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Morphotypes, polymorphism and peristome in the species of the genus Holcophylloceras SPATH, 1927. Hypothesis of the dimorphism in the species Holcophylloceras zignodianum (d’ORBIGNY, 1848)

¹ 8, rue de Garambault, 45190 Beaugency, bernard.joly15{at}wanadoo.fr. site WEB : http://phylloceras.free.fr/
² 12, rue du Fournet, 38120 Saint-Egrève, bruno.fonters{at}wanadoo.fr

The genus Holcophylloceras SPATH, 1927, including especially a majority of Jurassic species and some rare lower Cretaceous species, is easy to distinguish from other genera with constrictions such as Calliphylloceras SPATH, 1927 or Sowerbyceras PARONA & BONARELLI, 1895. In the genus Holcophylloceras the constrictions are clearly sigmoid or falcoid. Both characteristics can be observed on the same specimen. In the genus Calliphylloceras the constrictions are falcoid while they are sigmoid on Sowerbyceras. Actually these genera are closely allied and they represent evolutionary stages in a lineage characterized by more or less growth alteration. The lineage origin probably exists among Liassic Phylloceras. The genus Calliphylloceras shows a small growth alteration, while it is maximal in Sowerbyceras and it is intermediate in Holcophylloceras.

This note shows that species of the genus Holcophylloceras are also characterized by three morphotypes: prae-caucasica, caucasica and nausikaae; these morphotypes do not characterize any particular species. The first and the second morphotypes could be an adaptation to live in deep waters. They probably give more solidity to the shell. The initial object of this note was to study again all the morphotypes of the genus Holcophylloceras with a new material collected in the oxfordian "Terres noires".

The authors of this note have studied the relations between the morphotypes prae-caucasica, caucasica and the peristome of Holcophylloceras. Does the peristome study permit to envisage the existence of dimorphic couples? (sexual dimorphism?). The result has been disappointing. On the other hand many microconchs and macroconchs have been identified. In particular with zignodianum-mediterraneum it is possible to identify the "male form" (zignodianum = microconch) and the "female form" (mediterraneum = macroconch) which confirms the intuition of Beznosov [1958] by an other approach to the problem. Many authors asked them if they had to separate the forms with sigmoid constrictions from those with falcoid constrictions. It seems now, there is no doubt and both species zignodianum-mediterraneum have to be joined in the same taxon zignodianum which has priority over mediterraneum.

Last result of this study is the justification of the genus Holcophylloceras created by Spath.

Key Words: Holcophylloceras • Morphotype • prae-caucasica • caucasica • nausikaae • Polymorphism • Adabofoloceras • Peristome • Dimorphic couple • Macroconch • Microconch