Porpoceras, Buckman, 1911

Jattiot, Romain, Fara, Emmanuel, Brayard, Arnaud & Vennin, Emmanuelle, 2016, Revised stratigraphic range of the Toarcian ammonite genus Porpoceras Buckman, 1911, Geodiversitas 38 (4), pp. 505-513 : 508-512

Porpoceras gr. vortex ( Simpson, 1855) - verticosum Buckman, 1914

( Figs 4 B-H; 5)

Ammonites vortex Simpson, 1855: 60 .

Porpoceras vortex – Buckman 1911: pl. 29A. — Géczy & Szente 2007: pl. 8, fig. 4.

Porpoceras verticosum Buckman, 1914 : pl. 91.

Porpoceras gr. vortex - verticosum – Rulleau et al. 2013: 101, pl. 29, fig. 2; pl. 30, figs 1, 2; pl. 31, figs 1, 3, 5; pl. 32, figs 1, 3-6; pl. 33, figs 1, 2; pl. 34, fig. 1.

Porpoceras vortex – Kovàcs 2014: 50, pl. 2, figs 2, 7; pl. 4, fig. 5. DESCRIPTION

Very evolute shell with a cadicone whorl section on inner whorls, becoming quadratic to depressed on mature whorls. Every two, three or four primary ribs, one primary rib bears a ventro-lateral tubercle. Ribs are often grouped by two on the same tubercle. This ornamentation is constant throughout ontogeny. Each tubercle bears a very developed spine leaning on the following whorl and not visible on internal molds. Suture lines not visible on the specimens from the Airvault quarry.

MEASUREMENTS

See Table 1.

STRATIGRAPHIC AND PALAEOGEOGRAPHICAL OCCURRENCES From the uppermost part of the Serpentinum Zone (?) to the Variabilis Subzone; found in NW Europe, Siberia, Spitsbergen, North and South America.

DISCUSSION

According to Rulleau et al. (2013), it is extremely difficult to distinguish P. vortex from P. verticosum , even with a large number of specimens. They therefore decided to include these two species within the same group and we concur with this taxonomic opinion. Porpoceras differs from Catacoeloceras and Mucrodactylites by its well-developed ventro-lateral tubercles and ribs generally grouped by two on the same tubercle. The resembling genus Peronoceras is older and does not exhibit tubercles on inner whorls ( Rulleau et al. 2013).

Figures

FIG. 4. — A, Hildoceras sublevisoni Fucini, 1919 (UBGD 279080, Sublevisoni Subzone, Airvault); B-H, Porpoceras gr. vortex (Simpson, 1855) -verticosum Buckman, 1914 (B-E, UBGD 279078, Bifrons Horizon, Airvault; F-H, UBGD 279079, Bifrons Horizon, Airvault); I, Hildoceras bifrons (Bruguière, 1789) (UBGD 279081), Bifrons Horizon, Airvault); J, Haugia variabilis (d’Orbigny, 1844) (UBGD 279082), Variabilis subzone, Airvault). Scale bars: 10 mm.

Tables

TABLE 1. — Measurements (in mm) of Porpoceras gr. vortex (Simpson, 1855) -verticosum Buckman, 1914 specimens from Airvault quarry (UBGD 279076, UBGD 279077), from Belmont quarry (UBGD 279075), from Anduze, Gard (France, 1665 F-2) and of the holotypes of Porpoceras vortex (WM 153a) and Porpoceras verticosum (WM 220a) from Whitby, Yorkshire (England). Abbreviations: D, Diameter; H, whorl height; W, whorl width; U, umbilical diameter.

Genus	Species	Specimen	D	H	W	U	H/D	U/D	Remarks
Porpoceras	vortex-verticosum	UBGD 279076	59	13.3	–	34.8	0.23	0.59	–
Porpoceras	vortex-verticosum	UBGD 279077	57.3	11.5	–	35.2	0.20	0.61	–
Porpoceras	vortex-verticosum	UBGD 279075	74.7	18.7	–	43.5	0.25	0.58	–
Porpoceras	vortex-verticosum	1665 F-2; Rulleau et al. (2013: pl. 32, fig. 1)	78	17	20.5	47	0.22	0.60	–
Porpoceras	vortex	WM 153a; Buckman (1911: pl. 29A)	83	15	26	53	0.18	0.64	Holotype
Porpoceras	verticosum	WM 220a; Buckman (1914: pl. 91)	71	15	18.5	43	0.21	0.61	Holotype