| Albertosaurus | |
|---|---|
 | |
| Mounted cast in theMilwaukee Public Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Family: | †Tyrannosauridae |
| Subfamily: | †Albertosaurinae |
| Genus: | †Albertosaurus Osborn, 1905 |
| Species: | †A. sarcophagus |
| Binomial name | |
| †Albertosaurus sarcophagus Osborn, 1905 | |
| Synonyms | |
| |
Albertosaurus (/ælˌbɜːrtəˈsɔːrəs/; meaning "Alberta lizard") is agenus of largetyrannosauridtheropoddinosaur that lived in northwestern North America during the early to middleMaastrichtian age of theLate Cretaceous period, about 71 million years ago. Thetype species,A. sarcophagus, was apparently restricted inrange to the modern-dayCanadian province ofAlberta, after which the genus is named, although an indeterminate species ("cf.Albertosaurus sp.") has been discovered in theCorral de Enmedio andPackard Formations ofMexico.[1] Scientists disagree on the content of the genus and some recognizeGorgosaurus libratus as a second species.[2][3]
As a tyrannosaurid,Albertosaurus was abipedalpredator 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 theapex predator in its localecosystem. WhileAlbertosaurus was certainly large for a theropod, it was still much smaller than its larger and more famous relativeTyrannosaurus rex, growing up to 8–9 metres (26–30 ft) in length and weighing 1.7–3.0 metric tons (1.9–3.3 short tons).
Since the first discovery in 1884,fossils of more than 30 individuals have been recovered that provide scientists with a more detailed knowledge ofAlbertosaurus anatomy than what is available for most other tyrannosaurids. The discovery of 26 individuals in one particular site provides evidence ofgregarious behavior and allows for studies ofontogeny andpopulation biology. These are near impossible with lesser-known dinosaurs because their remains are rarer and more fragmentary when compared to those ofAlbertosaurus.
Albertosaurus was named byHenry Fairfield Osborn in a one-page note at the end of his 1905 description ofTyrannosaurus rex.[4] Its namesake isAlberta, theCanadian province established the very same year where the first remains were found. The generic name also incorporates theGreek wordσαυρος/sauros, meaning "lizard", which is the most common suffix in dinosaur names. Thetype species isAlbertosaurus sarcophagus and thespecific name is derived from the Ancient Greek term σαρκοφάγος (sarkophagos), meaning "flesh-eating", and having the sameetymology as thefuneral container with which it shares its name, which is a combination of the Greek words σαρξ/sarx ("flesh") andφαγειν/phagein ("to eat").[5] More than 30 specimens of all ages are known to science.[6][7]
Thetype specimen is a partial skull collected on June 9, 1884, from anoutcrop of theHorseshoe Canyon Formation alongside theRed Deer River in Alberta. It was recovered by an expedition of theGeological Survey of Canada, led by the famousgeologistJoseph 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.[8] The two skulls were assigned to the preexisting speciesLaelaps incrassatus byEdward Drinker Cope in 1892.[9] Although the nameLaelaps waspreoccupied by a genus ofmite and had been changed toDryptosaurus in 1877 byOthniel Charles Marsh, Cope stubbornly refused to recognize the new name created by his archrival. However,Lawrence Lambe used the nameDryptosaurus incrassatus instead ofLaelaps incrassatus when he described the remains in detail in 1903 and 1904,[10][11] which was a combination first coined byOliver Perry Hay in 1902.[12]
Shortly later, Osborn pointed out thatD. 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 ofD. aquilunguis, type species ofDryptosaurus, so Osborn gave them the new nameAlbertosaurus sarcophagus in 1905. He did not describe the remains in any great detail, citing Lambe's complete description the year before.[5] Both specimens, theholotype CMN 5600 and theparatype CMN 5601, are stored in theCanadian Museum of Nature inOttawa. By the early twenty-first century, some concerns had arisen that, due to the damaged state of the holotype,Albertosaurus might be anomen 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, orautapomorphy. The possession of an enlarged pneumatic opening in the back rim of the side of thepalatine bone proves thatAlbertosaurus is a validtaxon.[13]
On August 11, 1910, American paleontologistBarnum Brown discovered the remains of a large group ofAlbertosaurus at anotherquarry 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 thebone bed. Among the bones deposited in theAmerican Museum of Natural History collections in New York City are seven sets of rightmetatarsals, 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. PalaeontologistPhilip J. Currie of theRoyal Tyrrell Museum of Palaeontology rediscovered the bonebed in 1997 and resumed fieldwork at the site, which is now located insideDry Island Buffalo Jump Provincial Park.[14] 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 10 metres (33 feet) long. None of these individuals are known from complete skeletons and most are represented by remains in both museums.[6][15] 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 toontogeny. A total of 1,128Albertosaurus bones had been secured, which is the largest concentration of large theropod fossils known from the Cretaceous.[16]
