Toarcian (Early Jurassic) ammonoids from western North America

Abstract

Toarcian (Lower Jurassic) sedimentary rocks outcrop in western North America from southern Alaska to Nevada. The tectonic complexity of western North America, and of the Canadian Cordillera in particular, has, until recently, hampered biostratigraphic studies of this interval.Work in the Queen Charlotte Islands has identified one of the best preserved Toarcian sequences in western North America. Toarcian strata in the Queen Charlotte Islands have yielded a well preserved and diverse ammonite fauna that provides a framework for a regional ammonite zonation for western North America. Other localities in western North America, particularly in southern Alaska and northwestern British Columbia have provided additional material which has contributed to the development of an ammonite zonation. A diverse ammonite fauna has been collected from the Toarcian of western North America. Early Toarcian genera include Dactylioceras, Tiltoniceras, Taffertia, Cleviceras, and Hildaites. Middle Toarcian genera include Lytoceras, Peronoceras, Collina, Cleviceras, Harpoceras, Pseudolioceras, Polyplectus, Leukadiella, Paroniceras, Mercaticeras, Merlaites, Pseudomercaticeras, and Phymatoceras. Late Toarcian genera include Polyplectus, Grammoceras, Podagrosites, Pleydellia, Dumortieria, Phymatoceras, Yakounia, Sphaerocoeloceras, and Hammatoceras.