Albertosaurus sarcophagus
Albertosaurus sarcophagus (gr., "Alberta meat-eating lizard") is the only known species of the extinct genus Albertosaurus a tyrannosaurid theropod dinosaur that lived in western North America in the late Cretaceous period, more than seventy million years ago, in the early Maastrichtian. The nomenclatural type A. sarcophagus was restricted to the range of what is now the Canadian province of Alberta, to which the generic name refers. There is some disagreement in the scientific community regarding the number of species represented in the genus, which depending on the case would be one or two.
As a tyrannosaurid, Albertosaurus were bipedal predators with a large head, jaws with dozens of large teeth, and small "hands" two fingers It may have been the top predator in the food chain of its local ecosystem. Although relatively large for a theropod, Albertosaurus was much smaller than its famous relative Tyrannosaurus, probably weighing as much as a modern black rhinoceros.
Fossils of more than thirty individuals have been discovered, allowing scientists a more detailed understanding of the anatomy of Albertosaurus than that of other tyrannosaurids. The finding of twenty-six individuals together at one site points to evidence of group behavior and allows studies of their developmental biology, impossible with lesser-known animals.
Description
Albertosaurus was smaller than the giant tyrannosaurids better known as Tarbosaurus and Tyrannosaurus. Adults reached approximately 9 meters in length, while some older individuals would have reached 10 meters. Several independent estimates of its mass, obtained by different methods, suggest that an Albertosaurus adult would have weighed between 1.3 and 1.7 tons.
All tyrannosaurids, including Albertosaurus, shared a similar body appearance typical for a theropod. Albertosaurus was bipedal, swinging its heavy head and trunk with a long tail. However, tyrannosaurid forelimbs were extremely small for their size, having only two fingers. The hind limbs were long and ended in a four-toed foot. The first of these toes was very small and located at the back, with only the other three resting on the ground, with the middle finger longer than the others. Albertosaurus may have been able to reach speeds of 11 to 13 meters per second, about 40 to 48 km/h. At least for the smaller, juvenile individuals, higher speeds would be possible.
Skull and Teeth
The enormous skull of Albertosaurus, supported by a short S shaped neck, was about 1 meter long in the largest adults. It had wide openings called temporal fenestrae, which reduced the weight of the head, and provided areas for the attachment of muscles and sensory organs. Above the eyes were small bony ridges that may have been brightly colored in life and used in courtship to attract a mate.
Its long jaws contained more than sixty slightly curved teeth, many compared to larger tyrannosaurids, which had fewer teeth. Unlike most theropods, tyrannosaurids had heterodont dentition, that is, the teeth had different shapes depending on their position in the mouth. The premaxillary teeth, at the end of the upper jaw, were much smaller than the rest, compact and with a D-shaped section.
As in Tyrannosaurus, the maxillary teeth of Albertosaurus were generally adapted to resist the lateral forces exerted by fighting prey. The force of Albertosaurus's bite was less formidable, however; the maximum force exerted by the posterior teeth was 3,413 newtons.
William Abler observed in 2001 that the serrations on Albertosaurus teeth are so fine that they may be functionally a crack in the tooth. However, at the base of this crack is a round void space called a blister that functioned to spread the forces over a greater surface area, making it difficult for the 'crack' to break. formed by the serrations propagating through the tooth. In an examination of other ancient predators, such as a phytosaur and Dimetrodon, similar fractures have been found as striations, but no adaptations for the prevention of crack propagation. Tyrannosaurid teeth were used as clamping discs to pull meat from a body, rather than as knives that cut. The wear patterns of the teeth hint that head-shaking behavior may have been involved in tyrannosaurid feeding. When a tyrannosaurid removed a piece of meat, the force tended to push the tip of the tooth toward the front of the mouth, and the root attachment experienced tension on the posterior side and compression on the front. This would normally cause the tooth to crack posteriorly to the side, but blisters at the base of the grooves tend to disperse the crack-forming forces. This shape resembles the techniques used by guitar luthiers of "alternating regions of flexibility and stiffness to a stick of wood." Using a drill bit to create a "blister" and preventing the propagation of cracks through a material is also used to protect aircraft surfaces. Abler demonstrated that a plexiglass rod with notches and drilled holes was 25% stronger than one with only regularly placed incisions.
