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ARTICLE

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Albertosaurus (Dinosauria: Theropoda) material from an Edmontosaurus bonebed (Horseshoe Canyon Formation) near Edmonton; clarification of palaeogeographic distribution1 Phil R. Bell and Philip J. Currie

Abstract: Tyrannosaurid cranial bones — including a maxilla, dentary, and pterygoid — were collected from a monodominant Edmontosaurus bonebed in the Horseshoe Canyon Formation exposed near the city of Edmonton, Alberta, Canada. The maxilla can be identified as Albertosaurus sarcophagus based on the narrow interfenestral strut and relatively deep dental pits along the length of the palatal shelf. Cranial bones are interpreted to have come from a single large individual that was incorporated into the site during, or temporally close to, the formation of the final taphocoenosis. This discovery constitutes the northernmost record of A. sarcophagus, and helps to narrow the geographic gap of latest Cretaceous tyrannosaurs between Alberta and Alaska. The geographic distribution of A. sarcophagus — eclipsed only in areal extent by Tyrannosaurus rex in North America — attests to the adaptability of this species, which endured regional changes in climate that forced extirpation of many ornithischian taxa during deposition of the Horseshoe Canyon Formation. Résumé : Des ossements crâniens de tyrannosauridé, dont un maxillaire, un dentaire et un ptérygoïde, ont été recueillis d’un lit a` ossements a` monodominance d’Edmontosaurus dans la Formation de Horseshoe Canyon exposée a` proximité de la ville d’Edmonton (Alberta, Canada). Le maxillaire peut être identifié comme appartenant a` Albertosaurus sarcophagus a` la lumière de son élément interfenestral étroit et des fosses dentaires relativement profondes le long de la plateforme palatale. Les os crâniens sont interprétés comme provenant d’un seul individu, incorporé au site durant la formation de la taphocoenose finale, ou a` une époque rapprochée de cette dernière dans le temps. Cette découverte constitue le signalement le plus nordique d’A. sarcophagus et aide a` réduire l’écart géographique entre les tyrannosaures du Crétacé terminal de l’Alberta et de l’Alaska. La répartition géographique d’A. sarcophagus, dont l’étendue n’est surpassée que par celle de Tyrannosaurus rex en Amérique du Nord, témoigne de l’adaptabilité de cette espèce, qui a survécu a` des changements climatiques régionaux ayant forcé la disparition locale de nombreux taxons ornithischiens durant le dépôt de la Formation de Horseshoe Canyon. [Traduit par la Rédaction]

Introduction Large carnivorous dinosaurs of the family Tyrannosauridae are divisible into Tyrannosaurinae — a clade including, among others, Daspletosaurus, Tarbosaurus, and Tyrannosaurus — and Albertosaurinae, which includes Albertosaurus and Gorgosaurus (Currie 2003b). The Upper Cretaceous terrestrial sediments deposited along the margin of the Western Interior Seaway in Alberta preserve one of the richest, most diverse assemblages of tyrannosaurid fossils anywhere in the world. Presently, four taxa are recognised from the Late Cretaceous of Alberta: Albertosaurus sarcophagus, Daspletosaurus torosus, Gorgosaurus libratus, and Tyrannosaurus rex. Of these, only A. sarcophagus is definitively known from the Horseshoe Canyon Formation (herein, HCFm), which spans the Campanian–Maastrichtian boundary (Carr 2010; Eberth and Braman 2012). Gorgosaurus libratus, from the Dinosaur Park Formation, has been referred to as Albertosaurus libratus by some authors (Paul 1988; Carr et al. 2005; Carr and Williamson 2010); however, Albertosaurus is used in this paper strictly in reference to the HCFm taxon. In their review of Albertosaurus sarcophagus discoveries, Tanke and Currie (2010) noted a geographic range for this taxon that extended from near the town of Drumheller in southern Alberta