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In 1911, Barnum Brown, during the second year of the American Museum of Natural History's operations in Alberta, uncovered a fragmentary partialAlbertosaurus skull at the Red Deer River near Tolman Bridge (specimen AMNH 5222).[17]
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,[18] but this species has been considered identical toA. sarcophagus since 1970.[19] Parks' specimen (ROM 807) is housed in theRoyal Ontario Museum inToronto.[8]
NoAlbertosaurus 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.[8] Fossils have also been reported from the American states of Montana, New Mexico,Wyoming, andMissouri, but they are doubted to be fromA. sarcophagus and may not even belong to the genusAlbertosaurus.[20][7]
Two specimens from "cfAlbertosaurus ".sp" have been found inMexico (Packard Formation andCorral de Enmedio Formation).[1]
In 1913,paleontologistCharles H. Sternberg recovered another tyrannosaurid skeleton from the slightly olderDinosaur Park Formation in Alberta. Lawrence Lambe named this dinosaurGorgosaurus libratus in 1914.[21] Other specimens were later found in Alberta and the US state ofMontana. Finding no significant differences to separate the two taxa (due mostly to a lack of goodAlbertosaurus skull material),Dale Russell declared the nameGorgosaurus ajunior synonym ofAlbertosaurus, which had been named first, andG. libratus was renamedAlbertosaurus libratus in 1970. A species distinction was maintained because of the age difference. The addition extended the temporal range of the genusAlbertosaurus earlier by several million years and its geographic range southwards by hundreds of kilometres.[19]
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 aresister taxa, they are more closely related to each other than to any other species of tyrannosaurid. Recognizing this, Currie nevertheless recommended thatAlbertosaurus andGorgosaurus be kept as separate genera, as he concluded that they were no more similar thanDaspletosaurus andTyrannosaurus, which are almost always separated. In addition to this, several albertosaurine specimens have been recovered fromAlaska andNew Mexico. Currie suggested that theAlbertosaurus-Gorgosaurus situation may be clarified once these are fully described.[7] Most authors have followed Currie's recommendation,[20][15][22] but some have not.[23]
In 1930, Anatoly Nikolaevich Riabinin namedAlbertosaurus pericolosus based on a tooth from China that probably belonged toTarbosaurus.[24] In 1932,Friedrich von Huene renamedDryptosaurus incrassatus, not considered anomen dubium by him, toAlbertosaurus incrassatus.[25] Because he had identifiedGorgosaurus withAlbertosaurus, in 1970, Russell also renamedGorgosaurus sternbergi (Matthew & Brown 1922) intoAlbertosaurus sternbergi andGorgosaurus lancensis (Gilmore 1946) intoAlbertosaurus lancensis.[19] The former species is today seen as a juvenile form ofGorgosaurus libratus and the latter is seen as either identical toTyrannosaurus or representing a separate genus,Nanotyrannus. In 1988,Gregory S. Paul basedAlbertosaurus megagracilis on a small tyrannosaurid skeleton, specimen LACM 28345, from theHell Creek Formation of Montana.[26] It was renamedDinotyrannus in 1995,[27] but is now thought to represent a juvenileTyrannosaurus rex.[28] Also in 1988, Paul renamedAlectrosaurus olseni (Gilmore 1933) intoAlbertosaurus olseni,[26] but this has found no general acceptance. In 1989,Gorgosaurus novojilovi (Maleev 1955) was renamed by Bryn Mader and Robert Bradley asAlbertosaurus novojilovi.[29]
On two occasions, species based on validAlbertosaurus material were reassigned to a different genus,Deinodon. In 1922,William Diller Matthew renamedA. sarcophagus intoDeinodon sarcophagus.[30] In 1939, German paleontologistOskar Kuhn renamedA. arctunguis intoDeinodon arctunguis.[31]
Albertosaurus was a fairly large bipedal predator, but smaller thanTarbosaurus andTyrannosaurus rex. TypicalAlbertosaurus adults measured up to 8–9 m (26–30 ft) long[19][20][3] and weighed between 1.7 and 3.0 metric tons (1.9 and 3.3 short tons) in body mass.[32][3][33][2]
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 andtorso 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, thehallux, was very short and did not reach the ground. The third toe was longer than the rest.[20]Albertosaurus may have been able to reach walking speeds of 14–21 km/hour (8–13 mi/hour).[34] At least for the younger individuals, a high running speed is plausible.[14]
Two skin impressions fromAlbertosaurus are known, and both showscales. 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 centimeters apart from each other,[35] makingAlbertosaurus, along withCarnotaurus, the only known theropods with preserved feature scales.[36] Another skin impression is from an unknown part of the body. These scales are small, diamond-shaped, and arranged in rows.[35]
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The massiveskull ofAlbertosaurus, which was perched on a muscular, short, S-shaped neck, was about 1 metre (3.3 feet) long in the largest adults.[28] Wide openings in the skull, calledfenestrae, provided space formuscle attachment sites andsensory organs that reduced its overall weight. Its long jaws contained, both sides combined, 58 or more banana-shaped teeth. Larger tyrannosaurids possessed fewer teeth, butGorgosaurus had 62. Unlike most theropods,Albertosaurus and other tyrannosaurids wereheterodont, with teeth of different forms depending on their position in the mouth. Thepremaxillary teeth at the tip of the upper jaw, four per side, were much smaller than the rest, more closely packed, and D-shaped incross section.[20] Like withTyrannosaurus rex, the maxillary (cheek) teeth ofAlbertosaurus were adapted in general form to resist lateral forces exerted by a struggling prey animal. The bite force ofAlbertosaurus was less formidable, however, with the maximum force, by the back teeth, reaching 3,413 Newtons.[37] 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.[38]
