Albertosaurus
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Albertosaurus (Template:IPAc-en; meaning "Alberta lizard") is a genus of large tyrannosaurid theropod dinosaur that lived in northwestern North America during the early to middle Maastrichtian age of the Late Cretaceous period, about 71 million years ago. The type species, A. sarcophagus, was apparently restricted in range to the modern-day Canadian province of Alberta, after which the genus is named, although an indeterminate species ("cf. Albertosaurus sp.") has been discovered in the Corral de Enmedio and Packard Formations of Mexico.<ref name="sullivanlucas-alberto" /> Scientists disagree on the content of the genus and some recognize Gorgosaurus libratus as a second species.<ref name=Larson08/><ref name="paul2016"/>
As a tyrannosaurid, Albertosaurus was a bipedal predator with short arms, two-fingered hands, and a massive head with dozens of large, sharp teeth, a strong sense of smell, powerful binocular vision, and a bone crushing bite force. It may have even been the apex predator in its local ecosystem. While Albertosaurus was certainly large for a theropod, it was still much smaller than its larger and more famous relative Tyrannosaurus rex, growing up to Template:Convert in length and weighing Template:Convert.
Since the first discovery in 1884, fossils of more than 30 individuals have been recovered that provide scientists with a more detailed knowledge of Albertosaurus anatomy than what is available for most other tyrannosaurids. The discovery of 26 individuals in one particular site provides evidence of gregarious behavior and allows for studies of ontogeny and population biology. These are near impossible with lesser-known dinosaurs because their remains are rarer and more fragmentary when compared to those of Albertosaurus.
History of discoveryEdit
NamingEdit
Albertosaurus was named by Henry Fairfield Osborn in a one-page note at the end of his 1905 description of Tyrannosaurus rex.<ref>Template:Cite journal</ref> Its namesake is Alberta, the Canadian province established the very same year where the first remains were found. The generic name also incorporates the Greek word {{#invoke:Lang|lang}}/sauros, meaning "lizard", which is the most common suffix in dinosaur names. The type species is Albertosaurus sarcophagus and the specific name is derived from the Ancient Greek term σαρκοφάγος ({{#invoke:Lang|lang}}), meaning "flesh-eating", and having the same etymology as the funeral container with which it shares its name, which is a combination of the Greek words σαρξ/{{#invoke:Lang|lang}} ("flesh") and {{#invoke:Lang|lang}}/{{#invoke:Lang|lang}} ("to eat").<ref name="osborn1905">Template:Cite journal</ref> More than 30 specimens of all ages are known to science.<ref name=ericksonetal2006/><ref name=currie2003b/>
Early discoveriesEdit
The type specimen is a partial skull collected on June 9, 1884, from an outcrop of the Horseshoe Canyon Formation alongside the Red Deer River in Alberta. It was recovered by an expedition of the Geological Survey of Canada, led by the famous geologist Joseph Burr Tyrrell. Due to a lack of specialised equipment, the almost complete skull could only be partially secured. In 1889, Tyrrell's colleague Thomas Chesmer Weston found an incomplete smaller skull associated with some skeletal material at a location nearby.<ref name=TankeCurrie2010>Template:Cite journal</ref> The two skulls were assigned to the preexisting species Laelaps incrassatus by Edward Drinker Cope in 1892.<ref name=cope1892>Template:Cite journal</ref> Although the name Laelaps was preoccupied by a genus of mite and had been changed to Dryptosaurus in 1877 by Othniel Charles Marsh, Cope stubbornly refused to recognize the new name created by his archrival. However, Lawrence Lambe used the name Dryptosaurus incrassatus instead of Laelaps incrassatus when he described the remains in detail in 1903 and 1904,<ref>Template:Cite journal</ref><ref name=lambe1904>Template:Cite journal</ref> which was a combination first coined by Oliver Perry Hay in 1902.<ref>Hay, Oliver Perry, 1902 Bibliography and Catalogue of the Fossil Vertebrata of North America. Bulletin of the United States Geological Survey, N° 117, Government Printing Office. pp 868</ref>
