Taphonomy of three dinosaur bone beds in the Upper Cretaceous Two Medicine Formation of northwestern Montana; evidence for drought-related mortality

Taphonomic and sedimentologic attributes of three dinosaur bone beds discovered within the Campanian Two Medicine Formation of Montana suggest 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). All three assemblages are mono/paucispecific, parautochthonous concentrations of disarticulated and dissociated skeletal debris. 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. Styracosaurus sp. and Prosaurolophus sp. may have been gregarious, water-dependent taxa; during drought these particular taxa may have obligatorily congregated, either in herds, familial groups, or seasonal aggregates, in the vicinity of persistent water sources, The mono/paucispecific natures of the bone beds may reflect ecological segregation due to varying degrees of water-dependency, resource partitioning, or territorial/resource defense within a Late Cretaceous dinosaur community. The preservational bias suggested by previous workers for drought assemblages is seemingly substantiated within the richly fossiliferous strata of the Two Medicine Formation.