to Edmonton some 230 km to the north; however, no justification was given for the identification of specimens, particularly those in the northern part of this range. Perhaps most importantly, tyrannosaurid bones collected from the Edmonton locality — specifically the Edmontosaurus-dominated Danek Bonebed — come from a palaeolatitude of approximately 63°N (Scotese 1991; Brinkman 2003; D.A. Eberth, personal communication, 2014), making them some of the most northerly, potentially diagnostic tyrannosaurid material from North America. With the exception of the newly described Nanuqsaurus hoglundi from Alaska (Fiorillo and Tykoski 2014), tyrannosaurs reported from higher latitudes such as the Wapiti Formation in west-central Alberta (65°N) and the Prince Creek Formation (67°N–85°N) in Alaska are currently only identifiable to family level (Fanti and Miyashita 2009; Fanti et al. 2013). The aim of this paper is to assess the taxonomic identity of tyrannosaurid bones collected from the Danek Bonebed to help fill the geographic gap during the latest Cretaceous in western North America.

Institutional abbreviations TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UALVP, University of Alberta, Edmonton, Alberta, Canada.

Received 9 March 2014. Accepted 8 May 2014. Paper handled by Associate Editor Michael Ryan. P.R. Bell. Department of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia. P.J. Currie. Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada. Corresponding author: Phil R. Bell (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: 1–6 (2014) dx.doi.org/10.1139/cjes-2014-0050

Published at www.nrcresearchpress.com/cjes on xx xx xxxx.



Fig. 1. Location of Edmonton in Alberta, Canada, where tyrannosaurid elements were discovered in the hadrosaurdominated Danek Bonebed. Light grey region denotes the surface extent of the Horseshoe Canyon Formation.

Locality

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Tyrannosaurid material was collected by the Royal Tyrrell Museum (1989, 1991) and the University of Alberta (2006–2012) from a monodominant Edmontosaurus bonebed (informally known as the Danek Bonebed; TMP locality L2379) on Whitemud Creek within the city limits of Edmonton, Alberta (Fig. 1). The exposures at the Danek Bonebed are referred to the Horsethief Member (latest Campanian; Eberth and Braman 2012; Eberth and Bell, this issue) of the Horseshoe Canyon Formation. The bonebed host unit is a black, fissile, organic-rich siltstone up to 70 cm thick, which extends laterally for over 70 m and is interpreted as an overbank flood deposit (Bell and Campione, this issue). Bones are associated with abundant plant material, amber, and coalified logs. Disarticulated tyrannosaurid cranial bones were found associated with abundant, principally disarticulated material referrable to the hadrosaurine, Edmontosaurus regalis. Isolated tyrannosaurid teeth are also common and dispersed throughout the bonebed; however, they are not described here, as the majority of these are more likely shed teeth rather than from the individual represented by skeletal parts.

Description F2

Maxilla TMP 1989.017.0053 is an almost complete left maxilla (Fig. 2), measuring 49 cm anteroposteriorly. Precise quarry mapping had not been initiated in the Danek Bonebed when it was collected, but it would have been recovered from somewhere between X-axis lines H–K, and Y-axis lines 54–57. Based on maxillary tooth row length (436 mm), the animal is determined to have had a femur length of 933 mm, and a total body length of 8–8.5 m based on the allometric equations given by Currie (2003a). The distal end of the jugal process of the maxilla and the ascending ramus are

