The heteromorph ammonite genus Ancyloceras (Ancyloceratidae) in the Paris Basin (lower Aptian, Lower Cretaceous, NE France)

Abstract

In the present work, we examine a sample of 54 specimens of the heteromorph ammonite genus Ancyloceras d’Orbigny, 1842 from the Argiles à Plicatules Formation (lower Aptian, Lower Cretaceous) of the eastern Paris Basin (NE France). The results are as follows: (1) the sample is regarded as monospecific and assigned to the well-named species Ancyloceras varians d’Orbigny, 1842; (2) its variability is continuous and concerns as well as adult size, coiling and shell ornamentation. Adult size varies by a factor of two from one individual to another. The angle between the proversum and the retroversum is variable. The ornamentation on the spiral and the proversum varies between robust morphotype with trituberculate strong ribs separated by few smooth intercalaries ribs and slender morphotype with feebly tuberculate ribs. There are no arguments in favor of sexual dimorphism in Ancyloceras varians.