Palaeopathological changes in a population of Albertosaurus ...

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Palaeopathological changes in a population of Albertosaurus sarcophagus from the Upper Cretaceous Horseshoe Canyon Formation of Alberta, Canada1 Phil R. Bell

Abstract: Over the past decade, excavations of a monodominant theropod bonebed from the lower Maastrichtian beds of the Horseshoe Canyon Formation have recovered a minimum number of 26 individuals of the tyrannosaur, Albertosaurus sarcophagus. Examination of skeletal elements from the bonebed revealed a small number of abnormalities from at least two individuals. These include bony spurs (enthesophytes) of unknown origin on three pedal phalanges. Well-healed fracture calluses are present on two rib shafts and a gastralia element. The left dentary of one adult preserves both healed and unhealed parallel bite marks and a perforative lesion attributable to a partially healed, mechanically induced puncture wound. Unfortunately, the limited range in the types and frequencies of pathological changes provides only little information on the distribution of such phenomena but may be suggestive of the overall ‘‘health’’ of the population. Re´sume´ : Au cours de la dernie`re dećennie, des excavations d’un gisement d’ossements a` the´ropodes monodominant dans des lits du Maastrichtien infe´rieur de la Formation de Horseshoe Canyon ont produit un minimum de 26 exemplaires du tyrannosaure Albertosaurus sarcophagus. L’examen d’e´le´ments squelettiques provenant du gisement a re´ve´le´ un petit nombre d’anomalies chez au moins deux exemplaires, dont des e´perons osseux (enthe´sophytes) d’origine inconnue sur trois phalanges pe´dieuses. Des cals de fractures bien consolideés sont pre´sents sur deux diaphyses de coˆte et un e´le´ment de gastralium. L’os dentaire gauche d’un adulte conserve des marques de morsure paralle`les gue´ries et non gue´ries, ainsi qu’une le´sion perforante attribuable a` une plaie punctiforme d’origine mećanique partiellement gue´rie. Malheureusement, la gamme limiteé des types et des fre´quences de modifications pathologiques ne fournit que peu d’information sur la distribution de ces pheńome`nes, mais pourrait eˆtre un indicateur de la « sante´ » globale de la population. [Traduit par la Re´daction]

Introduction Palaeopathology is the study of disease and trauma in the fossil record, which provides insight into the lives of ancient organisms. Disease and trauma in extinct animals can only be identified when there is interaction between the causative agent (pathogen or trauma) and preservable hard parts (such as bones and teeth) of the individual. Such interaction may result in either the addition or removal of bone and may be expressed by variation in the size, density, or shape of the affected element (Rothschild and Martin 1993, 2006). These interactions may be pathognomic (unambiguously attributable to a given disease or traumatic episode) or non-pathognomic. The palaeopathology of theropod dinosaurs has been reviewed by several authors in recent years (Molnar 2001; Received 16 November 2009. Accepted 24 March 2010. Published on the NRC Research Press Web site at cjes.nrc.ca on 8 September 2010. Paper handled by Associate Editor H.-D. Sues. P.R. Bell. Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada (e-mail: [email protected]). 1This

article is one of a series of papers published in this Special Issue on the theme Albertosaurus.

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Rothschild and Tanke 2005). These studies identified injury (over disease or congenital anomalies) as the most common cause of pathology among theropods. Whereas individual records of isolated pathological changes in theropods are widely distributed in the literature (Molnar 2001 and references therein), there are few accounts of multiple pathological changes in individual animals (Larson 2001; Rega and Brochu 2001, 2002; Brochu 2003; Evans 2003; Bell 2008) and even fewer palaeopathological surveys of populations of single species, the latter due to the rarity of theropod bonebeds. Until now, only two palaeopathological surveys from individual theropod populations have been published: the Late Jurassic allosaurid Allosaurus fragilis from the Cleveland-Lloyd quarry in Utah (Hanna 2002) and the Late Cretaceous abelisaurid Majungasaurus crenatissimus from Madagascar (Farke and O’Connor 2007). At least two other bonebeds (the Late Triassic Coelophysis quarry at Ghost Ranch, New Mexico and a recently described bonebed of Daspletosaurus sp. from the Upper Cretaceous Two Medicine Formation of Montana (Currie et al. 2005)) are candidates for future palaeopathological surveys of penecontemporaneous theropod populations. Indeed, the presence of pathological bones has been noted, but not yet documented, from the Daspletosaurus bonebed (Currie et al. 2005). The Albertosaurus bonebed at Dry Island Buffalo Jump

