The endemic trends of liassic ammonite faunas of Portugal as the result of the opening up of a narrow epicontinental basin
Abstract During several stages the Sinemurian and Pliensbachian ammonite faunas of the Lusitanian Basin (coastal Portugal) have a pronounced endemic trend. These trends concern several lineages, without narrow phyletic relationship, belonging to Asteroceratinae ( Ptycharietites and Pompeckioceras ), Phricodoceratinae ( Pseudophricodoceras and Epideroceras ), Polymorphitidae ( Dayiceras ), and Eoderoceratidae ( Metaderoceras ). Frequently the species most derived from these endemic lineages display a pronounced morphological convergence. These species posses an involute shell with rather slender ribbing. This converging trend is probably the result of a morphofunctional factor. The Lusitanian endemic trends can only be understood within their biogeographical framework. The sequence of endemic events occurs during a critical phase in the geological history of the Lusitanian Basin: the settlement of a definite marine environment. Besides, this early extension of open marine conditions affects a narrow basin chopped up by several transform faults. It is within this particular paleogeographical framework that the main causes of these endemic events must be looked for.