Shortly later, Osborn pointed out that D. incrassatus was based on generic tyrannosaurid teeth, so the two Horseshoe Canyon skulls could not be confidently referred to that species. The Horseshoe Canyon skulls also differed markedly from the remains of D. aquilunguis, type species of Dryptosaurus, so Osborn gave them the new name Albertosaurus sarcophagus in 1905. He did not describe the remains in any great detail, citing Lambe's complete description the year before.<ref name=osborn1905/> Both specimens, the holotype CMN 5600 and the paratype CMN 5601, are stored in the Canadian Museum of Nature in Ottawa. By the early twenty-first century, some concerns had arisen that, due to the damaged state of the holotype, Albertosaurus might be a nomen dubium that could only be used for the type specimen itself because other fossils could not reliably be assigned to it. However, in 2010, Thomas Carr established that the holotype, the paratype, and comparable later finds all shared a single common unique trait, or autapomorphy. The possession of an enlarged pneumatic opening in the back rim of the side of the palatine bone proves that Albertosaurus is a valid taxon.<ref>Template:Cite journal</ref>
Dry Island bone bedEdit
On August 11, 1910, American paleontologist Barnum Brown discovered the remains of a large group of Albertosaurus at another quarry alongside the Red Deer River. Because of the large number of bones and the limited time available, Brown's party did not collect every specimen, but made sure to collect remains from all of the individuals that they could identify in the bone bed. Among the bones deposited in the American Museum of Natural History collections in New York City are seven sets of right metatarsals, along with two isolated toe bones that did not match any of the metatarsals in size. This indicated the presence of at least nine individuals in the quarry. Palaeontologist Philip J. Currie of the Royal Tyrrell Museum of Palaeontology rediscovered the bonebed in 1997 and resumed fieldwork at the site, which is now located inside Dry Island Buffalo Jump Provincial Park.<ref name=currie1998>Template:Cite journal (not printed until 2000)</ref> Further excavation from 1997 to 2005 turned up the remains of 13 more individuals of various ages, including a diminutive two-year-old and a very old individual estimated at over Template:Convert long. None of these individuals are known from complete skeletons and most are represented by remains in both museums.<ref name=ericksonetal2006/><ref name=ericksonetal2004/> Excavations continued until 2008, when the minimum number of individuals present had been established at 12 (on the basis of preserved elements that occur only once in a skeleton) and at 26 if mirrored elements were counted when differing in size due to ontogeny. A total of 1,128 Albertosaurus bones had been secured, which is the largest concentration of large theropod fossils known from the Cretaceous.<ref>Template:Cite journal</ref>
Other discoveriesEdit
In 1911, Barnum Brown, during the second year of the American Museum of Natural History's operations in Alberta, uncovered a fragmentary partial Albertosaurus skull at the Red Deer River near Tolman Bridge (specimen AMNH 5222).<ref>Carpenter, K., 1992, "Tyrannosaurids (Dinosauria) of Asia and North America", In: N. Mateer and P.-J. Chen (eds.) Aspects of nonmarine Cretaceous geology. China Ocean Press, Beijing, China, pp. 250–268</ref>
William Parks described a new species in 1928, Albertosaurus arctunguis, based on a partial skeleton lacking a skull that was excavated by Gus Lindblad and Ralph Hornell near the Red Deer River in 1923,<ref name=parks1928>Template:Cite journal</ref> but this species has been considered identical to A. sarcophagus since 1970.<ref name=russell1970/> Parks' specimen (ROM 807) is housed in the Royal Ontario Museum in Toronto.<ref name=TankeCurrie2010/>
