Les acanthopleurocératinés portugais et leurs relations avec les formes subboréales

Abstract

Key-words: Middle Carixian - Portugal- France (Cher) - Ammonitina - Evolution - Paleobiogeography The evolution of the Portuguese Acanthopleuroceratinae is similar to the celto-souabe succession such as it was described in the collects of the Cottards (Cher, France). A subspecies of one of the oldest Acanthopleuroceras (A. carinatum atlanticum) is abundant in the lower part of the Portuguese Ibex zone; this form is described here. The species is recognized in France by severa! nuclei associated with A. arietiforme (Cottards-22). Generaily the similarity between the successive French and Portuguese populations (A. maugenesti, A. valdani, A. alisiense. junior synonym of A. lepidum TIITCHER and TRUEMAN, 1925), is very good. This fact suggests their specifie identity. It is typical for A. lepidum of which the greatest populations allow the bioriletric comparaisons. In Portugal, the mesogean Tropidoceras are missing. This absence of the subboreal Acanthopleuroceras ancestors suggests the straight celto- souabe derivation of the Portuguese Acanthopleuroceras and not a similar local evolution. A. lepidum the last Acanthopleuroceras reaches the western coast of Canada (British Columbia) probably by the Arctic ocean.