doi:10.1139/E10-030

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Provincial Park in southern Alberta has yielded the remains of at least 26 individuals, ranging from 2 to 24 years in age (Erickson et al. 2006) and representing a standing population of large theropods from within a limited geographical region. The purpose of this study is to document the presence, type, and frequency of pathological elements within the Dry Island assemblage to permit future comparisons of pathological changes in other theropod populations, such as those just mentioned.

Material examined A total of 190 cranial and postcranial elements (excluding teeth) from Albertosaurus sarcophagus (Table 1) were visually examined for anomalous bone growth and (or) reduction. Pathological examples were compared against a subset of normal elements of Albertosaurus and other tyrannosaurids to characterize any deviation from the expected morphology. These elements represent a minimum of 26 individuals based on size and frequency of hindlimb elements representing animals ranging from 2 to 24 years in age (Erickson et al. 2006; Currie and Eberth, 2010). All elements were disarticulated and are now housed at the Royal Tyrell Museum of Palaeontology, Drumheller, Alberta (TMP) and the University of Alberta, Edmonton, Alberta (UALVP).

Descriptions and diagnoses Ribs and gastralia Two fragmentary ribs (TMP 1998.59.4, TMP 1998.59.5) show extensive callus formation due to healed fractures (Figs. 1A, 1B). In both specimens, an oblique pattern can be discerned indicative of a fracture (Aufderheide and Rodrı´guez-Martıń 1998). In TMP 1998.59.4, the two ends did not align correctly before healing, resulting in minor displacement of the shaft. The calluses are identified by the overtubulation (expansion–widening of the affected region relative to the ‘‘normal’’ shaft width) and irregular surface texture of the bone. An ‘‘open-weave’’ surface texture clearly demarcates the location of the fracture in TMP 1998.59.5. The analogous region in TMP 1998.59.4 is raised and has a dimpled, irregular surface. A healed fracture callus is also present on a single partial gastralia element (TMP 1998.63.96; Fig. 1C). Pedal phalanges Three pedal phalanges belonging to a minimum of two individuals preserve bony spurs on the ligament attachment sites (enthesophytes). Because enthesophytes grow in the direction of pull exhibited by the ligament or tendon at the enthesis, their development is regarded as an osteological response to repetitive stress and is positively correlated with age (Rogers et al. 1997; Benjamin et al. 2006). TMP 2000.45.15 is a right pedal phalanx IV-2 (Fig. 2A). A rounded spur of bone on the insertion site for the medial proximal collateral ligament is 5 mm long and 5 mm wide. The spur projects posteriorly (proximally) from the anterior margin of the depression that marks the border of the enthesis. The insertion pit for the medial collateral ligament on a right pedal phalanx II-1 (TMP 1999.50.14, Fig. 2B) pre-

