The horned dinosaur Centrosaurus apertus from the Belly River Group (Campanian) is represented by multiple articulated skulls and skeletons, and is particularly notable for its occurrence in dozens of large-scale monodominant bonebeds, which have been found in the Dinosaur Park Formation across southern Alberta and Saskatchewan. Here we present a detailed taphonomic analysis of the first large-scale Centrosaurus apertus bonebed (McPheeters bonebed) from the Oldman Formation of southeastern Alberta. The McPheeters bonebed rivals the richest bonebeds in the Dinosaur Park Formation in terms of bone density and size, and the complete disarticulation of elements. The bonebed occurs in an overbank facies and is dominated by small bone clasts, suggesting that only low energy water current contributed to the formation of the bonebed before its final burial event. Patterns of taphonomic modification suggest that bones experienced little weathering, breakage, or scavenging. In turn, these conclusions are compatible with an overall interpretation of rapid burial in humid conditions after the disarticulation of elements. These taphonomic features are virtually identical to those seen in the well-documented bonebeds of this species in the Dinosaur Park Formation, which are interpreted to represent mass death events caused by seasonal tropical storms and associated large-scale flooding. Late Cretaceous dinosaur species typically have small geographic and stratigraphic ranges defined by the extent of single geological formations. The new bonebed extends the distribution of Centrosaurus apertus to the upper Oldman Formation, which is interpreted as more inland than the coastally influenced Dinosaur Park Formation, and suggests that mass death events related to seasonal tropical storms occurred over a broader geographic area and in a greater range of paleoenvironments than previously documented.

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