Hadrosaurid (Edmontosaurus) bonebeds from the Horseshoe Canyon Formation (Horsethief Member) at Drumheller, Alberta, Canada: geology, preliminary taphonomy, and significance

Abstract

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. Résumé :Trois lits à ossements à monodominance d'hadrosauridés du Membre de Horsethief de la Formation de Horseshoe Canyon (Campanien sommital) dans le sud de l'Alberta (Canada) sont documentés. Chaque lit à ossements se trouve dans un dépôt d'épaisseur décimétrique de mudstone mal classé et granoclassé riche en fragments organiques. Ces gisements fossilifères sont interprétés comme ayant été transportés et déposés par des coulées de débris ou des coulées de masse de sédiments hyperconcentrées initiées par l'inondation des berges par des chenaux locaux. Chaque lit à ossements est dominé (>50 % des éléments identifiables) par les restes désarticulés à occasionnellement associés d'hadrosauridés hadrosaurinés, présumés être d'Edmontosaurus regalis. La majorité des éléments d'hadrosauridé à deux des sites (Bleriot Ferry et Prehistoric Park) proviennent de grands individus présumés être de taille adulte, alors que la majorité des éléments du site de Fox Coulee proviennent d'individus subadultes et juvéniles. Les éléments fossile...

Origins of dinosaur bonebeds in the Cretaceous of Alberta, Canada

“…Some of these terms are employed to describe Alberta's macrofossil dinosaur bonebeds, particularly where paleobehavioral and ontogenetic inferences are being made (e.g., Wood et al 1988;Sampson et al 1997;Ryan et al 2001;Eberth and Getty 2005;Fanti and Currie 2007;Currie et al 2008;Currie 2010, Eberth et al 2014;Evans et al 2015).…”

“…They consist of both vertebrate microfossil and macrofossil sites, with the latter comprising both monodominant and multitaxic assemblages (Langston 1975;Eberth et al 2013;Burns et al 2014). Paleoenvironmental associations for both vertebrate microfossil and macrofossil bonebeds include splay, pond, mass sediment flow, and paleochannel deposits (Eberth and Currie 2010;Eberth et al 2013;Evans et al 2015).…”

“…Unfortunately, these features are well documented at only a few sites (25 of 75 macrofossil bonebeds; Fig. 4) in the Dinosaur Park, Oldman, Horseshoe Canyon, lower St. Mary River, and Wapiti formations (Ryan et al 2001;Eberth and Getty 2005;Eberth and Currie 2010;Chiba et al 2015;Evans et al 2015). Root-mark discoloration is common on bone surfaces (Figs.…”

Introduction to the Special Issue commemorating the 30th anniversary of the Royal Tyrrell Museum of Palaeontology, with a summary of the museum’s early history and its research contributions

“…An analysis by Henderson and Tanke (2010) highlighted the fossiliferous richness of DPP in a different manner, by using historical collection data, the volume of rock contained within DPP, and erosion rates to calculate that about 10 500 dinosaur skeletons have been lost to erosion and that about 11 000 dinosaur skeletons remain to be exposed within the Park's boundaries. Taphonomic work by RTMP researchers and colleagues outside of the DPP area has included the Horseshoe Canyon Formation of southern Alberta (e.g., Straight and Eberth 2002;Eberth and Currie 2010;Eberth and Braman 2012;Evans et al 2015) and farther afield into the Lower Cretaceous of Utah Britt et al 2009), the Cretaceous of Argentina (e.g., Eberth et al 2000b), the Cretaceous and Jurassic of China and Mongolia (e.g., Eberth et al 2001aEberth et al , 2010bQi et al 2007), and the Permian of Germany (Eberth et al 2000a). Other notable taphonomic projects involving RTMP researchers include studies arguing that supposed mosasaur tooth marks on Late Cretaceous ammonites are remnants of limpet resting traces (Kase et al 1998) and using computer simulations to examine how weathering might affect the form of dinosaur tracks and the taxonomic implications of those alterations (Henderson 2006a).…”

“…of a Arrhinoceratops bonebed in the Horseshoe Canyon Formation near Tolman Bridge. The museum also has been involved with pay-to-dig programs that allowed members of the public to participate in bonebed excavations in the Drumheller area (see Evans et al 2015) and in DPP. Those programs also provided hands-on field training for numerous students who went on to careers in palaeontology, geology, and archaeology.…”