Albertosaurinae
Albertosaurines, or dinosaurs of the subfamily Albertosaurinae, lived in the Late Cretaceous of United States and Canada. The subfamily was first used by Philip J. Currie, Jørn H. Hurum and Karol Sabath as a group of tyrannosaurid dinosaurs. It was originally defined as "("Albertosaurus" + "Gorgosaurus")", including only the two genera. The group is sister clade to Tyrannosaurinae. In 2007, it was found that the group also contained "Maleevosaurus", often synonymized with "Tarbosaurus". However, this classification has not been accepted, and "Maleevosaurus" is still considered a juvenile "Tarbosaurus" or "Tyrannosaurus". Description. Albertosaurines are large, lightly built tyrannosaurids. Compared to  tyrannosaurines, they are lightly built, have shorter, flatter skulls, had shorter ilia, and had proportionally longer tibiae. Albertosaurines and tyrannosaurines share arms or about equal length, with the exception of "Tarbosaurus", which had short arms for its size. "Albertosaurus" was smaller than some other tyrannosaurids, such as "Tarbosaurus" and "Tyrannosaurus". Typical adults of "Albertosaurus" and "Gorgosaurus" measured up to long, while rare individuals of "Albertosaurus" could grow to over in length. Several independent mass estimates, obtained by different methods, suggest that an adult "Albertosaurus" weighed between and . "Gorgosaurus" estimates are higher, around , although greater estimates exist of about . All tyrannosaurids , including "Albertosaurus", shared a similar body appearance. Typically for a theropod, "Albertosaurus" was bipedal and balanced the heavy head and torso with a long tail. However, tyrannosaurid forelimbs were extremely small for their body size and retained only two digits. The hind limbs were long and ended in a four-toed foot. The first digit, called the hallux, was short and only the other three contacted the ground, with the third (middle) digit longer than the rest. "Albertosaurus" may have been able to reach walking speeds of 14−21 kilometres per hour (8−13 miles per hour). At least for the younger  individuals, a high running speed is plausible. Classification and systematics. The subfamily was first used by Philip J. Currie, Jørn H. Hurum and Karol Sabath as a group of tyrannosaurid dinosaurs. It was originally defined as "("Albertosaurus" + "Gorgosaurus")", including only the two genera. The group is sister clade to Tyrannosaurinae. In 2007, it was found that the group also contained "Maleevosaurus", often synonymized with "Tarbosaurus". However, this classification has not been accepted, and "Maleevosaurus" is still considered a juvenile "Tarbosaurus" or "Tyrannosaurus". This classification has not been accepted. "Albertosaurus" is a member of the theropod family Tyrannosauridae, in the  subfamily Albertosaurinae. Its closest relative is the slightly older "Gorgosaurus libratus" (sometimes called "Albertosaurus libratus"; see below). These two species are the only described albertosaurines; other undescribed species may exist. Thomas Holtz found "Appalachiosaurus" to be an albertosaurine in 2004, but his more recent unpublished work locates it just outside Tyrannosauridae, in agreement with other authors. The other major subfamily of tyrannosaurids is the Tyrannosauridae, including genera such as "Daspletosaurus", "Tarbosaurus" and "Tyrannosaurus". Relationship of "Gorgosaurus" and "Albertosaurus". The close similarities between "Gorgosaurus libratus" and "Albertosaurus sarcophagus" have led many experts to combine them into one genus over the years . "Albertosaurus" was named first, so by convention it is given priority over the name "Gorgosaurus", which is sometimes considered its junior synonym. William Diller Matthew and Barnum Brown doubted the distinction of the two genera as early as 1922. "Gorgosaurus libratus" was formally reassigned to "Albertosaurus" (as "Albertosaurus libratus") by Dale Russell in 1970, and many subsequent authors followed his lead. Combining the two greatly expands the geographical and chronological range of the genus "Albertosaurus". Other experts maintain the two genera as separate. Canadian paleontologist Phil Currie claims there are as many anatomical differences between "Albertosaurus" and "Gorgosaurus" as there  are between "Daspletosaurus" and "Tyrannosaurus", which are almost always kept separate. He also notes that undescribed tyrannosaurids discovered in Alaska, New Mexico and elsewhere in North America may help clarify the situation. Gregory S. Paul has suggested that "Gorgosaurus libratus" is ancestral to "Albertosaurus sarcophagus". Phylogeny. Albertosaurinae is a basal subfamily of tyrannosaurid. They were recognized in the 2014 analysis of the new genus "Nanuqsaurus", a derived tyrannosaurine, the sister group to Albertosaurinae. Albertosaurinae was recovered as including only "Albertosaurus" and "Gorgosaurus". "Albertosaurus" and "Gorgosaurus" are kept separate by most classifications, as should be according to Currie. The cladogram below  was found during the analysis of "Nanuqsaurus" by Anthony Fiorillo and Ronald Tykoski. Paleobiology. Coexistence of "Gorgosaurus" and "Daspletosaurus". In the middle stages of the Dinosaur Park Formation, "Gorgosaurus" lived alongside a rarer species of the tyrannosaurine "Daspletosaurus". This is one of the few examples of two tyrannosaurid genera coexisting. Similar-sized predators in modern predator guilds are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well