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Dec 15, 2014 - Horseshoe Canyon Formation, central Alberta) and correlations with strata in the Drumheller and Grande Prairie regions1 ... The stratigraphic position of the Danek Bonebed equates best with the uppermost ... More specifically, it provides an opportunity .... Exposures of the #8 coal seam (Edmonton) occur.

Canadian Journal of

Earth Sciences

Volume 51

An NRC Research Press Journal

2014

Une revue de NRC Research Press

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Revue canadienne des

sciences de la Terre

In cooperation with the Geological Association of Canada

Avec le concours de l’Association géologique du Canada

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ARTICLE Stratigraphy of the Danek Bonebed (Upper Cretaceous Horseshoe Canyon Formation, central Alberta) and correlations with strata in the Drumheller and Grande Prairie regions1

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David A. Eberth and Phil R. Bell

Abstract: Although considerable work has been conducted on the stratigraphy and dinosaur assemblages of the Horseshoe Canyon Formation of southern Alberta, equivalent strata and assemblages in central Alberta remain poorly understood. Data from the Danek Bonebed (Edmonton, Alberta) are beginning to fill this gap. The bonebed occurs 4 m above the #9 Big Island Coal Seam. This stratigraphic position lies just below the middle of the Horseshoe Canyon Formation in the Edmonton region, and also lies below a thick, stratigraphically significant non-coaly interval that is expressed throughout central and southern Alberta. The stratigraphic position of the Danek Bonebed equates best with the uppermost Horsethief Member of the Horseshoe Canyon Formation in the Drumheller region and the upper part of Unit 4 of the Wapiti Formation in the Grande Prairie region. In both Drumheller and Grande Prairie, the correlated position of the bonebed underlies a zone of marine transgression (Drumheller Marine Tongue), which, in turn, includes the Campanian–Maastrichtian boundary. In the context of Geologic Time Scale 2004, we infer a late Campanian age of 71.0–71.3 Ma for the bonebed. The Danek’s dinosaurian assemblage is limited taxonomically, but compares well with the Edmontosaurus regalis – Pachyrhinosaurus canadensis fossil assemblage zone in the Drumheller region. We propose that a mega-herbivore faunal assemblage, characterized by Edmontosaurus and Pachyrhinosaurus, extended continuously across the climatically wet coastal plain of latest Campanian southern and central Alberta, and likely extended northwest into the North Slope of Alaska, where it persisted into the early Maastrichtian. Résumé : Bien que beaucoup de travail ait été fait sur la stratigraphie et les assemblages de dinosaures de la Formation de Horseshoe Canyon du sud de l’Alberta, l’équivalence des strates et des assemblages dans le centre de l’Alberta demeure mal comprise. Des données provenant du « lit d’os » Danek (Edmonton, Alberta) commencent a` combler ce manque. Le lit d’os se trouve a` 4 m au-dessus du lit de charbon no 9, Big Island. Cette position stratigraphique est juste sous le milieu de la Formation de Horseshoe Canyon dans la région d’Edmonton; elle se trouve aussi sous un intervalle épais, d’importance stratigraphique, mais qui ne contient pas de charbon et qui est exprimé a` travers tout le centre et le sud de l’Alberta. La position stratigraphique du lit d’os Danek correspond le mieux au membre sommital Horsethief de la Formation de Horseshoe Canyon dans la région de Drumheller, et a` la partie supérieure de l’unité 4 de la Formation de Wapiti dans la région de Grande Prairie. À Drumheller et a` Grande Prairie, la position corrélée du lit d’os se trouve sous une zone de transgression marine (langue marine de Drumheller), laquelle, a` son tour comprend la limite Campanien–Maastrichtien. D’après l’échelle des temps géologiques 2004, nous concluons un âge de 71,0 a` 71,3 Ma pour le lit. L’assemblage dinosaurien Danek est limité en termes de taxonomie mais il se compare bien a` la zone d’assemblage fossilifère a` Edmontosaurus regalis – Pachyrhinosaurus canadensis dans la région de Drumheller. Nous proposons un méga-assemblage de faune herbivore, caractérisé par Edmontosaurus et Pachyrhinosaurus, qui s’étendait de manière continue a` travers la plaine côtière, a` climat humide, du Campanien sommital du sud et du centre de l’Alberta; ce mégaassemblage s’étendait probablement aussi vers le nord-ouest dans le versant nord de l’Alaska, où il a été préservé jusqu’au Maastrichtien précoce. [Traduit par le Rédaction]

