage; adult stage with strongly converging flanks bordered by a subangular ventrolateral shoulder from the tectiform venter. Growth lines coarse, strongly biconvex; ventrolateral shoulder with two spiral grooves. Outer suture line with symmetric, V-shaped prongs of the external lobe, a V-shaped lateral lobe, and a small rounded U 2 lobe.

Material examined

Holotype GERMANY • Rhenish Mountains , Oberscheld ( Grube Prinzkessel ); late Givetian (Red Ironstone); Welsch 1913 Coll.; illustrated by Wedekind (1918: pl. 20 fig. 1) and House & Ziegler (1977: pl. 5 figs 29–31); SMF.Mbg.2316 . ( Fig. 25 )

Additional material

GERMANY • 1 specimen; Rhenish Mountains , Oberscheld ( Grube Anna ); late Givetian (Red Ironstone); Koch Coll.; MB.C.22182 • 1 specimen; Rhenish Mountains , Oberscheld (Grube Sahlgrund, zwischen 370- und 420 m Sohlen); late Givetian (Red Ironstone); Kegel 1927 Coll.; MB.C.22180 .

Description

Holotype SMF.Mbg.2316 is a rather well-preserved but incomplete specimen with 74 mm conch diameter in red ironstone ( Fig. 25 ). It allows the study of the adult conch as well as portions of the inner whorls. Its last volution is extremely discoidal and subevolute. It is widest at the rounded umbilical margin, from where the flanks strongly converge towards the tectiform venter. This shape of the venter is developed only during the last half volution; the venter is still rounded at 180 degrees before the largest diameter. The ventrolateral shoulder bears two longitudinal grooves on each side. The inner whorls are very evolute and it appears that the whorl profile is crescent-shaped throughout most of the ontogeny.

The suture line of the holotype has a wide external lobe with a median saddle attaining more than half of the lobe depth ( Fig. 27A ). The prongs of the external lobe are V-shaped and symmetric. They are separated from the V-shaped lateral lobe by a rather narrow, rounded ventrolateral saddle. The U 2 lobe is rather small and rounded.

MB.C.22182 is an incomplete specimen with 88 mm conch diameter in red ironstone; the last volution belongs almost completely to the body chamber ( Fig. 26A ). It has an extremely discoidal and subevolute conch; the whorl profile is compressed with shallow umbilical wall, rounded umbilical margin, and convergent flanks that are widest at some distance from the umbilicus. The venter is narrowly rounded in the penultimate whorl, but developing into a subacute shape with a rounded keel at the end of the last volution. The inner whorls display coarse growth lines which become finer on the last volution. Runzelschicht is present in the dorsal whorl zone; it consists of fine wrinkles with slightly prorsiradiate direction.

MB.C.22180 is an incomplete specimen with 69 mm conch diameter in iron-rich micritic limestone, with several calcite veins cutting through ( Fig. 26B ). It shows very similar conch proportions; the whorl profile is also compressed with shallow umbilical wall, rounded umbilical margin, and convergent flanks. The narrowly rounded venter is separated from the flanks by a subangular margin accompanied by a shallow spiral groove. In the penultimate whorl, the venter develops into a subacute shape with a rounded keel at the end of the last volution. The inner whorls show a rather well-preserved ornament; the growth lines are coarse in the inner whorls and slightly finer on the last volution.

Remarks

Pharciceras galeatum is, in the adult stage, easily separable from all other pharciceratids by its tectiform venter. A clear separation using conch characters in small specimens may be difficult, but the compressed whorl profile and the rather narrow venter may serve as good criteria. The suture line provides another character for a separation, as P. galeatum possesses V-shaped ventrolateral prongs and a V-shaped lateral lobe.

= 1 mm. Pharciceras galeatum has some similarity to Oxypharciceras chebbiense Bockwinkel, Becker & Aboussalam, 2017, but differs in the shape of the venter, which is distinctively oxyconic from ca 60 mm dm in O. chebbiense but tectiform in P. galeatum .

Figures

Fig. 25. Pharciceras galeatum Wedekind, 1918, specimen SMF.Mbg.2316 (Welsch 1913 Coll.) from Oberscheld (Prinzkessel Mine); reproduction from Wedekind (1918: pl. 20 fig. 1). Scale bar units = 1 mm.

Fig. 26. Pharciceras galeatumWedekind, 1918. A. Specimen MB.C.22182 (Koch Coll.) from Oberscheld (Anna Mine). B. Specimen MB.C.22180 (Kegel 1927 Coll.) from Oberscheld (Sahlgrund Mine). Scale bar units = 1 mm.

Fig. 27. Pharciceras galeatum Wedekind, 1918. A. Suture line of holotype SMF.Mbg.2316 (Welsch 1913 Coll.) from Oberscheld (Prinzkessel Mine), at ww =15.0 mm, wh =9.0 mm. B–E. Ontogeneric trajectories of the cardinal conch parameters. Abbreviations: see Material and methods. Scale bar units