Discovery and research
The first Albertosaurus was found by Joseph Burr Tyrrell during geological surveys in 1884. Albertosaurus was named in 1905 by Henry Fairfield Osborn of the American Museum of Natural History, in a very brief note at the end of his description of Tyrannosaurus rex. The name honors Alberta, the Canadian province in which the first remains were found. The generic name also incorporates the Greek term σαύρος, sauros, meaning "lizard", the most common suffix in dinosaur names.
The nomenclatural type of Albertosaurus is A. sarcophagus, also named by Osborn in 1905. The name means "flesh-eater" and has the same etymology as the sarcophagus funerary container, with which it shares its name: a combination of the ancient Greek words σάρξ, sarx, meaning " meat" and φάγειν, fagein, meaning "to eat". More than thirty specimens of various ages are known.
First discoveries
The type specimen is a partial skull, collected in 1884 from an outcrop along the Red Deer River in Alberta. This specimen, found on June 9, 1884, was recovered by a Geological Survey of Canada expedition led by famed geologist Joseph Burr Tyrrell. Due to the lack of specialized equipment the almost complete skull could only be partially secured. In 1889, Tyrell's colleague Thomas Chesmer Weston found an additional smaller incomplete skull and some other skeletal material at a nearby location. This specimen is now kept at the Canadian Museum of Nature. The two skulls were assigned to the species Laelaps incrassatus by Edward Drinker Cope in 1892. However, by 1877 the name Laelaps had already been assigned to a genus of mites, so it was replaced and renamed Dryptosaurus by Cope's rival, Othniel Charles Marsh, although Cope refused to accept the new name. Lawrence Lambe moved Laelaps incrassatus to the genus Dryptosaurus, where he described the remains in detail between 1903 and 1904, a combination first coined by Oliver Perry Hay in 1902. Finally, since the D. incrassatus was based solely on generic tyrannosaurid teeth that could not easily indicate their assignment to a particular species. The Horseshoe Canyon skulls differ markedly from the remains of Dryptosaurus aquilunguis, the nomenclatural type of Dryptosaurus, so Osborn created the new name Albertosaurus sarcophagus for them in 1905. He did not describe the remains in great detail, citing Lambe's full description the previous year. Both specimens, the holotype CMN 5600 and the paratype 'CMN 5601 , are stored at the Canadian Museum of Nature in Ottawa. At the beginning of the 21st century there has been some concern regarding the poor condition of the holotype, which could make Albertosaurus a nomen dubium, a "dubious name" it could only be used for the type specimen itself, because other fossils could not be reliably assigned to it. However, in 2010 Thomas Carr established that the holotype, the paratype and the later comparable specimens found shared a unique common feature or autapomorphy, the possession of a wide pneumatic opening on the posterior edge of one side of the palatine bone, demonstrating that Albertosaurus was a valid taxon.
Dry Island's Bed of Bones
In 1910, American paleontologist Barnum Brown unearthed the remains of a large group of Albertosaurus in another quarry along the Red Deer River. Due to the large number of bones and the limited time available, Brown's team did not manage to collect every specimen, but they made sure to collect the bones of all the individuals present. Among many other bones deposited in the collections of the American Museum of Natural History in New York, noteworthy are seven sets of right metatarsals, along with two isolated toe bones that do not match any of the metatarsals in size. This indicates the presence of at least nine individuals in the quarry. The Royal Tyrrell Museum of Palaeontology rediscovered the site in 1997 and resumed field work, at the site now located in Dry Island Buffalo Jump Provincial Park. Subsequent excavations from 1997 to 2005 yielded the remains of thirteen more individuals than many ages, from a small two-year-old to an old specimen that is estimated to be more than ten meters in length. None of these individuals are known from complete skeletons, and most are represented by remains in both museums. Excavations continued until 2008, when the minimum number of individuals present had been established at twelve, based on the elements preserved that appear only once in a skeleton, and in twenty-six if remains that differ in size due to ontogeny are counted. A total of 1,128 Albertosaurus bones have been recovered, the largest concentration of large theropod fossils known from the Cretaceous.