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serves an enthesophyte that is 3 mm long and 3 mm wide. The enthesophyte projects anteriorly (distally) from the posterior margin of the insertion pit and terminates as a rounded nub. A second right pedal phalanx II-1 (TMP 2000.45.44, Fig. 2C) has an elongate enthesophyte on the dorsal margin of the medial proximal collateral ligament insertion scar. The spur is 10 mm long, oriented parallel to the long axis of the element and projects perpendicular to the bone surface. The surface of the enthesophyte is slightly irregular compared with the surrounding enthesial surface. Enthesophyte formation has been linked to a number or causes, particularly in response to inflammation of the enthesis caused by seronegative spondyloarthropathies (Benjamin and McGonagle 2001) or diffuse idiopathic skeletal hyperostosis (Resnick et al. 1975). Repetitive stresses on the tendon or ligament attachment site have also been linked to enthesophyte formation (Benjamin et al. 2006). Alternatively, their development may be in the absence of any clear causes, and it has been suggested that certain individuals may be genetically predisposed to form enthesophytes (Rogers et al. 1997). Given the wide range of etiologies and the absence of other evidence (e.g., behaviour, associated pathological changes), the enthesophytes described here must be regarded as of unknown etiological origin. Dentary An adult left dentary (TMP 2003.45.84) exhibits several abnormalities that are interpreted here as healed and unhealed bite marks. The first mark is situated on the labial surface of the dentary below the eighth alveolus. It is 60 mm long and 5 mm wide at its widest point and is oriented dorsoposteriorly (Fig. 3B). It is U-shaped in section and tapers towards its dorsoposterior extent. The groove itself is smooth-walled, and there is no evidence of exostosis or lytic (erosive) edges. A second region on the posterior half of the lateral wall of the dentary preserves three parallel, dorsoventrally oriented ridges (Fig. 3C). The ridges increase in length and spacing from anterior to posterior, measuring 20, 60, and 80 mm in length, respectively, and are 30 and 65 mm apart. Each is raised in low relief from the surrounding unaffected bone and has a narrow, median groove that extends along part of the length of each ridge. The edges of each lesion are ‘‘wrinkled’’ into disorganized bone textures, which extend from 2 to 4 mm from either side of the median groove. Similar lesions may be caused by stress fractures or plastic deformation; however, based on their size, parallel orientation, and the presence of raised, reactive bone growth, these marks are interpreted as healed bite marks made by another tyrannosaur. The first groove described here more likely represents an unhealed bite that was incurred perimortem or post mortem. Approximately 30 mm posterior to the longest lesion is an opening that completely perforates the dentary wall (Figs. 3C, 3D). The perforation is ovoid, measuring 5 mm long and is oriented dorsoventrally. A 6 mm long, V-shaped groove extends from the ventral margin of the opening. The bone in this area is diagenetically fractured, obscuring the surface details of the bone. However, the lesion itself is smooth-walled and clearly not post mortem in origin. When viewed lingually, the opening is recessed within an oval depression (Fig. 3D). The depression is also smooth and oriPublished by NRC Research Press

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1265 Table 1. Inventory of Albertosaurus elements examined. Element Skull Mandible Scapulae Forelimb Manual phalanges Manual unguals Ribs Gastralia Pelvis Femora Tibiae Fibulae Metatarsals Pedal phalanges Pedal unguals Cervical vertebra Dosal vertebrae Caudal vertebrae Chevrons

Total examined 27 7 4 3 4 1 30 23 2 2 0 4 7 42 13 1 2 6 4

Pathological

Catalogue No.

1

RTMP 2003.45.84

2 1

RTMP 1998.59.4, RTMP 1998.59.5 RTMP 1998.63.96

2

RTMP 2000.45.15, RTMP 2000.45.44

Fig. 1. Palaeopathological changes in Albertosaurus sarcophagus. (A) Rib fragment (TMP 1998.59.5). (B) Rib fragment (TMP 1998.59.4). (C) Gastralia element (TMP 1998.63.96). Scale bar = 2 cm.

Fig. 2. Palaeopathological changes in Albertosaurus sarcophagus. (A) Right pedal phalanx IV-2 (TMP 2000.45.15). (B) Right pedal phalanx II-1 (TMP 1999.50.14). (C) Right pedal phalanx II-1 (TMP 2000.45.44). Arrows indicate the location of enthesophytes. Scale bar = 2 cm.

ented parallel to the long axis of the opening, which is approximately 20 mm long and 8 mm wide. There is no evidence of osteomyelitis. Perforative lesions can be caused mechanically (e.g., impact) or by the death and resorption of bone caused by disease. Wolff et al. (2009) described similar smooth-walled lesions on a number of tyrannosaur mandibles (including TMP 2003.45.84), which they attributed to a trichomonosis-like disease. However, Trichomonas gallinae, which causes avian trichomonosis, does not elicit an osteological response (Stabler 1941; Narcisi et al. 1991), and thus its presence in tyrannosaurs must be considered dubious. Rega and Brochu (2001) considered similar lesions in Tyrannosaurus rex (FMNH PR2081) as possibly fungal or neoplastic in origin. Perforative lesions on other tyrannosaur specimens and the allosauroid Sinraptor dongi have also been referred to tooth-strike trauma by Tanke and Currie

(1998). The morphology of the smooth-walled perforative lesion in TMP 2003.45.84 is consistent with a mechanically produced puncture analogous to trephination (Campillo 1991), suggesting trauma and related healing. The relationPublished by NRC Research Press

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Fig. 3. Albertosaurus sarcophagus, left dentary (TMP 2003.45.64). (A) Lateral view. Close-ups (B) unhealed tooth mark; (C) healed tooth marks (arrows) and penetrative lesion (For); and (D) the penetrative lesion in lingual aspect. Scale bar for A = 10 cm. All other scale bars = 2 cm.

ship of this lesion to the aforementioned healed bites may implicate face biting as the cause of puncture.