In 2001,William Abler observed thatAlbertosaurustooth serrations resemble acrack in the tooth ending in a round void called an ampulla.[39] 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.[39] However, the presence of the ampulla distributed these forces over a largersurface area and lessened the risk of damage to the tooth under strain.[39] 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.[39] The use of adrill to create an "ampulla" of sorts and prevent the propagation of cracks through material is also used to protect aircraft surfaces.[39] 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.[39] Unlike tyrannosaurs, more ancient predators, likephytosaurs andDimetrodon, had no adaptations to prevent the crack-like serrations of their teeth from spreading when subjected to the forces of feeding.[39]
Albertosaurus is a member of the theropodfamily Tyrannosauridae, specifically the subfamily Albertosaurinae. Its closest relative is the slightly olderGorgosaurus libratus (sometimes calledAlbertosaurus libratus; see below).[40] These two species are the only described albertosaurines, but other undescribed species may exist.[7]Thomas Holtz foundAppalachiosaurus to be an albertosaurine in 2004,[20] but his more recent unpublished work places it as a basal eotyrannosaurian just outside of Tyrannosauridae,[41] in agreement with other authors.[23]
The other major subfamily of tyrannosaurids is Tyrannosaurinae, which includes members likeDaspletosaurus,Tarbosaurus, andTyrannosaurus. 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 andphalanges).[28][40]
Below is the cladogram of Tyrannosauridae based on thephylogenetic analysis conducted by Loewenet al. in 2013.[42]
Most age categories ofAlbertosaurus are represented in thefossil record. Using bonehistology, 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 knownAlbertosaurus 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 2 metres (6.6 feet) long. The 10 metres (33 feet) 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, anS-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 122 kilograms (269 pounds) 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 toAlbertosaurus had similar growth rates, although the much largerTyrannosaurus rex grew at almost five times this rate (601 kilograms [1,325 pounds] per year) at its peak.[6] The end of the rapid growth phase suggests the onset ofsexual maturity inAlbertosaurus, although growth continued at a slower rate throughout the animals' lives.[6][15] Sexual maturation while still actively growing appears to be a shared trait among small[43] and large[44] dinosaurs, as well as in large mammals like humans andelephants.[44] 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.[22][44]
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.[45]
Most knownAlbertosaurus individuals were aged 14 years or older at the time of death. Juvenile animals are rarely fossilized for several reasons, mainlypreservation 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.[46] YoungAlbertosaurus 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 oflife history, rather than bias, and that fossils of juvenileAlbertosaurus are rare because they simply did not die as often as adults did.[6]
A hypothesis ofAlbertosaurus life history postulates thathatchlings 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 adultAlbertosaurus, 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 ofsenescence. 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, andrhinoceros. 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 allAlbertosaurus specimens are known from the exact same locality.[6][22][47]
The Dry Island bonebed discovered by Barnum Brown and his crew contains the remains of 26Albertosaurus, the most individuals found in one locality of any large Cretaceous theropod and the second-most of any large theropod dinosaur behind theAllosaurus assemblage at theCleveland-Lloyd Dinosaur Quarry inUtah. 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.[6]
The near-absence ofherbivore remains and the similar state of preservation common to the many individuals at theAlbertosaurus bonebed quarry led Currie to conclude that the locality was not a predator trap, such as theLa Brea Tar Pits inCalifornia, and that all of the preserved animals died at the same time. Currie claims this as evidence of pack behavior.[14] Other scientists are skeptical, observing that the animals may have been driven together by a drought, flood, or other reasons.[6][46][48]
