Extropharciceras becheri (von Buch, 1832)

Figs 32–34; Tables 19–20

Ammonites Becheri von Buch, 1832: 171, pl. 2 fig. 2.

Ammonites Becheri – Beyrich 1837a: 31, pl. 1 figs 7–8; 1884: 211.

Goniatites (Prolecanites) Becheri – Frech 1888: 28, pl. 2 fig. 4.

Prolecanites Becheri – Frech 1897: 177 e, text-fig. 2e, pl. 35 fig. 11; 1902: 64, pl. 3 fig. 13, text-fig. 21e.

Pharciceras Becheri – Wedekind 1918: 127.

Pharciceras becheri – Matern 1931: 90.

Extropharciceras becheri – Bockwinkel et al. 2009: 95.

non Pharciceras becheri – Petter 1959: 137, pl. 7 fig. 5, text-fig. 33e. — Kullmann & Ziegler 1970: 78, pl. 1 fig. 5, text-figs 2c, 3c, 4.

Diagnosis

Extropharciceras with thinly pachyconic, evolute conch at 20 mm dm (ww /dm ~ 0.60; uw / dm ~ 0.45), thickly discoidal, subevolute conch at 40 mm dm (ww / dm ~0.50; uw/ dm ~ 0.40) and thinly discoidal, subevolute conch at 80 mm dm (ww / dm ~0.35; uw / dm ~0.35). Whorl profile changing from strongly depressed in the juvenile stage (ww/ wh ~ 2.30) to weakly compressed in the adult stage (ww / wh ~ 0.95). Whorl profile crescent-shaped with continuously rounded venter up to 35 mm dm; adult stage with slightly flattened, convergent flanks bordered by a pronounced ventrolateral shoulder from the slightly flattened venter. Growth lines fine, strongly biconvex; ventrolateral shoulder with two spiral grooves in the middle growth stage. Outer suture line with symmetric, lanceolate prongs of the external lobe, a V-shaped lateral lobe and small, V-shaped U 2, U 4 and U 6 lobes.

Material examined

Neotype GERMANY • Rhenish Mountains, Oberscheld (Grube Volpertseiche); late Givetian (Red Ironstone); Lotz 1901–1902 Coll.; MB.C.3609. (Fig. 32A)

Additional material

GERMANY • 1 specimen; Rhenish Mountains, Oberscheld (Grube Anna); late Givetian (Red Ironstone); Koch Coll.; MB.C.3613 • 1 specimen; Rhenish Mountains, Oberscheld; late Givetian (Red Ironstone); MB.C.30247 • 1 specimen; Rhenish Mountains, Oberscheld (Grube Prinzkessel, Liegendes Lager der 130 m Sohle); late Givetian (Red Ironstone); Lotz 1901–1902 Coll.; MB.C.30248.

Remarks to the type material

The original specimen described by von Buch (1832) was from the Goldfuss collection and could not be traced; it is missing in the Bonn collection and probably lost. Therefore, a neotype (MB.C. 3609) from the type area is proposed here. The selection of this neotype stabilises the use of the species name, which was mainly coined by Frech (1888, 1897 – 1902, 1902).

As the species description provided by von Buch (1832) can hardly be used for a definition of the species, we herein refer to the description by Beyrich (1837a, 1837b), which allows a characterisation of the species. As the original specimen figured by Beyrich could not be traced, we propose herein a neotype from the Volpertseiche Mine near Oberscheld.

[The original specimen of von Buch (1832) came from the Rinzenbach Mine near “Edbach” (or “Eybach”). This mine was located between Eibach and Oberscheld and was later included in the Königszug Mine.]

Description

Three specimens are selected for description and illustration:

Neotype MB.C.3609: well-preserved specimen with nearly 100 mm conch diameter in the contact zone between a pillow diabase and an iron-rich sparitic limestone (Fig. 32A).

MB.C.3613: slightly deformed specimen with 52 mm conch diameter in iron-rich sparitic limestone; the last quarter of the last volution belongs to the body chamber (Fig. 32B).