No Albertosaurus fossils were found from 1926 to 1972, but there has been an increase in findings since then. Apart from the Dry Island bonebed, six more skulls and skeletons have since been discovered in Alberta and are housed in various Canadian museums. Specimen RTMP 81.010.001 was found in 1978 by amateur paleontologist Maurice Stefanuk. RTMP 85.098.001 was found by Stefanuk on June 16, 1985. RTMP 86.64.001 was found in December 1985. RTMP 86.205.001 was found in 1986. RTMP 97.058.0001 was found in 1996 and then there is CMN 11315. Unfortunately, none of these skeletons were found with complete skulls.<ref name=TankeCurrie2010/> Fossils have also been reported from the American states of Montana, New Mexico, Wyoming, and Missouri, but they are doubted to be from A. sarcophagus and may not even belong to the genus Albertosaurus.<ref name=holtz2004/><ref name=currie2003b/>
Two specimens from "cf Albertosaurus ".sp" have been found in Mexico (Packard Formation and Corral de Enmedio Formation).<ref name="sullivanlucas-alberto">Listed as "cf. Albertosaurus sp." "Corral De Enmedio and Packard Formations, Cabollona Group, Sonora, Mexico," in Sullivan and Lucas (2006). Page 16.</ref>
Gorgosaurus libratusEdit
In 1913, paleontologist Charles H. Sternberg recovered another tyrannosaurid skeleton from the slightly older Dinosaur Park Formation in Alberta. Lawrence Lambe named this dinosaur Gorgosaurus libratus in 1914.<ref name=lambe1914>Template:Cite journal</ref> Other specimens were later found in Alberta and the US state of Montana. Finding no significant differences to separate the two taxa (due mostly to a lack of good Albertosaurus skull material), Dale Russell declared the name Gorgosaurus a junior synonym of Albertosaurus, which had been named first, and G. libratus was renamed Albertosaurus libratus in 1970. A species distinction was maintained because of the age difference. The addition extended the temporal range of the genus Albertosaurus earlier by several million years and its geographic range southwards by hundreds of kilometres.<ref name=russell1970/>
In 2003, Philip J. Currie, benefiting from much more extensive finds and a general increase in anatomical knowledge of theropods, compared several tyrannosaurid skulls and came to the conclusion that the two species are more distinct than previously thought. As the two species are sister taxa, they are more closely related to each other than to any other species of tyrannosaurid. Recognizing this, Currie nevertheless recommended that Albertosaurus and Gorgosaurus be kept as separate genera, as he concluded that they were no more similar than Daspletosaurus and Tyrannosaurus, which are almost always separated. In addition to this, several albertosaurine specimens have been recovered from Alaska and New Mexico. Currie suggested that the Albertosaurus-Gorgosaurus situation may be clarified once these are fully described.<ref name=currie2003b/> Most authors have followed Currie's recommendation,<ref name=holtz2004/><ref name=ericksonetal2004/><ref name=ricklefs2007>Template:Cite journal</ref> but some have not.<ref name=carretal2005/>
Other speciesEdit
In 1930, Anatoly Nikolaevich Riabinin named Albertosaurus pericolosus based on a tooth from China that probably belonged to Tarbosaurus.<ref>Template:Cite journal</ref> In 1932, Friedrich von Huene renamed Dryptosaurus incrassatus, not considered a nomen dubium by him, to Albertosaurus incrassatus.<ref>Von Huene, F., 1932 Die fossile Reptil-Ordnung Saurischia: ihre Entwicklung und Geschichte. Monographie für Geologie und Palaeontologie, Parts I and II, ser. I, 4: 1–361</ref> Because he had identified Gorgosaurus with Albertosaurus, in 1970, Russell also renamed Gorgosaurus sternbergi (Matthew & Brown 1922) into Albertosaurus sternbergi and Gorgosaurus lancensis (Gilmore 1946) into Albertosaurus lancensis.<ref name=russell1970/> The former species is today seen as a juvenile form of Gorgosaurus libratus and the latter is seen as either identical to Tyrannosaurus or representing a separate genus, Nanotyrannus. In 1988, Gregory S. Paul based Albertosaurus megagracilis on a small tyrannosaurid skeleton, specimen LACM 28345, from the Hell Creek Formation of Montana.<ref name=paul1988>Template:Cite book</ref> It was renamed Dinotyrannus in 1995,<ref name=olshevsky1995>Template:Cite journal</ref> but is now thought to represent a juvenile Tyrannosaurus rex.<ref name=currie2003a/> Also in 1988, Paul renamed Alectrosaurus olseni (Gilmore 1933) into Albertosaurus olseni,<ref name=paul1988/> but this has found no general acceptance. In 1989, Gorgosaurus novojilovi (Maleev 1955) was renamed by Bryn Mader and Robert Bradley as Albertosaurus novojilovi.<ref>Template:Cite journal</ref>