There is plentiful evidence for gregarious behaviour among herbivorous dinosaurs, includingceratopsians andhadrosaurs.[49] However, only rarely are so many dinosaurian predators found at the same site. Small theropods, likeDeinonychus[50] andCoelophysis,[51] have been found in aggregations, as have larger predators, such asAllosaurus andMapusaurus.[52] There is some evidence ofgregarious behaviour in other tyrannosaurids as well, as fragmentary remains of smaller individuals were found alongside "Sue", theTyrannosaurus mounted in theField Museum of Natural History inChicago, and a bonebed in theTwo Medicine Formation of Montana contains at least three specimens ofDaspletosaurus preserved alongside several hadrosaurs.[53] These findings may corroborate the evidence for social behaviour inAlbertosaurus, although some or all of the above localities may represent temporary or unnatural aggregations.[14] Others have speculated that, instead of social groups, at least some of these finds representKomodo dragon-like mobbing of carcasses, where aggressive competition leads to some of the predators being killed and evencannibalized.[46] The evidence of cannibalism was later reported in 2024 by Coppock and Currie. The evidence consisted of tooth traces found on the medial surface of a pubis attributed to an albertosaurine, which given the lack of other albertosaurine species in the assemblage almost certainly reflected anotherA. sarcophagus. These bite traces show no signs of healing and thus strongly indicate that they were made post-mortem and represent feeding marks as opposed to injuries sustained during intraspecific combat.[54]
Currie has also speculated on the pack-hunting habits ofAlbertosaurus. The leg proportions of the smaller individuals were comparable to those ofornithomimids, which were probably among the fastest dinosaurs. YoungerAlbertosaurus 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.[14] Juveniles may also have had different lifestyles than adults, filling predatorniches between the enormous adults and the smaller contemporaneous theropods, the largest of which were twoorders of magnitude smaller than adultAlbertosaurus in mass.[20] A similar situation is observed in modern Komodo dragons, with hatchlings beginning life as smallinsectivores before growing to become the dominant predators on their islands.[55] 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.[56]
In 2009, researchers hypothesized that smooth-edged holes found in the fossil jaws oftyrannosaurid dinosaurs, such asAlbertosaurus, were caused by a parasite similar toTrichomonas gallinae, which infects birds.[57] They suggested that tyrannosaurids transmitted the infection by biting each other and that the infection impaired their ability to eat.[57]
In 2001, Bruce Rothschild and others published a study examining evidence forstress fractures andtendon avulsions intheropod dinosaurs and the implications for their behavior. They found that only one of the 319Albertosaurus 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 inAlbertosaurus than in thecarnosaurAllosaurus.[58] ROM 807, the holotype ofA. arctunguis (now referred toA. sarcophagus), had a 2.5 by 3.5 cm (0.98 by 1.38 in) deep hole in theiliac blade, although the describer of the species did not recognize this as pathological. The specimen also contains someexostosis on the fourth left metatarsal. In 1970, two of the fiveAlbertosaurus sarcophagus specimens with humeri were reported by Dale Russel as having pathological damage to them.[59]
In 2010, the health of the Dry IslandAlbertosaurus 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-calledenthesophytes, 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 aMajungasaurus population of which it was established, in 2007, that 19% of individuals showed bone pathologies.[60]
Most fossils ofAlbertosaurus sarcophagus are known from the upperHorseshoe Canyon Formation in Alberta. These younger units of thisgeologic formation date to the earlyMaastrichtian age of theLate Cretaceous period, about 70 to 68 million years ago. Immediately below this formation is theBearpaw Shale, amarine formation representing a section of theWestern 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 aslagoons,estuaries, andtidal flats. Numerouscoal seams represent ancientpeatswamps. Like most of the othervertebrate fossils from the formation,Albertosaurus remains are found in deposits laid down in thedeltas andfloodplains of large rivers during the later half of Horseshoe Canyon times.[61]
Thefauna 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-likeAspidorhynchus made up the fish fauna. Mammals includedmultituberculates and themarsupialDidelphodon. The saltwaterplesiosaurLeurospondylus has been found in marine sediments in the Horseshoe Canyon, while freshwater environments were populated byturtles,Champsosaurus, andcrocodilians likeLeidyosuchus andStangerochampsa. Dinosaurs dominate the fauna, especially hadrosaurs, which make up half of all dinosaurs known. These include the generaEdmontosaurus,Saurolophus, andHypacrosaurus. Ceratopsians andornithomimids were also very common, together making up another third of the known fauna. Along with much rarerankylosaurians andpachycephalosaurs, all of these animals would have been prey for a diverse array of carnivorous theropods, includingtroodontids,dromaeosaurids, andcaenagnathids.[61][62] Intermingled with theAlbertosaurus remains of the Dry Island bonebed, the bones of the small theropodAlbertonykus were found.[63] AdultAlbertosaurus were theapex predators in their environment, with intermediate niches possibly filled by juvenileAlbertosaurus.[61]
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