understood. In 1970, Dale Russell hypothesized the more common "Gorgosaurus" actively hunted fleet-footed hadrosaurs, while the rarer  and more troublesome ceratopsians and ankylosaurians (horned and heavily armoured dinosaurs) were left to the more heavy built "Daspletosaurus". However, a specimen of "Daspletosaurus" (OTM 200) from the contemporaneous Two Medicine Formation of Montana preserves the digested remains of a juvenile hadrosaur in its gut region, and another bonebed contains the remains of three "Daspletosaurus" along with the remains of at least five hadrosaurs. Unlike some other groups of dinosaurs, neither genus was more common at higher or lower elevations than the other. However, "Gorgosaurus" appears more common in northern formations like Dinosaur Park, with species of "Daspletosaurus" more abundant  to the south. The same pattern is seen in other groups of dinosaurs. Chasmosaurine ceratopsians and hadrosaurine hadrosaurs are also more common in the Two Medicine Formation of Montana and in southwestern North America during the Campanian, while centrosaurine and lambeosaurines dominate in northern latitudes. Holtz has suggested this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and hadrosaurines. At the end of the later Maastrichtian stage, tyrannosaurines like "Tyrannosaurus rex", hadrosaurines like "Edmontosaurus" and chasmosaurines like "Triceratops" were widespread throughout western North America, while lambeosaurines were rare and albertosaurines and centrosaurines had gone extinct. Growth. Gregory Erickson and colleagues  have studied the growth and life history of tyrannosaurids using bone histology, which can determine the age of a specimen when it died. A growth curve can be developed when the ages of various individuals are plotted against their sizes on a graph. Tyrannosaurids grew throughout their lives, but underwent tremendous growth spurts for about four years, after an extended juvenile phase. Sexual maturity may have ended this rapid growth phase, after which growth slowed down considerably in adult animals. Examining five "Gorgosaurus" specimens of various sizes, Erickson calculated a maximum growth rate of about per year during the rapid  growth phase, slower than in tyrannosaurines like "Daspletosaurus" and "Tyrannosaurus", but comparable to "Albertosaurus". "Gorgosaurus" spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years. This, along with the complete lack of predators intermediate in size between huge adult tyrannosaurids and other small theropods, suggests these niches may have been filled by juvenile tyrannosaurids. This pattern is seen in modern Komodo dragons, whose hatchlings start off as tree-dwelling insectivores and slowly mature into massive apex predators capable of taking down large vertebrates. During growth, through thickening  the tooth morphology changed so much that, had the association of young and adult skeletons on the Dry Island bonebed not proven they belonged to the same taxon, the teeth of juveniles would likely have been identified by statistical analysis as those of a different species. Life history. Most known "Albertosaurus" individuals were aged 14 years or more at the time of death. However, the youngest individual known is a juvenile two-year-old, and the oldest is an adult 28-year-old. The two-year-old was discovered in the Dry Island bonebed, weighed an estimated , and was about  in length. The 28-year-old specimen from the same quarry measured long. Juvenile animals are rarely found as fossils 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. Young "Albertosaurus" are relatively large for juvenile animals, but their remains are still rare in the fossil record compared with 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. A hypothesis of "Albertosaurus" life history postulates that hatchlings died in large numbers, but have not been preserved in the fossil record due to 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 of foot 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 eventually, the 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 sizes. 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 one locality. "Albertosaurus" and "Gorgosaurus" individuals both grow the most rapidly during a four-year period  until they are approximately 16. Pack behaviour. 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, who had not yet reached the growth phase. 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 like 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 behaviour. Other scientists are skeptical, observing that the animals may have been driven together by drought, flood or for other reasons. There is plentiful evidence for gregarious behaviour among  herbivorous dinosaurs, including ceratopsians and hadrosaurs. However, only rarely are so many dinosaurian predators found at the same site. Small theropods like "Deinonychus", "Coelophysis" and "Megapnosaurus" ("Syntarsus rhodesiensis") have been found in aggregations, as have larger predators like "Allosaurus" and "Mapusaurus". There is some evidence of gregarious behaviour in other tyrannosaurids as well. 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. 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. 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 cannibalized. Even though "Albertosaurus" are commonly found in packs, there is no evidence of group behavior in the close relative "Gorgosaurus".