On two occasions, species based on valid Albertosaurus material were reassigned to a different genus, Deinodon. In 1922, William Diller Matthew renamed A. sarcophagus into Deinodon sarcophagus.<ref>Template:Cite journal</ref> In 1939, German paleontologist Oskar Kuhn renamed A. arctunguis into Deinodon arctunguis.<ref>Kuhn, O., 1939 Saurischia — Fossilium catalogus I, Animalia, Pars 87. 's-Gravenhage, W. Junk, 1939, 124 pp</ref>
DescriptionEdit
Albertosaurus was a fairly large bipedal predator, but smaller than Tarbosaurus and Tyrannosaurus rex. Typical Albertosaurus adults measured up to Template:Convert long<ref name=russell1970>Template:Cite journal</ref><ref name=holtz2004>Template:Cite book</ref><ref name="paul2016"/> and weighed between Template:Convert in body mass.<ref name=christiansenfarina2004>Template:Cite journal</ref><ref name="paul2016">Template:Cite book</ref><ref>Template:Cite journal</ref><ref name=Larson08>Template:Cite book</ref>
Albertosaurus shared a similar body appearance with all other tyrannosaurids, Gorgosaurus in particular. Typical for a theropod, Albertosaurus was bipedal and balanced its large, heavy head and torso with a long, muscular tail. However, tyrannosaurid forelimbs were extremely small for their body size and retained only two functional fingers, the second being longer than the first. The legs were long and ended in a four-toed foot on which the first toe, the hallux, was very short and did not reach the ground. The third toe was longer than the rest.<ref name=holtz2004/> Albertosaurus may have been able to reach walking speeds of 14–21 km/hour (8–13 mi/hour).<ref>Template:Cite journal</ref> At least for the younger individuals, a high running speed is plausible.<ref name=currie1998/>
Two skin impressions from Albertosaurus are known, and both show scales. One patch was found associated with some gastralic ribs and the impression of a long, unknown bone, indicating that the patch is from the belly. The scales are pebbly and gradually become larger and somewhat hexagonal in shape. Also preserved are two larger feature scales, placed 4.5 cm apart from each other,<ref name=":0">Template:Cite journal</ref> making Albertosaurus, along with Carnotaurus, the only known theropods with preserved feature scales.<ref>Template:Cite journal</ref> Another skin impression is from an unknown part of the body. These scales are small, diamond-shaped, and arranged in rows.<ref name=":0" />
Skull and teethEdit
The massive skull of Albertosaurus, which was perched on a muscular, short, S-shaped neck, was about Template:Convert long in the largest adults.<ref name=currie2003a>Template:Cite journal</ref> Wide openings in the skull, called fenestrae, provided space for muscle attachment sites and sensory organs that reduced its overall weight. Its long jaws contained, both sides combined, 58 or more banana-shaped teeth. Larger tyrannosaurids possessed fewer teeth, but Gorgosaurus had 62. Unlike most theropods, Albertosaurus and other tyrannosaurids were heterodont, with teeth of different forms depending on their position in the mouth. The premaxillary teeth at the tip of the upper jaw, four per side, were much smaller than the rest, more closely packed, and D-shaped in cross section.<ref name=holtz2004/> Like with Tyrannosaurus rex, the maxillary (cheek) teeth of Albertosaurus were adapted in general form to resist lateral forces exerted by a struggling prey animal. The bite force of Albertosaurus was less formidable, however, with the maximum force, by the back teeth, reaching 3,413 Newtons.<ref>Template:Cite journal</ref> Above the eyes were short bony crests that may have been brightly coloured in life and possibly used, by males in particular, in courtship to attract a mate.<ref name="ageofdinosaursalbertosaurus">"Albertosaurus." In: Dodson, Peter; Britt, Brooks; Carpenter, Kenneth; Forster, Catherine A.; Gillette, David D.; Norell, Mark A.; Olshevsky, George; Parrish, J. Michael; & Weishampel, David B. The Age of Dinosaurs. Lincolnwood, Illinois: Publications International, Ltd., 1993. pp. 106–107. Template:ISBN.</ref>
