Middle Jurassic ammonite evolution in the Andean Province and emigration to Tethys.

Abstract

Middle Jurassic phyletic clades and grades are documented, with major radiations in late Aalenian-earliest Bajocian and late Bathonian, coincident with high sea levels regionally and globally. Several major clades probably originated in the Andes, including the Neuquen Embayment, and spread to western Tethys via the Hispanic Corridor ("proto-Atlantic'). The most important are: Sonniniidae with Euhoploceras derived from Andean Puchenquia (Gerthiceras), Sphaeroceratidae derived with Chondroceras from East-Pacific and mostly Andean Emileia (Chondromileia), and ? Reineckeiinae(?) perhaps derived with Reineckeia (Rehmannia) from Neuqueniceras (Frickites). The eastward passage through the Hispanic Corridor is supported by the ecotonic overlap of Andean/East-Pacific with west Tethyan faunas in the Mixteca terrane, south Mexico. The Macrocephalitinae derived from the Eurycephalitinae in the south Pacific and spread westward into Tethys. Major migrations through the Hispanic Corridor coincided with high eustatic sea levels which presumably removed barriers in the cratonic seaway.-from Authors