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

broken, and there is some damage to the anterolingual surface. Thirteen alveoli are preserved, although the anterior-most edge of the maxilla is missing and presumably would have supported at least one more alveolus, bringing the total number of teeth to 14. In lateral view, the alveolar margin is smoothly convex ventrally. Germ teeth are present in alveoli 2–5, 7–9, and 11. The broken roots of teeth are also still present in alveoli 10 and 12. Serrations are chisel-shaped and typically tyrannosaurid in morphology (Currie et al. 1990), numbering 10 per 5 mm on both the anterior and posterior carinae. There are two horizontal rows of shallow depressions that extend along the lateral surface of the maxilla. Depressions are most distinct above alveoli 4–7. Numerous neurovascular openings are roughly arranged into two horizontal lines; one occurring close to the alveolar margin, the other approximately halfway between the alveolar margin and the ventral margin of the antorbital fossa. The antorbital fossa is smooth in comparison to the more textured lateral surface of the maxilla. The interfenestral strut, although broken, is anteroposteriorly narrow. Despite damage to the ascending ramus and interfenestral strut, enough is preserved to show that the maxillary fenestra was large, D-shaped, and about as long as it was high. The ventral margin of the maxillary fenestra approaches but does not contact the antorbital fossa margin as in other tyrannosaurids (Carr et al. 2005). The maxillary fenestra reaches the margin of the antorbital fossa anterodorsally but not anteroventrally. The elliptical promaxillary fenestra is anterior to the maxillary fenestra at the anterior limit of the antorbital fossa and opens anteromedially into the medial surface of the maxilla. In lateral view, it is partially obscured by the ascending ramus. Medially, the promaxillary recess forms a broad elliptical cavity that occupies approximately half of the preserved portion of the ascending ramus. The palatal shelf is almost straight. The ventral surface of the palatal shelf is distinctly pitted along its entire length for housing the tips of the dentary teeth (Currie 2003b). No interdental plates were preserved intact. Dentary UALVP52743 is a poorly preserved right dentary (Fig. 3A) col- F3 lected in 2009 from quarry coordinate M.9, 35.2 (approximately 20–25 m west of TMP 1989.017.0053). The specimen is badly crushed, missing both distal (symphyseal) and proximal extremities, measures 45 cm long, and has a minimum depth of 11.5 cm. These measurements, although incomplete, suggest that the jaw is from an animal that had a femur that was about a metre long, suggesting it is from an individual that is about the same size as TMP 1989.17.53. Owing to its fractured state, only the lingual surface has been prepared, and unfortunately, little can be said of its original morphology. The interdental plates and all teeth are missing, but at least 11 alveoli can be counted; however, more were likely present in the jaw. Pterygoid A partial tyrannosaur pterygoid (TMP 1989.017.0015, Figs. 3B, 3C) found near the maxilla consists of a broken quadrate process of the pterygoid. The bone is broader lateromedially than it is high and is concave in posterior aspect. The quadrate ala is a relatively thin sheet of bone, thickest ventrally, although the lateral and ventral edges are broken, obscuring its original shape. Anteriorly, the quadrate contact forms a shallow recess across the ventrolateral surface, which continues to the preserved margins of the quadrate process. This recess is also marked by numerous small crenulations that extend dorsoventrally and lateromedially. The basipterygoid process is missing, but the base is preserved on the anteromedial portion of the quadrate process. The bone near the base of the basipterygoid process forms the thickest and most robust part of the quadrate process. Just medial to the basipterygoid process is a vertically oriented spur of bone with which the Published by NRC Research Press


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Fig. 2. Albertosaurus sarcophagus left maxilla (TMP 1989.17.53) in (A, B) lateral and (C, D) medial views. Alveoli are numbered i–xiii. Scale = 5 cm. A. sarcophagus autapomorphies are indicated by Arabic numerals. 1, narrow interfenestral strut; 2, deep pits ventral to palatal shelf. Ld, lateral depressions; Mf, maxillary fenestra; Pal, palatal contact (palatal shelf); ProF, promaxillary fenestra; Pror, promaxillary recess.

anterior ramus of the pterygoid would have been continuous; however, it too is broken.

Discussion and conclusions Of the tyrannosaurid elements recovered from the Danek Bonebed, only the maxilla (TMP 1989.017.0053) is considered potentially diagnostic to species level; therefore, taxonomic assignment is based largely on this specimen. TMP 1989.017.0053 can be identified as Tyrannosauridae based on the maxillary fenestra, which approaches the ventral margin of the antorbital fossa (Carr 1999; Carr and Williamson 2004, 2010; Carr et al. 2005). TMP 1989.017.0053 lacks several tyrannosaurine synapomorphies, including (i) maxillary fenestra that approaches or is anteromedial to the anterior margin of the antorbital fossa (Currie et al. 2003; Carr and Williamson 2010), and (ii) anteroposterior length of the maxillary fenestra that exceeds the distance between the anterior margins of the antorbital fenestra and antorbital fossa (Carr et al. 2005; Carr and Williamson 2010). Within Albertosaurinae, Albertosaurus and Gorgosaurus can be differentiated by the relatively anteroposteriorly narrow interfenestral strut in Albertosaurus (Carpenter 1992; Carr 2010). Currie (2003b) also regarded the presence of relatively deep pits along the length of the ventral surface of the palatal process as autapomorphic for Albertosaurus. However, Carr (2010) disagreed because this feature cannot be observed in the type series (the jaws are closed in both the type and paratype, preventing observation of the medial surface of the maxilla). Nevertheless, this feature is present in other Albertosaurus specimens unequivocally assigned to that taxon (TMP 1981.10.1, TMP 1995.25.83, TMP 1998.63.88) and is therefore regarded as characteristic of Albertosaurus. The presence of both