Introduction The Edmontosaurus-rich Danek Bonebed occurs along Whitemud Creek in Edmonton and serves as an important window through which latest Cretaceous non-marine environments and ecosystems can be assessed. More specifically, it provides an opportunity to compare upper Cretaceous dinosaur assemblages from the Horseshoe Canyon Formation (HCFm) in the Edmonton region with those better known assemblages from the Drumheller and Grande Prairie regions (300 km south and 400 km west, respectively). Here we combine subsurface data from southern and

central Alberta with vintage (1930s) outcrop-based coal studies (conducted in Edmonton; Beach 1934; Taylor 1934) to place the bonebed in a reliable local and regional stratigraphic context. We also correlate and compare the bonebed’s dinosaur assemblage with those that have been documented in the Drumheller and Grande Prairie regions (e.g., Fanti 2009; Fanti and Catuneanu 2010; Ryan et al. 2011; Eberth and Braman 2012; Eberth et al. 2013; Bell et al. 2014). In both the Drumheller and Edmonton regions, coal seams and swarms of the HCFm have been referenced in scientific publications using sequential numbers that increase in value up-section

Received 28 March 2014. Accepted 21 August 2014. Paper handled by Associate Editor Victoria Arbour. D.A. Eberth. Royal Tyrrell Museum of Palaeontology, Drumheller, AB T0J 0Y0, Canada. P.R. Bell. School of Environmental and Rural Science, University of New England, Armidale 2351, NSW, Australia. Corresponding author: David A. Eberth (e-mail: [email protected]). 1This article is part of a Special Issue entitled “The Danek Edmontosaurus Bonebed: new insights on the systematics, biogeography, and palaeoecology of Late Cretaceous dinosaur communities”. Can. J. Earth Sci. 51: 975–981 (2014) dx.doi.org/10.1139/cjes-2014-0069

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(Allan 1921; Beach 1934; Gibson 1977). Thus, in the scientific literature, identical terms (e.g., #1 coal) are sometimes used in reference to unrelated coals in the two regions. To reduce confusion in the following text, coal seam and swarm numbers are often followed by a parenthetical reference to the area being discussed (e.g., #6–7 coal zone (Drumheller)).

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Setting The Danek Bonebed is exposed along Whitemud Creek in the southwestern part of Edmonton (Fig. 1). Although detailed locality data are not presented here due to issues of fossil resource sensitivity, qualified researchers may access the data from the vertebrate collections programs of the Royal Tyrrell Museum of Palaeontology (TMP) and the University of Alberta Laboratory of Vertebrate Paleontology (UALVP). Excavations for vertebrate fossils have been conducted on-and-off at the bonebed since its discovery in 1989, first by TMP staff and, more recently, by UALVP members. The HCFm stratigraphic section that hosts the site is limited in thickness (⬃5 m) and lateral exposure ( 1) Troodontidae Troodon sp. (tooth; n = 1) Ornithomimidae Genus and species? (pedal phalanx; n = 1) Hadrosaurinae Edmontosaurus regalis (many skeletal elements; n = 100s) Chasmosaurinae Genus and species? (horn core; n = 1)

Inferred age of the Danek Bonebed Using a variety of 40Ar/39Ar and U–Pb radiometric ages and biostratigraphic data — all calibrated using Geologic Time Scale 2004 (Gradstein et al. 2004) — Eberth et al. (2013) inferred a time scale for the HCFm that assigns an age of ⬃71.0–71.3 Ma to the #8– 9 coal zone (Drumheller). Fanti and Catuneanu (2010), Eberth and Braman (2012), and Eberth et al. (2013) also proposed that the marine transgression in the middle of the Horseshoe Canyon and Wapiti formations has an age of 70.4–70.6 Ma, coinciding with, or

just postdating, the Campanian–Maastrichtian Boundary (70.6 Ma; Ogg et al. 2004; Figs. 3 and 4). Given that the Danek Bonebed correlates with the #8–9 coal zone (Drumheller), and that it predates the DMT (Drumheller), we propose that the Danek Bonebed was deposited ⬃71.0–71.3 Ma. Davies et al. (2014) conducted LAICPMS and TIMS U–Pb analyses of zircons from a tuff that occurs a few metres below the Danek Published by NRC Research Press

