World distribution of middle Jurassic ammonites (Upper Aalenian to Middle Bathonian): relationships between biodiversity and palaeogeography

Abstract

Abstract The break up of the Pangea takes place in the Jurassic; the palaeoceanographic consequences are the opening of seaways, particularly at the place of the future Atlantic and Indian oceanic areas. During the Toarcian, and from the late Aalenian to the middle Bathonian, the so-called “Hispanic corridor” (or “Atlantic seaway”) exists between the “western Tethys” and the “American Pacific border”, through the “Caribbean Tethys”. Two additional seaways which play as by-passes of the Pangea are proposed, one along the northern border of Laurasia (Boreal sea), and a second along the southern border of Gondwana (South Pacific Sea); however, if these two last could be effectively used for faunal exchanges and migrations, they are poorly documented. Near the end of the Aalenian, the Ammonitina sub-order undergoes a strong faunal turnover. The last Hammatocerataceae, a well known and diversified Liassic superfamily, gives birth to three distinct superfamilies which will dominate among others till the end of the Jurassic: Stephanocerataceae, Perisphinctaceae and Haplocerataceae. The analysis of the worldwide corresponding radiation of these three major taxa puts in light differences and similarities between the biogeographic provinces usually recognised. The Tethyan, Pacific and Boreal domains, and their associated epicratonic platforms are divided into sixteen “palaeobiogeographical provinces”; the listed ammonite taxa concern twenty-three subfamilies which range in fifteen biozones, from the late Aalenian to the middle Bathonian. The counting of the species of each subfamily within the several palaeobiogeographic provinces emphasises faunal similarities in terms of total biodiversity between the several provinces. To perform, a “hierarchical ascendant classification” is used: the results are illustrated in the shape of a relational tree where the nearest branches correspond to the provinces that show the most similar biodiversity. The northwestern Tethyan provinces (European platforms) share strong similarities between them and with the provinces of the southwest Tethyan margin (Maghreb). The Circum-Pacific provinces gather themselves, underlining their great faunal similarities. Finally, provinces that show a strong endemism are isolated; these are the Boreal areas and the southeast Tethyan margin. The total time variation of the diversity is expressed by the counting of species in each biozone from the late Aalenian to the middle Bathonian. The primary and global signal of the diversity evolution obtained is then independently related to each palaeobiogeographic province. The comparison between the two signals shows the differences of the time evolution of the diversity in the several provinces. The maxima of diversity are often diachronous in the several listed provinces: early Bajocian in North America; late Bajocian on the northwest European platforms. Finally, the palaeogeographic distribution of each ammonite subfamily is used to emphasise the evidence for the several seaways that would exist between the several provinces. The maps which have been constructed for the considered ammonite taxa show that peculiar seaways like the “Hispanic corridor” (Caribbean Tethys), the “South Gondwana” (South Pacific Sea) and the “North Laurasia” (Boreal Sea) bypasses could have been used by ammonites to invade several provinces. Nevertheless, some groups have a geographic distribution of which the causes still remain problematic.