The Liassic ichthyosaurStenopterygius cf.quadriscissus from the lower Toarcian of Dobbertin (northeastern Germany) and some considerations on lower Toarcian marine reptile palaeobiogeography

Abstract

An incomplete skull of the lower Toarcian ichthyosaurStenopterygius cf.quadriscissus is described from the lower Toarcian of Dobbertin (Mecklenburg, northeastern Germany). It represents both the northeasternmost occurrence of this ichthyosaur genus and the first diagnostic specimen from East Germany. It therefore extends the palaeobiogeographic range ofStenopterygius considerably and demonstrates that this ichthyosaur also inhabited the Germanic Basin east of the Rhenish Massif by Early Jurassic times. A palaeobiogeographical pattern is evident within the genusStenopterygius, with the ubiquitous speciesS. longifrons andS. hauffianus on one hand, andS. megalorhinus andS. quadriscissus on the other hand, which appear not to have ranged northwest of the London-Brabant Massif. It is suggested that, whereas the Rhenish Massif was not an effective barrier for dispersal of ichthyosaurs in Western Europe during early Toarcian times, the London-Brabant Massif played a rather significant role, as is also shown by the fossil record of other marine reptile groups. A provincialism of early Toarcian marine reptiles is suggested for Western Europe, with a northwestern province which contains the British occurrences, an intermediate, Subgermanic province in France and the Benelux countries, and a southeastern province in the Germanic Basin. The British and Germanic provinces are each characterized by a typical assemblage of ichthyosaurs, plesiosaurs, and marine crocodiles, wheres the intermediate Subgermanic region shows an intermingling of faunal elements.