In 2001, William Abler observed that Albertosaurus tooth serrations resemble a crack in the tooth ending in a round void called an ampulla.<ref name="abler-2001">Abler, W.L. 2001. A kerf-and-drill model of tyrannosaur tooth serrations. p. 84–89. In: Mesozoic Vertebrate Life. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press.</ref> Tyrannosaurid teeth were used as holdfasts for pulling flesh off a body, so when a tyrannosaur pulled back on a piece of meat, the tension could cause a purely crack-like serration to spread through the tooth.<ref name="abler-2001" /> However, the presence of the ampulla distributed these forces over a larger surface area and lessened the risk of damage to the tooth under strain.<ref name="abler-2001" /> The presence of incisions ending in voids has parallels in human engineering. Guitar makers use incisions ending in voids to, as Abler describes, "impart alternating regions of flexibility and rigidity" to wood that they work on.<ref name="abler-2001" /> The use of a drill to create an "ampulla" of sorts and prevent the propagation of cracks through material is also used to protect aircraft surfaces.<ref name="abler-2001" /> Abler demonstrated that a plexiglass bar with incisions called "kerfs" and drilled holes was more than 25% stronger than one with only regularly placed incisions.<ref name="abler-2001" /> Unlike tyrannosaurs, more ancient predators, like phytosaurs and Dimetrodon, had no adaptations to prevent the crack-like serrations of their teeth from spreading when subjected to the forces of feeding.<ref name="abler-2001" />
Classification and systematicsEdit
Albertosaurus is a member of the theropod family Tyrannosauridae, specifically the subfamily Albertosaurinae. Its closest relative is the slightly older Gorgosaurus libratus (sometimes called Albertosaurus libratus; see below).<ref name=currieetal2003>Template:Cite journal</ref> These two species are the only described albertosaurines, but other undescribed species may exist.<ref name=currie2003b>Template:Cite journal</ref> Thomas Holtz found Appalachiosaurus to be an albertosaurine in 2004,<ref name=holtz2004/> but his more recent unpublished work places it as a basal eotyrannosaurian just outside of Tyrannosauridae,<ref name=holtzdml2005>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> in agreement with other authors.<ref name=carretal2005>Template:Cite journal</ref>
The other major subfamily of tyrannosaurids is Tyrannosaurinae, which includes members like Daspletosaurus, Tarbosaurus, and Tyrannosaurus. Compared with the more robust tyrannosaurines, albertosaurines had slender builds, with proportionately smaller skulls and longer bones of the lower legs (tibia) and feet (metatarsals and phalanges).<ref name=currie2003a/><ref name=currieetal2003/>
Below is the cladogram of Tyrannosauridae based on the phylogenetic analysis conducted by Loewen et al. in 2013.<ref name=Loewen13>Template:Cite journal</ref>
PalaeobiologyEdit
Growth patternEdit
Most age categories of Albertosaurus are represented in the fossil record. Using bone histology, the age of an individual animal at the time of death can often be determined, allowing growth rates to be estimated and compared with other species. The youngest known Albertosaurus is a two-year-old discovered in the Dry Island bonebed, which would have weighed about 50 kilograms (110 lb) and measured slightly more than Template:Convert long. The Template:Convert specimen from the same quarry is 28 years old, the oldest and largest one known. When specimens of intermediate age and size are plotted on a graph, an S-shaped growth curve results, with the most rapid growth occurring in a four-year period ending around the sixteenth year of life, a pattern also seen in other tyrannosaurids. The growth rate during this phase was Template:Convert per year, based on an adult weighing 1.3 tonnes. Other studies have suggested higher adult weights, which would affect the magnitude of the growth rate, but not the overall pattern. Tyrannosaurids similar in size to Albertosaurus had similar growth rates, although the much larger Tyrannosaurus rex grew at almost five times this rate (Template:Convert per year) at its peak.<ref name=ericksonetal2006/> The end of the rapid growth phase suggests the onset of sexual maturity in Albertosaurus, although growth continued at a slower rate throughout the animals' lives.