Pharciceras tridens

Pharciceras tridens (Sandberger & Sandberger, 1850)

Figs 14–16; Tables 6–7

Goniatites tridens Sandberger & Sandberger, 1850: 66, pl. 4 fig. 2.

Pharciceras tridens – Hyatt 1884: 336. — Matern 1931: 93. — Korn & Klug 2002: text-fig. 131.

Prolecanites tridens – Frech 1897: 177 e, figs 1f, 2a; 1902: 64, text-fig. 21a, pl. 3 fig. 12.

non Goniatites tridens Sandberger & Sandberger 1850: pl. 9 fig. 2.

non Pharciceras tridens – Frech 1888: 29, pl. 2 fig. 5. — Wedekind 1918: 128, text-fig. 37a. — Korn & Klug 2002: text-fig. 134a, h.

? Pharciceras tridens – Termier & Termier 1950: pl. 150 figs 31–32, 34–36. — Petter 1959: 133, pl. 7 figs 10, 12, 12a, text-figs 18, 33k. — Bogoslovsky 1969: pl. 26 fig. 1, text-fig. 69b. — Kullmann & Ziegler 1970: 77, pl. 1 figs 2–3, text-figs 2a, 3a, 4. — Bensaïd 1974: 110, pl. 4 fig. 8, pl. 5 fig. 11, text-figs 7, 18, 19c. — Göddertz 1987: 181, pl. 13 figs 3–4, text-fig. 29. — Bockwinkel et al. 2009: 76, text-figs 6a–g, 7a.

Diagnosis

Species of Pharciceras with thinly discoidal, subevolute conch at 30 mm dm (ww / dm ~ 0.45; uw/ dm ~0.42) and thinly discoidal, subevolute conch at 50 mm dm (ww/ dm ~ 0.40; uw / dm ~ 0.40). Whorl profile weakly depressed at 30 mm dm (ww/ wh ~1.45) and weakly depressed at 50 mm dm (ww / wh ~ 1.20); whorl expansion rate low to moderate. Venter continuously rounded. Growth lines coarse, strongly biconvex; ventrolateral shoulder with two spiral grooves. Outer suture line with symmetric, lanceolate prongs of the external lobe, a tongue-shaped lateral lobe, a narrow U-shaped U 2 lobe, and a shallow and wide U 4 lobe.

Material examined

Lectotype GERMANY • Rhenish Mountains, Oberscheld; late Givetian (Red Ironstone); nr. 29 (Wiesbaden Museum); illustrated by Sandberger & Sandberger (1850 –1856: pl. 4 fig. 2), re-illustrated here in Fig. 14; Wiesbaden Museum nr. 29.

Additional material

GERMANY • 5 specimens; Rhenish Mountains, Oberscheld (Westfeld der Grube Königszug, Firste der 120–150 m Sohle); late Givetian (Red Ironstone); Ahlburg Coll.; MB.C.3618, MB.C.3634, MB.C.3647, MB.C.3663, MB.C.3670 • 1 specimen; Rhenish Mountains, Oberscheld (Grube Anna); late Givetian (Red Ironstone); Koch Coll.; MB.C.3638.

Description

Five specimens are selected for description and illustration:

Lectotype 29 in the Wiesbaden Museum: Rather poorly preserved, corroded specimen with 26 mm conch diameter in iron-rich limestone (Fig. 14).

MB.C.3618: incomplete specimen with 54 mm conch diameter in iron-rich micritic limestone (Fig. 15A).

MB.C.3647: incomplete specimen with 46 mm conch diameter in iron-rich micritic limestone with body chamber occupying the last volution (Fig. 15B).

MB.C.3663: Fully chambered but widely covered with shell, incomplete specimen with 31 mm conch diameter in micritic, haematitic limestone, largely filled with sparry calcite (Fig. 15C).

MB.C.3634: incomplete specimen with 27 mm conch diameter in micritic, haematitic limestone (Fig. 15D).

Lectotype 29 is a specimen of less than 30 mm diameter and allows the study of conch shape and suture line of the preadult stage. The conch is discoidal and subevolute with a crescent-shaped whorl profile. The shell is largely removed and only at some parts preserved with rather coarse growth lines. The suture line has U-shaped prongs of the external lobe and a tongue-shaped lateral lobe.

The four specimens MB.C.3634, MB.C.3663, MB.C.3647 and MB.C.3618 allow the study of conch geometry, ornament and suture line between 27 and 57 mm conch diameter. The general conch morphology is similar during this interval, but the conchs become slenderer (ww/ dm decreasing from ~0.47 to ~0.39), paralleled by flattening of the whorl profile (ww/ wh decreasing from ~1.45 to ~1.20). All specimens show double ventrolateral grooves.

Specimen MB.C.3618 shows the suture line, which has a very wide external lobe with a median saddle attaining about 60% of the lobe depth and lanceolate prongs. The lateral lobe is much shorter than the external lobe; it is tongue-shaped with a narrowly rounded base. On the flanks and umbilical margin follow two very small, rounded umbilical lobes (Fig. 16A).

Remarks

Unfortunately, the lectotype of Pharciceras tridens is a rather small and poorly preserved specimen with only 26 mm conch diameter (Fig. 14); therefore, it may be difficult to compare this with the other species of the genus, which are defined by larger type material. The type specimen allows at least to figure two characters that can be used to characterise the species, the first is the rather wide umbilicus (uw / dm ~ 0.40) and the tongue-shaped, rounded or blunt lateral lobe.

Only one species from the Red Ironstone has an umbilicus as wide as that in P. tridens, it is P. beyrichi sp. nov. but this differs in its much stouter conch with a much broader whorl profile (ww/ dm ~ 0.45 and

ww / wh ~ 1.40 in P. tridens but ww /dm ~ 0.55 and ww /wh ~ 1.75 in P. beyrichi sp. nov.). Furthermore, the ventrolateral grooves are much deeper in P. beyrichi sp. nov.

The species name Pharciceras tridens has frequently been used in the past decades for all specimens with a rather wide umbilicus and little differentiated suture line. For a secure classification of these specimens, a comprehensive revision would be necessary, which, however, is outside the scope of the present study. These mentions in the literature are therefore marked with a question mark in the list of synonyms.