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The tangled dimorphism within the ammonite Stephanoceratidae of the Late Bajocian

Pavia G.

Volumina Jurassica

2006

Vol. 4, no. 1

242-243

Stephanoceratidae ammonoids achieved maximum diversification in the Upper Bajocian. At the topmost Lower Bajocian, two phyletic lineages produced important taxa which divided into a net of widespread supraspecific entities. The first one refers to the dimorphic Lokuticeras regarded by Galacz (1994) as the origin of the long-ranging subfamily Cadomitinae, from which the typically Late Bajocian Garantianinae derived at the beginning of the substage. The second lineage started from the dimorphic Phaulostephanus which was proposed by Pavia (2000) as the ancestor of an archipelago of minor taxa, such as Parastrenoceras, Subcollina, the polyphyletic superfamily Perisphinctaceae, the corresponding dimorphic genera Caumontisphinctes and Infraparkinsonia as possible stem of Parkinsoniidae. All these stephanoceratids are grouped in the literature into detailed dimorphic pairs, some of which do not need any particular comments, whereas others still lie in a undefined, not totally shared position. The following case-history could clarify the situation: * Cadomitinae: the coupling of Cadomites (M) and Polyplectites (m) is confirmed by all authors. * Garantianinae: Metz (1992) and Gauthier et al. (2002) clearly demonstrated that the inner whorls of Orthogarantiana (M) are homologous to the adult phragmocones of Strenoceras (m); consequently the previous combination Orthogarantiana-Torrensia (Pavia 1983) cannot be valid any longer. A distinct group of forms (cf. Pavia 1973, pl. 19) shows mature features which recall the microconchs of the genus Parkinsonia, in particular open umbilicus and very short apertural lappets: the possibility to transfer these forms to the parkinsonid family must be taken into account. Pseudogarantiana is accepted as the microconchiate counterpart of later garantianinids derived from Orthogarantiana, such as Hlawiceras and other supraspecific names. Moreover, an accurate taxonomic revision is necessary as the number of microconchiate taxa is too low when compared with that of the macroconchs. Benatinites, Parastrenoceras, Subcollina: they seem to be derived from Phaulostephanus by iterative evolution in different times. As to their dimorphism, (M) Benatinites matches the (m) Lugariceras (Schlögl et al. 2006). On the contrary the dimorphism of Parastrenoceras and Subcollina is far to be clarified; the microconchiate Torrensia is a good candidate, at least for Subcollina with which it shares the zigzag pattern of the ventral ribs.