<ref name=ericksonetal2006>Template:Cite journal</ref><ref name=ericksonetal2004/> Sexual maturation while still actively growing appears to be a shared trait among small<ref name=ericksonetal2007>Template:Cite journal</ref> and large<ref name=lee_werning2008>Template:Cite journal</ref> dinosaurs, as well as in large mammals like humans and elephants.<ref name=lee_werning2008/> This pattern of relatively early sexual maturation differs strikingly from the pattern in birds, which delay their sexual maturity until after they have finished growing.<ref name=ricklefs2007/><ref name=lee_werning2008/>
During growth, thickening of the tooth morphology changed so much that, had the association of young and adult skeletons on the Dry Island bonebed not proven that they belonged to the same taxon, the teeth of juveniles would likely have been identified by statistical analysis as those of a different species.<ref>Template:Cite journal</ref>
Life historyEdit
Most known Albertosaurus individuals were aged 14 years or older at the time of death. Juvenile animals are rarely fossilized for several reasons, mainly preservation bias, where the smaller bones of younger animals were less likely to be preserved by fossilization than the larger bones of adults, and collection bias, where smaller fossils are less likely to be noticed by collectors in the field.<ref name=roachbrinkman2007>Template:Cite journal</ref> Young Albertosaurus are relatively large for juvenile animals, but their remains are still rare in the fossil record when compared to adults. It has been suggested that this phenomenon is a consequence of life history, rather than bias, and that fossils of juvenile Albertosaurus are rare because they simply did not die as often as adults did.<ref name=ericksonetal2006/>
A hypothesis of Albertosaurus life history postulates that hatchlings died in large numbers, but have not been preserved in the fossil record because of their small size and fragile construction. After just two years, juveniles were larger than any other predator in the region, aside from adult Albertosaurus, and more fleet-footed than most of their prey animals. This resulted in a dramatic decrease in their mortality rate and a corresponding rarity of fossil remains. Mortality rates doubled at age twelve, perhaps the result of the physiological demands of the rapid growth phase, and then doubled again with the onset of sexual maturity between the ages of fourteen and sixteen. This elevated mortality rate continued throughout adulthood, perhaps due to the high physiological demands of procreation, including stress and injuries received during intraspecific competition for mates and resources, and the eventual, ever-increasing effects of senescence. The higher mortality rate in adults may explain their more common preservation. Very large animals were rare because few individuals survived long enough to attain such size. High infant mortality rates, followed by reduced mortality among juveniles and a sudden increase in mortality after sexual maturity, with very few animals reaching maximum size, is a pattern observed in many modern large mammals, including elephants, African buffalo, and rhinoceros. The same pattern is also seen in other tyrannosaurids. The comparison with modern animals and other tyrannosaurids lends support to this life history hypothesis, but bias in the fossil record may still play a large role, especially since more than two-thirds of all Albertosaurus specimens are known from the exact same locality.<ref name=ericksonetal2006/><ref name=ricklefs2007/><ref>Template:Cite journal</ref>
Social behaviourEdit
The Dry Island bonebed discovered by Barnum Brown and his crew contains the remains of 26 Albertosaurus, the most individuals found in one locality of any large Cretaceous theropod and the second-most of any large theropod dinosaur behind the Allosaurus assemblage at the Cleveland-Lloyd Dinosaur Quarry in Utah. The group seems to be composed of one very old adult, eight adults between 17 and 23 years old, seven sub-adults undergoing their rapid growth phases at between 12 and 16 years old, and six juveniles between the ages of 2 and 11 years old that had not yet reached the growth phase.<ref name=ericksonetal2006/>