Discussion and conclusions Six pathological elements were identified among 190 partial and complete bones of Albertosaurus sarcophagus representing at least 26 individuals. Pathological changes were found in at least two individuals and document healed fractures (n = 3), enthesophytes of unknown etiology (n = 2), and healed bites (n = 1). A perforation on the dentary of one adult Albertosaurus (TMP 2003.45.64) appears to be a partially healed, mechanically induced puncture, perhaps related to face-biting behaviour. The rarity of pathological bones from the Albertosaurus bonebed can be interpreted as a reflection of the relative health of the population. Although multiple injuries and lesions have been found in individual skeletons of Gorgosaurus (Larson 2001; Bell 2008), Tyrannosaurus (Rega and Brochu 2001; Brochu 2003), and Allosaurus (Hanna 2002), these probably represent exceptional cases. Among theropods, pathological changes are best known in the Tyrannosauridae (Molnar 2001). Fractures dominate this record and are most commonly found on the thoracic ribs and fibulae (Molnar 2001, Rothschild and Tanke 2005). Although no injured fibulae were identified, fractured ribs were relatively common at the Albertosaurus bonebed; however, this is no

doubt influenced by the large number of ribs collected from the thanatocoenosis. Other published accounts of pathological changes in Albertosaurus include undiagnosed humeral injuries (Russell 1970) and stress fractures (Rothschild and Tanke 2005), neither of which were identified from the bonebed. Although phalangeal enthesophytes have not previously been identified in Albertosaurus, enthesophytes are present on the left metatarsal IV of a specimen of A. sarcophagus (Parks 1928; Russell 1970). Few studies have addressed pathological changes and their distributions in theropod populations in part because of the rarity of theropod bonebeds (Table 2). Hanna (2002) listed and described 30 pathological elements of Allosaurus, some with multiple lesions, from the Cleveland-Lloyd quarry. Although this quarry has yielded remains of more than 40 individuals of Allosaurus, the total number of examined specimens and the minimum number of individuals (MNI) from which the pathological elements came were not provided by that author. Farke and O’Connor (2007) identified eight pathological bones from a total of 181 postcranial elements representing a minimum of 21 individuals from a single population of Majungasaurus. These bones (comprising seven vertebrae and one pedal phalanx) were distributed among a minimum number of four individuals. That is, 19% of the population showed some type of bone abnormality. Although it may be tempting to speculate on the reasons Published by NRC Research Press

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No. of pathological elements 30 8 6 No. of elements examined ? 181 190 Taxon Allosaurus fragilis Majungasaurus crenatissimus Albertosaurus sarcophagus

Table 2. Comparison of pathologies in populations of large theropods.

Total MNI 40+ 21 26

Pathological MNI ? 4 2

Pathological % of Population ? 19 7.7

Reference Hanna (2002) Farke and O’Connor (2007) Bell (this study)

behind the low incidence of pathology in the Dry Island assemblage, it is inappropriate to do so in the absence of similar pathological studies for a wider range of taxa.

Acknowledgements I thank Philip Currie and Eva Koppelhus for the invitation to contribute to this special issue and without whom I never would have had the opportunity to sacrifice years of backbreaking labour at the bonebed. Thanks also must go to the many volunteers who have similarly given their time to help in these excavations. I personally thank Mick and Elaine Morse on whose property we have camped, for their unwavering hospitality and generosity over the years. Brandon Strilitsky (TMP) provided access to the specimens in his care and Lara Shychoski generously lent me her camera equipment. Both rib fragments and gastralia were collected by Darren Tanke whose keen eye has saved many pathological specimens from being discarded over the years. Philip Currie provided useful comments on an earlier version of the manuscript. Helpful reviews by Bruce Rothschild, Larry Martin, and Associate Editor Hans-Dieter Sues improved the final version.

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