<i>Edmontosaurus</i> from the Rocky Mountain foothills, Alberta, and its chronostratigraphic position in the Late Cretaceous Brazeau Formation and correlative units in western Canada

The Upper Cretaceous Brazeau Formation (early Campanian–early Maastrichtian) near Hinton, AB, preserves dinosaur fossils associated with fluvial and lacustrine palaeoenvironments during a transgressive–regressive cycle within the Western Interior Seaway. Here, the first description of vertebrate remains, hadrosaurid footprints, and radiometric age constraints from the Brazeau Formation are presented. Multiple dinosaur elements were recovered from a new bonebed, including a left postorbital and a partial right postorbital representing Edmontosaurus sp. based on an enlarged postorbital fossa. These are the first dinosaur elements diagnosable to the genus level to have been described from the Brazeau Formation; vertebrate remains have not previously been described from this formation. Eight tephras lie between ∼24 and ∼263 m above the Bennett Bonebed. Seven yielded statistically indistinguishable weighted-average laser ablation zircon 206 Pb/ 238 U dates between 70.14 ± 0.38 Ma (23.75 m) and 69.60 ± 0.35 Ma (∼262.60 m). A Bayesian age-depth model based on the zircon U–Pb dates yields a bonebed age of 70.17 ± 0.42|0.38 Ma and a median sedimentation rate of ∼450 m/Myr across the ∼240 m of measured section. The age of the Bennett Bonebed is, therefore, equivalent to the upper Tolman Member of the Horseshoe Canyon Formation, placing it in the middle Maastrichtian, ∼1.3 Ma after Edmontosaurus regalis is proposed to have been extirpated from Alberta, and within the &gt;2 Ma hiatus of the genus from the fossil record of northwestern North America.