The near-absence of herbivore remains and the similar state of preservation common to the many individuals at the Albertosaurus bonebed quarry led Currie to conclude that the locality was not a predator trap, such as the La Brea Tar Pits in California, and that all of the preserved animals died at the same time. Currie claims this as evidence of pack behavior.<ref name="currie1998"/> Other scientists are skeptical, observing that the animals may have been driven together by a drought, flood, or other reasons.<ref name=ericksonetal2006/><ref name=roachbrinkman2007/><ref name=eberthmccrea2001>Template:Cite journal(published abstract only)</ref>
There is plentiful evidence for gregarious behaviour among herbivorous dinosaurs, including ceratopsians and hadrosaurs.<ref name=horner1997>Template:Cite book</ref> However, only rarely are so many dinosaurian predators found at the same site. Small theropods, like Deinonychus<ref name=maxwellostrom1995>Template:Cite journal</ref> and Coelophysis,<ref name=raath1990>Template:Cite book</ref> have been found in aggregations, as have larger predators, such as Allosaurus and Mapusaurus.<ref name=coriacurrie2006>Template:Cite journal</ref> There is some evidence of gregarious behaviour in other tyrannosaurids as well, as fragmentary remains of smaller individuals were found alongside "Sue", the Tyrannosaurus mounted in the Field Museum of Natural History in Chicago, and a bonebed in the Two Medicine Formation of Montana contains at least three specimens of Daspletosaurus preserved alongside several hadrosaurs.<ref name=currieetal2005>Template:Cite book</ref> These findings may corroborate the evidence for social behaviour in Albertosaurus, although some or all of the above localities may represent temporary or unnatural aggregations.<ref name=currie1998/> Others have speculated that, instead of social groups, at least some of these finds represent Komodo dragon-like mobbing of carcasses, where aggressive competition leads to some of the predators being killed and even cannibalized.<ref name=roachbrinkman2007/> The evidence of cannibalism was later reported in 2024 by Coppock and Currie.<ref>Template:Cite journal</ref>
Currie has also speculated on the pack-hunting habits of Albertosaurus. The leg proportions of the smaller individuals were comparable to those of ornithomimids, which were probably among the fastest dinosaurs. Younger Albertosaurus were probably equally fleet-footed or at least faster than their prey. Currie hypothesized that the younger members of the pack may have been responsible for driving their prey towards the adults, who were larger and more powerful, but also slower.<ref name=currie1998/> Juveniles may also have had different lifestyles than adults, filling predator niches between the enormous adults and the smaller contemporaneous theropods, the largest of which were two orders of magnitude smaller than adult Albertosaurus in mass.<ref name=holtz2004/> A similar situation is observed in modern Komodo dragons, with hatchlings beginning life as small insectivores before growing to become the dominant predators on their islands.<ref name=auffenberg1981>Template:Cite book</ref> However, as the preservation of behaviour in the fossil record is exceedingly rare, these ideas cannot readily be tested. In 2010, Currie, though still favouring the hunting pack hypothesis, admitted that the concentration could have been brought about by other causes, such as a slowly rising water level during an extended flood.<ref>Template:Cite journal</ref>
PalaeopathologyEdit
In 2009, researchers hypothesized that smooth-edged holes found in the fossil jaws of tyrannosaurid dinosaurs, such as Albertosaurus, were caused by a parasite similar to Trichomonas gallinae, which infects birds.<ref name=Wolff2009>Template:Cite journal</ref> They suggested that tyrannosaurids transmitted the infection by biting each other and that the infection impaired their ability to eat.<ref name=Wolff2009/>
