Plate Tectonics and Paleobiogeography: Early Jurassic (Pliensbachian) Endemism and Diversity

Abstract

A diversity gradient and endemic patterns that are a function of latitude are evident in the ammonoid biogeography of the Pliensbachian. High diversity, Tethyan faunas are dominated by the families Hildoceratidae and Dactylioceratidae whereas the less diverse faunas of the northern part of the Northern Hemisphere are dominated by the Amaltheidae and Liparoceratidae. The biogeography of North American Pliensbachian faunas is examined quantitatively to provide constraints on the tectonic movement of suspect terranes along the western margin of the craton. Standards of reference by which to measure displacement are provided by low-diversity, autochthonous Borealfaunas on the craton as far south as southern Alberta, and higher diversity, Tethyanfaunas on the Sonomia terrane in western Nevada, which was accreted in Triassic time. Diversity and similarity coefficients are high for Sonomia and three suspect terranes (Wrangellia, Stikinia, and Quesnellia); this reflects their mutual Tethyan affinities and the pervasive presence of the east Pacific ammonite Fanninoceras. Much weaker, but still signifcant similarity coefficients between the faunas on the suspect terranes and the Boreal Craton fauna result from the gradational area of BorealTethyan overlap passing across the northern regions of the suspect terranes. These biogeographic data constrain the latitudinal position of the suspect terranes with respect to the North American continent and also indicate that the suspect terranes were in the Northern rather than the Southern Hemisphere and in the eastern rather than the western Pacific during the Pliensbachian. Evident migration of various molluscs between the eastern Pacific and western Tethyan oceans suggests the presence of a rifted seaway, the Hispanic Corridor, through the Caribbean and along the North Atlantic, that preceded spreading and the formation of oceanic crust.