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Can. J. Earth Sci. Vol. 51, 2014

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Fig. 4. Chronostratigraphic distribution of dinosaur assemblage zones in the Horseshoe Canyon Formation as recognized from Drumheller to Edmonton. Modified from Eberth et al. (2013). Abbreviations as in Figs. 2 and 3.

Bonebed on Whitemud Creek. They report a maximum age of 71.9 Ma for the bonebed, an age that is somewhat older than our proposed age of ⬃71.0–71.3 Ma. This discrepancy may reflect an older crystallization history for the zircons, errors in calibrating the chronostratigraphic framework of Eberth et al. (2013), or both. An 40Ar/39Ar age of 72.58 ± 0.09 Ma was reported from a tuff that overlies Edmontosaurus regalis specimen UALVP 53722 from the Wapiti Formation near Grande Prairie (Bell et al. 2014; Fig. 3). According to F. Fanti (personal communication, 2014), this tuff occurs in the lower to middle portion of Unit 4 (Grande Prairie) and thus underlies the correlated stratigraphic position of the Danek Bonebed. Nonetheless, the stratigraphic position and age of this tuff remains compatible with our proposed age range of 71.0– 71.3 Ma for the Danek Bonebed, as well as the results of Davies et al. (2014).

Dinosaurian composition and biostratigraphy of the Danek Bonebed The Danek Bonebed has yielded a limited assemblage of dinosaurs (Table 1). The assemblage is dominated overwhelmingly by hadrosaur elements and, so far, all identifiable hadrosaur elements belong to the hadrosaurine, E. regalis (Bell and Campione 2014). This suggests that the bonebed is best classified as a monodominant assemblage (cf., Eberth et al. 2007) of E. regalis. Monodominant bonebeds of E. regalis are common in the HCFm of the Drumheller area (Eberth et al. 2013), and they are particularly abundant from the top of the Drumheller Member (#6–7 coal zone, Drumheller) upward through to the top of the Horsethief Member (top of the #8–9 coal zone, Drumheller; Eberth et al. 2013: figs. 3 and 5). Because the Danek Bonebed correlates with the #8– 9 coal zone at the top of the Horsethief Member (see above) and both intervals yield the remains of E. regalis, we interpret these occurrences as coeval and Edmontosaurus as part of a mega-

herbivore fauna that likely was distributed continuously between these regions. Eberth et al. (2013) identified three discrete dinosaur assemblage zones in the Horseshoe Canyon Formation of the Drumheller region. From oldest to youngest these are Edmontosaurus regalis – Pachyrhinosaurus canadensis zone, Hypacrosaurus altispinus – Saurolophus osborni zone, and Eotriceratops xerinsularis zone. All known Danek Bonebed dinosaur taxa are also found in the Edmontosaurus regalis – Pachyrhinosaurus canadensis dinosaur assemblage zone (cf., Eberth et al. 2013: fig. 3 and table 1), which corresponds to the stratigraphic extent of the combined Drumheller and Horsethief members (⬃71.0–72.5 Ma). Edmontosaurus regalis is absent in the two overlying fossil assemblage zones, and the Danek Bonebed contains no evidence of the three diagnostic taxa from these overlying zones (Hypacrosaurus altispinus, Saurolophus osborni, and Eotriceratops xerinsularis). Accordingly, the Danek Bonebed’s dinosaur assemblage corresponds well compositionally and stratigraphically with the Edmontosaurus regalis – Pachyrhinosaurus canadensis assemblage zone in the Drumheller region, which has been assessed an age of ⬃71.0–72.4 Ma (Eberth et al. 2013), but may have been even older if it encompassed the lowest portions of the Horseshoe Canyon Formation in the Edmonton region (Figs. 3 and 4). Given that the Edmontosaurus regalis – Pachyrhinosaurus canadensis assemblage zone includes forms that preferred, or were at least tolerant of, well-watered to saturated coastal plain landscapes (Eberth and Braman 2012; Eberth et al. 2013), and that the Danek Bonebed appears to have occupied a similarly wet coastal plain setting (Beach, 1934), we anticipate that ongoing discoveries at the bonebed, or other localities at this stratigraphic position, should reveal dinosaurs and other vertebrates that were also present in the Drumheller area at this same time. Published by NRC Research Press

