Taphonomy of Three Dinosaur Bone Beds in the Upper Cretaceous Two Medicine Formation of Northwestern Montana: Evidence for Drought-Related Mortality

Two bone beds, Canyon Bone Bed and Dino Ridge Quarry, have yielded the near-exclusive remains of a new species of Styracosaurus (Family Ceratopsidae); the third bone bed, Westside Quarry, is dominated by a new species of Prosaurolophus (Family Hadrosauridae). Evidence supporting a drought hypothesis includes: 1) a seasonal, semiarid paleoclimate, 2) associated caliche horizons, 3) aqueous depositional settings, 4) apparent age distributions characteristic of modern drought mortality (CBB and DRQ), and 5) the intraformational recurrence of low-diversity bone beds. Several alternative scenarios were considered, but drought proved most reasonable in light of the enhanced probability of preserving drought assemblages, and the species-selective and recurrent nature of modern drought mortality