Article: Latest Toarcian ammonoids from the North American Cordillera
Latest Toarcian (Early Jurassic) strata crop out in southern Alaska, southern Yukon, British Columbia, south-western Alberta, and eastern Oregon. Work in the Queen Charlotte Islands, British Columbia has proved a relatively complete Toarcian sequence which has provided the basis for a North American Toarcian ammonite zonation. The Upper Toarcian Yakounensis Zone is characterized by a diverse ammonite fauna, previously assigned to the Middle Toarcian. Species of Hammatoceras, Dumortieria, Sphaerocoeloceras, Pleydellia, Holcophylloceras, and Pseudolioceras allow correlation with the younger part of the Levesquei Zone of north-west Europe. The new genus Yakounia and seven new species (Yakounia yakounensis, Y. pacifica, Y. freboldi, Y. silvae, Pleydellia maudensis, P. crassiornata and Dumortieria? phantasma) are introduced, all of which are endemic to western North America. A global regression during the Late Toarcian may have restricted migration between the eastern Pacific and western Tethys resulting in the development of endemic taxa.