Douvilleiceras

Description. Type Ammonites mammillatus Schlotheim, 1813. According to Wright (1996) and Sharikadze et al. (2004), a semi-evolute genus with a depressed, reniform or polygonal whorl section. Venter broad, rounded, or slightly flattened. Umbilicus open and deep, stepped, with a high, abrupt wall. Ornamentation of strong, undivided radial ribs. In juveniles (2 cm), they bear on each flank one umbilical tubercle, one lateral, and one ventrolateral, as in Epicheloniceras, from which the genus descends. The ribs collapse on the siphon, giving the impression of a ventral groove, although the venter remains flat or convex in the intercostal spaces. Next comes a multituberculate stage called clavatus, where the ventrolateral tubercle divides into clavi that move down the rib. The ventral end of the ribs may become protruding in some species. With growth, the clavi take on a drop shape and the ventral groove disappears: this is the moniliform stage, from the Latin monilis (rosary). In the adult stage, the tubercles disappear, leaving smooth ribs. These ribs may even vanish completely in the senile stage. Prins (2006) provides photographs of the progressive appearance of tubercles in juveniles.

Species. This cosmopolitan genus existed from the early Albian (tardefurcata zone) to the base of the dentatus zone (middle Albian). Kennedy & Klinger (2005) classify most of published species (around forty!) into two long-lived variable forms: D. mammillatum (Schlotheim, 1813), with similar strong ribs (photo on the left), and D. inaequinodum (Quenstedt, 1849), which displays large ventrolateral tubercles and irregular ribs with weaker intermediates. We follow these authors, except for two species that we believe deserve to be retained. D. leightonense Casey, 1962, from the basal Albian, has a broad ventral sulcus and a rapidly diminishing ornamentation. D. clementinum (d'Orbigny, 1841) is a late form from the upper benettianus zone that can reach 80 cm. It is the only Douvilleiceras that survives up to the lower dentatus zone. More recently, Futakami & Haggart (2018) proposed a less drastic revision, which retains seven species.

Remarks. In all Douvilleiceras, the umbilical and lateral tubercles are small and conical on the internal mold, but they are the bases of long, septate spines on the test. These spines are rarely preserved or are lost during preparation, but the best preparators sometimes manage to keep those in the fine specimens from Courcelles (Aube). Some specimens have their venter covered by a shield, a less robust secondary layer that covers the clavi and extends down each flank to the lateral tubercles. This too is rarely preserved or is removed to reveal the clavi.

D. clementinum (4)	Clavatus stage 95 mm	Mid-growth 149 mm	Subadult 250 mm	Adult 380 mm
D. inaequinodum (3)	Irregular ribs	Variant alternans	Preserved spines
D. leightonense (1)	Malagasy specimen
D. mammillatum (3)	Clavatus stage	Moniliform stage	Adult stage

Genus Douvilleiceras de Grossouvre, 1894

Suborder Ancyloceratina – Superfamily Douvilleicerataceae – Family Douvilleiceratidae – Subfamily Douvilleiceratinae