Amauroceras Buckman, 1913

Bardin, Jérémie, Rouget, Isabelle & Cecca, Fabrizio, 2013, Late Pliensbachian (Early Jurassic) ammonites from Lac de Charmes (Haute-Marne, France): Systematic, biostratigraphy and palaeobiogeography, Geodiversitas 35 (2), pp. 309-334 : 322-324

Genus Amauroceras Buckman, 1913

Proamaltheus Lange, 1932: 238 . — Type species: Amaltheus wertheri ( Lange, 1932) .

TYPE SPECIES. — Ammonites ferrugineus Simpson, 1855 (by subsequent designation of Buckman [1913]) STRATIGRAPHICAL DISTRIBUTION. — Late Pliensbachian, Margaritatus chronozone, Stockesi subchronozone up to Spinatum chronozone , Hawskerense subchronozone. Haute-Marne: bed 3.

DIAGNOSIS. — Ľe adult size is small, ribbing is very weak or absent all along the growth. Ľe keel is very low; chevrons are extremely weak and disappear at the end of the growth. Ľe whorl section is elliptical to ovate.

DISCUSSION

According to Howarth (1958) and Tintant et al. (1961), the closest taxon to the genus Amauroceras is Amaltheus wertheri ( Lange, 1932) . Buckman (1913) considered these species as “degenerated Amaltheus ”. Some authors ( Dommergues et al. 2008) included the species wertheri within Amauroceras . While the adult size of A. wertheri is similar to that of Amauroceras specimens, its ribs are pronounced and its keel is that of Amaltheus . Amauroceras has been considered the microconch of Amaltheus ( Callomon 1981; Comas-Rengifo 1985; Meister 1988) but while the total biostratigraphic ranges of these two genera seem to be similar, their occurrence together in a same bed is not the rule.Furthermore their relative abundances are different. Ľe most common hypothesis is to consider the specimens of Amauroceras as “dwarf Amaltheus ” resulting from a hypomorphosis ( Reilley et al. 1997).

Amauroceras ferrugineum ( Simpson, 1855) ( Fig. 5C, D, G, H )

Ammonites ferrugineus Simpson, 1855: 79 .

Ammonites amaltheus laevis Quenstedt, 1856: 328, pl. 42, fig. 1.

Amauroceras ferrugineum – Buckman 1919: pl. 142, figs1-5. — Howarth 1958: 24, pl. 4, figs 9-14. — Meister 1988: pl.3, fig. 9.

TYPE MATERIAL. — Simpson (1855) figured two syntypes, Buckman has selected one of them (Whitby museum, WHITM: SIM 482.2) as the lectotype (1919: pl. 142).

SPECIMENS AVAILABLE. — UPMC-139, 140.

STRATIGRAPHICAL DISTRIBUTION. — Late Pliensbachian, Margaritatus chronozone, Stockesi subchronozone up to Spinatum chronozone , Hawskerense subchronozone. Haute-Marne: bed 2 to 3.

GEOGRAPHIC DISTRIBUTION. — England and Scotland ( Howarth 1958); South- East France, Causses ( Monestier 1913, 1916, 1928; Mattéi 1985; Meister 1989; Morard 2004); eastern France, Burgundy ( Tintant et al. 1961) and Haute-Marne (this work); Northwest Germany ( Jordan 1971); South-West Germany ( Frentzen 1934; Urlichs 1977); South-East Germany (Hoffman et al. 2007).

DESCRIPTION

Involute shell (⅔ of the previous whorl recovered) reaching the diameter of 50 mm. Ornamentation is reduced to striae. Ľe ventral area is angular with a gentle keel.

DISCUSSION

Ľis species differs from Amaltheus engelhardti (d’Orbigny, 1844) and Amauroceras lenticulare ( Young & Bird, 1828) because of its less involute shell and its smaller size.

Figures

FIG. 5. — Amaltheus margaritatus Montfort, 1808:A, B, specimen UPMC-104, Outcrop 1, bed 3; E, F, specimen UPMC-147, Outcrop 1, bed 5. Amaltheus gibbosus (Schlotheim, 1820); I, J, specimen UPMC-148, Outcrop 1, bed 5; Amauroceras ferrugineum (Simpson, 1855); C, D, specimen UPMC-140, Outcrop 1, bed 3; G, H, specimen UPMC-139, Outcrop 1, bed 3. Amaltheus salebrosum Hyatt, 1867; K, L, specimen UPMC-225, Outcrop 1, bed 7. Pleuroceras transiens (Frentzen, 1937); M, O, specimen UPMC-174, Outcrop 1, beds 8, 9. Scale bars: 1 cm. The arrows indicate the beginning of the body chamber.