Lower Jurassic Amaltheidae (Ammonitina) in North America: Paleobiogeography and tectonic implications

The amaltheids are restricted temporally to the late Pliensbachian and geog raphically to the northern part of the northern hemisphere. Amaltheus stoke si is the only species that occurs in all areas of North America where amal theids are found. The craton north of the Canada-U.S.A. border yields the m ost diverse amaltheid fauna, including six of the seven taxa known in North America. On Quesnellia and Stikinia, there are no endemic amaltheids, and diversity is low; A. stokesi increases in abundance northwards where, in St ikinia, A. margaritatus makes rare appearances. Wrangellia, with its rich P liensbachian Tethyan and east Pacific faunas, is almost devoid of amaltheid s, but its amaltheid fauna does include two specimens of A. viligaensis, an eastern Russian species that is unknown elsewhere in North America. Craton al amaltheid faunas have more in common with those of northwest Europe than eastern Eurasia, suggesting that the Arctic and northern North Atlantic co nstituted the main dispersal route. Paleobiogeographic patterns on the majo r allochthonous terranes argue against terrane rotation and in support of p ost-Pliensbachian northward displacement relative to the North American cra ton. In addition, the presence of western Pacific faunal elements on Wrange llia suggests a more significant longitudinal displacement relative to the craton for this terrane compared to that for Quesnellia and Stikinia. The C hilliwack terrane of southwestern British Columbia is a Pliensbachian paleo biogeographic anomaly.