Three monodominant hadrosaurid bonebeds in the Horsethief Member of the Horseshoe Canyon Formation (uppermost Campanian) in southern Alberta, Canada, are documented. Each bonebed is hosted by a decimetre-thick deposit of poorly sorted and graded organic-fragment-rich mudstone. These fossil deposits are interpreted as having been carried and deposited by debris flows or hyperconcentrated mass sediment flows initiated by overbank flooding from local channels. Each bonebed is dominated (>50% of identifiable elements) by the disarticulated to occasionally associated remains of hadrosaurine hadrosaurids, inferred to be Edmontosaurus regalis. The majority of hadrosaurid elements at two of the sites (Bleriot Ferry and Prehistoric Park) are from large, presumably adult-sized individuals, whereas the majority of elements from the Fox Coulee site are from subadults and juveniles. Fossil elements from all the sites exhibit similar taphonomic signatures suggestive of a high degree of biostratinomic modification including: (i) thorough disarticulation of carcasses, (ii) a large amount of breakage, (iii) modest amounts of size sorting, and (iv) minimum to modest occurrences of abrasion, and scratch and tooth marks. These signatures indicate that carcasses were exposed for significant amounts of time on the floodplain, where they rotted, were scavenged and trampled, and were exposed to moving water prior to final burial. The size of each bonebed together with the density of bones suggest that the biocoenoses comprised large groups of hadrosaurids, and bone size distributions suggest the possibility of age-segregated populations. The monodominant nature of the assemblages combined with homogenous taphonomic signatures within and between sites suggests that these bonebed assemblages are best interpreted as the result of mass kills rather than attrition, with recurring tropical storm-induced coastal-plain flooding postulated as a likely mechanism for what killed and eventually buried these dinosaurs.

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