Revised Formulation of the Late Triassic Land Vertebrate “Faunachrons” of Western North America: Recommendations for Codifying Nascent Systems of Vertebrate Biochronology

Abstract

Recently developed rigorous and testable stratigraphic models, improved vertebrate systematics, and radioisotopic calibration allow the Late Triassic land vertebrate “faunachrons” of western North America to be extensively revised and codified. Recognizing that reliable biochronology depends on well-documented and testable biostratigraphic models, we propose a simple set of three units to serve as a biochronology “starter kit”: one local biostratigraphic unit (a multiple taxon interval teilzone or biozone), one regional or global chronostratigraphic unit (an estimated holochronozone), and one biochronologic unit (an estimated holochron). These units are applied to the Otischalkian, Adamanian, Revueltian, and Apachean “faunachrons,” with all units being defined by multiple phytosaur taxa representing the appearance of monophyla. Best estimates for the holochrons are 227–224Ma (earliest Norian) for the Otischalkian; 224–215Ma (early to earliest middle Norian) for the Adamanian; 215–207Ma (earliest middle Norian to Rhaetian) for the Revueltian; and 207–202Ma (Rhaetian) for the Apachean.