Evopharciceras formosum, Korn & Bockwinkel, 2021

Korn, Dieter & Bockwinkel, Jürgen, 2021, The pharciceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea), European Journal of Taxonomy 771, pp. 1-79 : 36-38

Evopharciceras formosum gen. et sp. nov.

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Figs 28–29 ; Tables 15–16

Diagnosis

Evopharciceras gen. nov. with thinly discoidal, evolute conch at 35 mm dm (ww / dm ~ 0.35; uw/ dm ~0.50) and extremely discoidal, evolute conch at 65 mm dm (ww /dm ~ 0.30; uw/ dm ~0.50). Whorl profile weakly depressed with convergent flanks and broadly rounded venter, umbilical wall steep. Growth lines rather coarse, strongly biconvex; ventrolateral shoulder with two spiral grooves in the middle growth stage. Outer suture line with high median saddle, symmetric, lanceolate prongs of the external lobe, a lanceolate, weakly pouched lateral lobe, a very narrow, acute and small U 2 lobe and a V-shaped U 4 lobe.

Etymology

Named after the Latin ‘ formosum ’, meaning ‘beautiful’, because of the decorated shell.

Material examined

Holotype GERMANY • Rhenish Mountains , Oberscheld (Grube Beilstein); late Givetian (Red Ironstone); Lotz 1901–1902 Coll.; MB.C.30241 . ( Fig. 28A )

Paratype GERMANY • 1 specimen; Rhenish Mountains , Oberscheld (Grube Anna); late Givetian (Red Ironstone); Koch Coll.; MB.C.22173 .

Description

Paratype MB.C.22173: Laterally deformed, but otherwise well-preserved specimen with 35 mm conch diameter in iron-rich micritic limestone. Most of the specimen is covered with shell; a quarter of the last volution belongs to the body chamber ( Fig. 28B ). It has a discoidal and evolute conch with low coiling rate ( Fig. 28B ); the whorl profile is subcircular with shallow umbilical wall, broadly rounded flanks and a broadly rounded venter. One faint ventrolateral spiral groove on each side separates flanks and venter. The ornament includes lamellar growth lines, which extend with low dorsolateral projection on the umbilical margin, a shallow lateral sinus, a prominent, narrow ventrolateral projection and a deep ventral sinus.

The larger holotype MB.C.30241 is a laterally strongly deformed specimen with about 65 mm conch diameter in haematitic limestone, showing the suture line ( Fig. 28A ). It has a similar conch geometry, but is slightly slenderer. It displays the suture line, which has a rather narrow external lobe with high median saddle and narrow and slightly pouched, lanceolate prongs. The lateral lobe is also very narrow and lanceolate; the U 2 lobe is very narrow and the U 4 lobe is V-shaped and wider than the U 2 lobe ( Fig. 29A ).

Remarks

Evopharciceras formosum gen. et sp. nov. is similar to Extropharciceras metallicum sp. nov. but differs in the slightly stouter conch and the slightly narrower umbilicus. The suture line provides more specific criteria for the separation of the two species: while the E 2, L and U 2 lobes are very narrow and lanceolate in Evopharciceras gen. nov., they are wider and tongue- or V-shaped in Extropharciceras .

Figures

Fig. 28. Evopharciceras formosum gen. et sp. nov. A. Holotype MB.C.30241 (Lotz 1901–1902 Coll.) from Oberscheld (Beilstein Mine). B. Paratype MB.C.22173 (Koch Coll.) from Oberscheld (Anna Mine). Scale bar units = 1 mm.

Fig. 29. Evopharciceras formosum gen. et sp. nov. A. Suture line of holotype MB.C.30241 (Lotz 1901– 1902 Coll.) from Oberscheld (Beilstein Mine), at ww = 14.8 mm, wh= 14.2 mm. B–E. Ontogeneric trajectories of the cardinal conch parameters. Abbreviations: see Material and methods. Scale bar units = 1 mm.