Eberth and Bell

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Implications for dinosaurian paleobiogeography Eberth et al. (2013) proposed that the faunal changes between the three dinosaur assemblage zones in the Drumheller region were driven principally by changes in seasonal rainfall and substrate drainage (but not temperature). Accordingly, they interpreted the assemblage changes as reflecting migrations of some ornithischians into or out of the region rather than as extinctions or evolutionary events. Our correlation of the Danek Bonebed fossil assemblage with the Edmontosaurus regalis – Pachyrhinosaurus canadensis zone in Drumheller suggests that this mega-herbivore fauna was distributed continuously from Drumheller to Edmonton. Although these assemblage zones have not been documented yet in the Grande Prairie region, the lithostratigraphic correlation of the Horseshoe Canyon Formation and Wapiti Formation proposed by Fanti and Catuneanu (2010) and supported here correlates the Danek Bonebed horizon with the upper portion of Unit 4 (Fig. 3). To our knowledge, identifiable dinosaurs have not been reported yet from that horizon, but E. regalis and Pachyrhinosaurus lakustai are known from strata below that interval (estimated age range of 72.58–73.2 Ma (Fanti 2009: fig. 2, WP1, WP2, PC; Bell et al. 2014)). Thus, we speculate that a long-lived late Campanian megaherbivore fauna, characterized by the genera Edmontosaurus and Pachyrhinosaurus, became widely established across coastal plain settings of southern and central Alberta at least 73 Ma, and persisted until ⬃71 Ma, at which time changing climatic conditions forced changes in the fauna (Eberth et al. 2013). Edmontosaurus and Pachyrhinosaurus are also present in the upper Prince Creek Formation in the North Slope of Alaska (Gangloff and Fiorillo 2010; Fiorillo and Tykoski 2012). This stratigraphic interval appears to have an age range of ⬃69–72 Ma (Fiorillo and Tykoski 2012), and thus, the same late Campanian mega-herbivore fauna may have persisted in that region for a longer time than in southern Alberta. Based on the evidence discussed here, we speculate that an Edmontosaurus–Pachyrhinosaurus mega-herbivore fauna became established across the vast coastal lowland region of western Canada to Alaska ⬃73 Ma, survived in southern and central Alberta until the end of the Campanian, but may have persisted in northern Alaska into the early Maastrichtian.

Summary The Danek Bonebed lies 4 m above the #9 Big Island Coal Seam in Edmonton. We interpret the bonebed’s age as latest Campanian (⬃71.0–71.3 Ma) or slightly older. We correlate its stratigraphic position with the #8–9 coal zone at the top of the Horsethief Member of the Horseshoe Canyon Formation in the Drumheller region and the upper part of Unit 4 of the Wapiti Formation in the Grande Prairie region. The Danek dinosaur assemblage corresponds well with the Edmontosaurus regalis – Pachyrhinosaurus canadensis assemblage zone in the Drumheller region. When combined with biostratigraphic data from the Wapiti Formation in the Grande Prairie region, we infer the presence of a late Campanian Edmontosaurus– Pachyrhinosaurus mega-herbivore fauna across the climatically wet coastal plain setting that stretched 700 km from the Drumheller region northwest to the Grande Prairie region. Emerging biostratigraphic data from the North Slope of Alaska suggest that this fauna existed in that region as well, and may have survived there into the early Maastrichtian.

Acknowledgements D.A.E. thanks D.-q. Chen, W. Langenberg, P. Currie, D. Braman, E. Koppelhus, V. Arbour, M. Burns, J. Davies, and especially F. Fanti for helpful comments, assistance, and humor relating to this manuscript. D.A.E. and P.R.B. thank the reviewers W. Langenberg and

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J. Scott for helpful criticisms and comments, but we assume responsibility for all errors big and small.

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