A succession of fossil localities from the Hanna Basin, south-central Wyoming, brackets the boundary between Torrejonian and Tiffanian North American Land Mammal “Ages” (NALMAs). Unusually high rates of deposition in mid-Paleocene time in the Hanna Basin led to a greatly expanded section relative to classic mid-Paleocene sedimentary accumulations. Outcrops of the Hanna Formation, in an area of badlands in the northeast corner of the Hanna Basin called “The Breaks,” yield abundant vertebrate fossils. Mammalian fossils from a 550 m-thick interval in The Breaks document the latest Torrejonian (To3) though middle Tiffanian (Ti3) NALMAs. A 55 m-thick overlap zone between faunas typical elsewhere of To3 or earliest Tiffanian (Ti1) lies centrally within this interval.
The entire overlap zone in The Breaks represents the earliest parts of Ti1 as based upon presence of Plesiadapis praecursor and Nannodectes intermedius, index taxa for the Tiffanian. This does not affect the traditional definition of the To–Ti boundary. It does, however, extend ranges of several mammals typically considered exclusively Torrejonian into early Tiffanian time.
The mammalian fauna from The Breaks is one of the most diverse earliest Tiffanian faunas yet described, with 72 species of mammals recognized. The high diversity facilitates correlation with less diverse faunas of western North America. This is especially valuable for faunas of late Torrejonian or early Tiffanian age that lack critical index taxa (i.e., members of the Plesiadapidae) necessary for assigning a definitive age to the fauna. Furthermore, recognition of an overlap zone, the fauna of which is formally defined as The Breaks local fauna, within earliest parts of Ti1 provides greater age resolution for faunas near the Torrejonian–Tiffanian boundary. The greater detail about first and last appearances of mammalian species near the To–Ti boundary has resulted in complications to biostratigraphic zonation that have been undetectable elsewhere in thinner sections.