MB.C.30247: sectioned specimen with 92 mm conch diameter in haematitic limestone (Fig. 33A).

Neotype MB.C.3609 with nearly 100 mm conch diameter has an extremely discoidal and subevolute conch with a moderate coiling rate (Fig. 32A); the whorl profile is pear-shaped in the last volution with a shallow umbilical wall, a broadly rounded umbilical margin, rather strongly convergent flanks, a subangular ventrolateral shoulder and a rounded venter. The ventrolateral shoulder is much more rounded half a volution earlier (at 73 mm dm). A ventrolateral groove on each side is well-preserved another half whorl earlier (at 52 mm dm). Shell remains are preserved in the inner whorls and also on the last volution, but here only on the umbilical wall and the inner flanks. The course of the fine growth lines on the last volution is biconvex with a rather high and rounded dorsolateral projection. Inner whorls show coarser growth lines, which possess a very prominent, narrow ventrolateral projection.

MB.C.3613 has, at a diameter of 52 mm, a discoidal and subevolute conch and a whorl profile with rather steep umbilical wall, a rounded umbilical margin, convergent flanks and a broadly rounded venter (Fig. 32B). The growth lines can be best seen in the area 270–360 degrees before the preserved aperture; their course is strongly biconvex with very prominent, very narrow ventrolateral projection. The growth lines are here superimposed by the Runzelschicht, which consists of fine wrinkles with radial direction. The outer suture line can be seen on the last preserved volution; at a conch diameter of about 39 mm it shows a wide external lobe with nearly parallel flanks and four V-shaped lobes on the flank, which continuously become smaller towards the umbilicus (Fig. 33B).

The sectioned specimen MB.C.30247 allows the study of the ontogenetic changes of conch geometry from the earliest whorls up to a diameter of 92 mm (Fig. 33A). The most conspicuous morphological change concerns the whorl profile, which is depressed in the whorls of the early and preadult stage but compressed in the adult stage. The whorl profile is widest in a growth interval between 4.5 and 18 mm conch diameter, when the ww / wh ratio exceeds a value of 2.00. Up to a conch diameter of 38 mm, the whorl profile is crescent-shaped with a very broad venter. Thereafter, the whorls become higher with narrowing of the venter. The last half whorl, between 68 and 92 mm dm, shows slightly flattened, convergent flanks that are slightly incurved in the ventrolateral area. At the largest diameter, the ventrolateral shoulders are pronounced and separate the flanks from the weakly flattened venter. The uw / dm ratio increases in early ontogeny up to 6 mm dm to a value of 0.50; thereafter, a nearly continuous decrease to a value of 0.35 in the terminal stage can be observed. The coiling rate decrease in early ontogeny and slowly increases to a maximum value of 1.82 at 92 mm dm (Fig. 33C–F).

Remarks

The description and the figure of “ Ammonites Becheri ” by von Buch (1832) gives little information about the morphology of the species. Beyrich (1837a, 1837b) provided a better illustration; it shows a species with evolute inner convolutions and quite high adult whorl (Fig. 34B). Finally, Frech (1888) gave a more detailed description and illustration (Fig. 34A); the species Extropharciceras becheri is redefined here based on this description and figure.

Extropharciceras becheri differs from E. metallicum sp. nov. from the Roteisenstein in that it has considerably wider whorls and the adult stage with a flattened venter. Differentiation from some species of Pharciceras is more difficult if the suture line is not visible. But here, too, a demarcation can be ensured by the conch form; E. becheri shows the combination of very evolute inner whorls and a flat venter, which is not known in the species of Pharciceras from the Roteisenstein.

Extropharciceras becheri has some similarity in the shape of the conch to Allopharciceras maximum from the Anti-Atlas of Morocco (Bockwinkel et al. 2017), but differs in the ontogeny of the suture, which shows irregular insertion of umbilical lobes in A. maximum.