of these autapomorphic characters (Fig. 2) therefore permits confident assignment of TMP 1989.017.0053 to Albertosaurus sarcophagus. This identification documents the northernmost record of tyrannosaurid material unequivocally assigned to A. sarcophagus and helps define the geographical range of this taxon within the HCFm. In the Drumheller region of southern Alberta, the HCFm is divisible into three distinct biozones based on the distribution of ornithischians, which correspond to changes in climate (particularly rainfall and substrate drainage; Eberth et al. 2013). In contrast to the ornithischian assemblages, the only known tyrannosaurid, Albertosaurus sarcophagus, spans all except the lower (Strathmore and Drumheller) and uppermost (Whitemud and upper Carbon) members of the HCFm (Eberth and Braman 2012; Eberth et al. 2013). The total temporal span for Albertosaurus is 4.8 Ma and subsequently subsisted in both the warm–wet and cool – seasonally dry climatic regimes that persisted at times during deposition of the HCFm. Therefore, while climate change appears to have been a major factor driving local residence and extirpation of some ornithischian taxa, the sole tyrannosaurid, A. sarcophagus, was unaffected by such changes in the HCFm. The Danek Bonebed has been placed at the end of a warm–wet phase in line with the No. 8–9 coal zone at the top of the Horsethief Member, well within the known temporal distribution of Albertosaurus in the Drumheller region (Eberth et al. 2013; Eberth and Bell, this issue). Considering the large geotemporal range for Albertosaurus and its apparent tolerance of a wide range of climatic regimes, the discovery of A. sarcophagus in Edmonton, some 230 km north of its previously confirmed northern range limit (in the area of Dry Island Buffalo Jump Provincial Park), is perhaps not surprising. Published by NRC Research Press




Fig. 3. (A) Right tyrannosaurid dentary UALVP52743 in medial view. Alveoli are numbered i–xi from the first visible (preserved) alveolus. Scale bar = 5 cm. Partial left quadrate process of the pterygoid (TMP 1989.17.15) in (B) posterior and (C) anterior aspect. Scale bar = 2 cm. AntR, broken base of anterior ramus; Bpt, broken base of basipterygoid process; Qa, quadrate ala.

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With few exceptions, the distributions of North American tyrannosaurids are based on a limited number of specimens, resulting in overly simplistic representations of their geographic ranges as points on a map rather than actual ecologically meaningful spatial distributions (Fig. 4). Even particularly well-sampled strata, such as the Dinosaur Park Formation — where dozens of taxonomically informative tyrannosaurid specimens have been recovered — have limited utility in geographic range reconstructions, because exposures are limited to a few tens of square kilometres (Henderson and Tanke 2010). Discussion of dinosaur provincialism (Lehman 1987), particularly with respect to the latest Cretaceous of the Western Interior, has become increasingly topical and more widely accepted in recent years (Lehman 2001; Sampson and Loewen 2010; Sampson et al. 2010; Carr et al. 2011; Longrich et al. 2013). Nevertheless, such discussions are hampered by the fact that geographic ranges are based frequently on specimen counts of one, which artificially enhances (or blurs) the perceived endemism in these taxa. The geographic range of A. sarcophagus, which covered an estimated 25 000 km2 of coast-

