Why did North American rock sequences become important standards for Triassic time?

North American rock sequences became important standards for Triassic time primarily due to the limitations of the traditional Alpine succession, which faced challenges in its application as a global standard.1

In response to these issues, paleontologists—most notably E. Timothy Tozer and Norman J. Silberling—proposed a new zonal scheme based on relatively complete and in-place sequences found in Arctic Canada, northeastern British Columbia, and the western United States. This North American framework was bolstered by the discovery of hundreds of new localities between 1955 and 1980, which provided precisely defined stratotypes for all recognized North American biozones.1

Furthermore, the region proved vital because more than half of the world’s known genera of ammonoids occurred in North America, highlighting the cosmopolitan nature of the group. Today, this zonal scheme is accepted by most authorities as the standard for Triassic global biostratigraphy, allowing researchers to accurately place Alpine and Boreal zones into their proper chronological sequence.1