In 2001, Bruce Rothschild and others published a study examining evidence for stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. They found that only one of the 319 Albertosaurus foot bones checked for stress fractures actually had them and none of the four hand bones did. The scientists found that stress fractures were "significantly" less common in Albertosaurus than in the carnosaur Allosaurus.<ref name="rothschild-dino">Rothschild, B., Tanke, D. H., and Ford, T. L., 2001, Theropod stress fractures and tendon avulsions as a clue to activity: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 331–336.</ref> ROM 807, the holotype of A. arctunguis (now referred to A. sarcophagus), had a Template:Convert deep hole in the iliac blade, although the describer of the species did not recognize this as pathological. The specimen also contains some exostosis on the fourth left metatarsal. In 1970, two of the five Albertosaurus sarcophagus specimens with humeri were reported by Dale Russel as having pathological damage to them.<ref name="molnar-pathology">Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 337–363.</ref>
In 2010, the health of the Dry Island Albertosaurus assembly was reported upon. Most specimens showed no sign of disease. On three phalanges of the foot, strange bony spurs that consisted of abnormal ossifications of the tendons, so-called enthesophytes, were present, but their cause is unknown. Two ribs and a belly-rib showed signs of breaking and healing. One adult specimen had a left lower jaw showing a puncture wound and both healed and unhealed bite marks. The low number of abnormalities compares favourably with the health condition of a Majungasaurus population of which it was established, in 2007, that 19% of individuals showed bone pathologies.<ref>Template:Cite journal</ref>
PalaeoecologyEdit
Most fossils of Albertosaurus sarcophagus are known from the upper Horseshoe Canyon Formation in Alberta. These younger units of this geologic formation date to the early Maastrichtian age of the Late Cretaceous period, about 70 to 68 million years ago. Immediately below this formation is the Bearpaw Shale, a marine formation representing a section of the Western Interior Seaway. The Inland Sea was receding as the climate cooled and sea levels subsided towards the end of the Cretaceous, thus exposing land that had previously been underwater. It was not a smooth process, however, and the seaway would periodically rise to cover parts of the region throughout Horseshoe Canyon before finally receding altogether in the years after. Due to the changing sea levels, many different environments are represented in the Horseshoe Canyon Formation, including offshore and near-shore marine habitats and coastal habitats, such as lagoons, estuaries, and tidal flats. Numerous coal seams represent ancient peat swamps. Like most of the other vertebrate fossils from the formation, Albertosaurus remains are found in deposits laid down in the deltas and floodplains of large rivers during the later half of Horseshoe Canyon times.<ref name=eberth1997>Template:Cite book</ref>
The fauna of the Horseshoe Canyon Formation is well-known, as vertebrate fossils, including those of dinosaurs, are very common. Sharks, rays, sturgeons, bowfins, gars, and the gar-like Aspidorhynchus made up the fish fauna. Mammals included multituberculates and the marsupial Didelphodon. The saltwater plesiosaur Leurospondylus has been found in marine sediments in the Horseshoe Canyon, while freshwater environments were populated by turtles, Champsosaurus, and crocodilians like Leidyosuchus and Stangerochampsa. Dinosaurs dominate the fauna, especially hadrosaurs, which make up half of all dinosaurs known. These include the genera Edmontosaurus, Saurolophus, and Hypacrosaurus. Ceratopsians and ornithomimids were also very common, together making up another third of the known fauna. Along with much rarer ankylosaurians and pachycephalosaurs, all of these animals would have been prey for a diverse array of carnivorous theropods, including troodontids, dromaeosaurids, and caenagnathids.<ref name=eberth1997/><ref>Template:Cite journal</ref> Intermingled with the Albertosaurus remains of the Dry Island bonebed, the bones of the small theropod Albertonykus were found.<ref name=LC08>Template:Cite journal</ref> Adult Albertosaurus were the apex predators in their environment, with intermediate niches possibly filled by juvenile Albertosaurus.<ref name=eberth1997/>