line on the Western Interior (based on the north–south distance of Drumheller to Edmonton and the east–west distance of ⬃100 km from the palaeoshoreline to the westernmost occurrence of A. sarcophagus), is one of the largest for any tyrannosaurid and unique in being established on the basis of at least 14 skulls and (or) skeletons plus an additional bonebed occurrence (Tanke and Currie 2010). This distribution is eclipsed only among North American tyrannosauroids by Tyrannosaurus rex (based on at least 30 partial or complete skeletons), which ranged through some 8° of latitude from southern Alberta and Saskatchewan south to Wyoming and the Dakotas during the latest Maastrichtian — the largest range of any tyrannosauroid to date (Fig. 4). Even the abundant Asian taxon Tarbosaurus, which is known from an unparalleled 71 articulated skeletons (Currie 2009), has a limited geographic range of ⬃5200 km2. In all cases, these represent minimum values inherently limited by surface exposures of the respective geological units and their correlative strata. Better defining the geographic range of A. sarcophagus is therefore important for future tests of dinosaur provincialism and also enables one-to-one comparisons between the Edmonton Published by NRC Research Press


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Fig. 4. Palaeobiogeography of North American tyrannosauroids during the latest Cretaceous. For ease, geographic ranges and distributions are shown into three time slices (75, 70, and 65 Ma, respectively); however, some simplifications of biogeographic relationships are implicit. For example, Albertosaurus and Bistahieversor span the Campanian–Maastrichtian boundary. Specimen counts (n) tabulated from Holtz (2004), Carr and Williamson (2004, 2010), Currie (2005), Currie et al. (2005), Tanke and Currie (2010), and Fiorillo and Tykoski (2014). Palaeogeographic maps reproduced with permission from Colorado Plateau Geosystems, Inc. (http://cpgeosystems.com/paleomaps.html).

exposures of the HCFm with those further south near Drumheller and contemporaneous deposits of the upper (units 4 and 5; sensu Fanti and Catuneanu 2009) Wapiti Formation, which crops out in west-central Alberta. Moreover, it may be predicted that specimens attributable to Albertosaurus may be identified in the future from units 4 and 5 of the Wapiti Formation, where only indeterminate tyrannosaurids have yet been identified (Fanti and Miyashita 2009; Fanti et al. 2013). The three identifiable tyrannosaur bones from the Danek Bonebed cannot be unequivocally attributed to the same individual. The maxilla and dentary — the only two bones identifiable on the quarry maps — were not associated; however, there is also almost no association between any of the Edmontosaurus elements. Nevertheless, the three bones can all be said to have come from roughly the same-sized individual(s). Lastly, it is perhaps more parsimonious to assume the remains belong to a single animal, as catastrophic assemblages rarely preserve multiple large theropods (Currie et al. 2005). Tyrannosaur remains are frequently found in multidominant or mixed bonebeds; however, the genesis of these deposits fall under very different taphonomic regimes (attritional rather than catastrophic accumulations; Eberth et al. 2007; Rogers and Kidwell 2007) to monodominant bonebeds. So how did an Albertosaurus end up within a hadrosaur-dominated taphocoenosis? Even within monodominant vertebrate accumulations, other taxa are frequently encountered as minor components. This may have been caused by overprinting of an earlier assemblage by a subsequent event (and incorporation of additional carcasses), or because they were incorporated into the assemblage at the same time under the same conditions as part of the original biocoenosis (Eberth et al. 2007). The tyrannosaurid bones exhibit taphonomic features (disarticulated, low abrasion–weathering, similar element size range) identical to those of the hadrosaur material (Bell and Campione, this issue) suggesting incorporation of tyrannosaurid bones was synchronous with the remainder of the assemblage.

Acknowledgements TMP 1989.017.0053 was found and collected by David Fisk, Liz Fear, and Kent Wallis; UALVP52743 was found and collected by Eva Koppelhus (University of Alberta) and the second author. Jim Gardner (Royal Tyrrell Museum of Palaeontology) provided access to specimens. Federico Fanti (University of Bologna, Italy) provided valuable insight into the well-log data. Thanks to Eric Snively (formerly at the University of Alberta) for insightful dis-

cussions and reviewing earlier versions of the manuscript. Specimen illustrations are by the first author. Helpful comments from Ali Polat (Editor), Michael Ryan (Associate Editor), Thomas Carr, and an anonymous reviewer are greatly appreciated.

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