Other discoveries
A specimen originally named A. arctunguis was also found near the Red Deer River, based on a partial skeleton with the skull missing, excavated by Gus Lindblad and Ralph Hornell in 1923, and is currently on deposit at the Royal Ontario Museum in Toronto, Canada. But this species is considered identical to A. sarcophagus since 1970. Between 1926 and 1972, no Albertosaurus fossils were found, but since the 1970s there has been a steady increase in known material. Six more skulls and skeletons have since been found in Alberta off Dry Island and deposited in other Canadian museums. These specimens are RTMP 81.010.001, found in 1978 by amateur paleontologist Maurice Stefanuk; RTMP 85,098,001, found by Stefanuk on June 16, 1985; RTMP 86.64.001, December 1985; RTMP 86.205.001 in 1986; RTMP 97.058.0001 in 1996, and CMN 11315. Due to vandalism and accidents, however, no complete, undamaged skulls were found among these finds. Various fossils have been reported from US states such as Montana, New Mexico, and Wyoming, but probably do not represent A. sarcophagus and may not even belong to the genus Albertosaurus.
All identifiable fossils of Albertosaurus sarcophagus are known from the Horseshoe Canyon Formation in Alberta. This formation dates from the early Maastrichtian epoch of the Late Cretaceous, some 70 to 73 million years ago. Many other dinosaurs have been found there, including smaller theropods such as Ornithomimus, Chirostenotes, and various dromaeosaurids, and a wide variety of herbivores such as ankylosaurs, ceratopsians, pachycephalosaurs, and hadrosaurids.
In 1913, paleontologist Charles Hazelius Sternberg discovered another tyrannosaurid skeleton in slightly older sediments in Alberta. This dinosaur was named Gorgosaurus libratus in 1914 by Lawrence Lambe. Having found a few differences to separate the two genera, Dale Russell declared Gorgosaurus a synonym of Albertosaurus in 1970, creating the new combination Albertosaurus libratus. This fact led to the expansion of the temporal range several million years ago, and the geographic range hundreds of km to the south, of this genus.
More recent examination of Albertosaurus and Gorgosaurus has cast doubt on Russell's proposed synonymy. In 2003, Phil Currie and his colleagues examined the skulls of the two species and concluded that both genera should be maintained, although they acknowledged that the two genera are sister taxa and that the distinction is therefore highly arbitrary. However, according to Currie, Albertosaurus and Gorgosaurus are no more similar than Daspletosaurus and Tyrannosaurus are. >, almost always considered independent genera. In addition, several undescribed specimens of albertosaurids have been found in other parts of North America, including Alaska and New Mexico, so Currie has recommended keeping these two genera separate until more information is obtained. Many authors since then have followed Currie's recommendation, but others have not.
Invalid species
Some other species of Albertosaurus have been named but have subsequently been found to be invalid. William Parks described a partial skeleton found in Alberta as Albertosaurus arctunguis in 1928, but it is now considered a synonym of A. sarcophagus. The Parks specimen, ROM 807, is held at the Royal Ontario Museum in Toronto.
Other than A. sarcophagus, A. arctunguis and A. libratus, several other species of Albertosaurus have been named. All of them are now seen as more modern synonyms of other species or as nomina dubia, and are not assigned to Albertosaurus. In 1930, Anatoly Nikolaevich Riabinin named it Albertosaurus pericolosus based on a tooth from China, probably belonging to Tarbosaurus. Friedrich von Huene renamed it Dryptosaurus incrassatus, not considered a nomen dubium by him, as Albertosaurus incrassatus in 1932. Due to having identified Gorgosaurus with Albertosaurus, Russell, in 1970, also renamed Gorgosaurus sternbergi Matthew & Brown 1922 as Albertosaurus sternbergi and Gorgosaurus lancensis Gilmore 1946 as Albertosaurus lancensis. The former species is now seen as a juvenile form of Gorgosaurus libratus and the latter is identified as Tyrannosaurus or representing the separate genus known as Nanotyrannus.
The species Albertosaurus megagracilis named in 1988 by Gregory S. Paul based on a skeleton of a small tyrannosaurid, specimen LACM 28345, from the Hell Creek Formation of Montana It was renamed Dinotyrannus in 1995, but is now believed to have been a juvenile of Tyrannosaurus. Also in 1988, Paul renamed it Alectrosaurus olseni Gilmore 1933 as Albertosaurus olseni, but this was not accepted by most. In 1989, Gorgosaurus novojilovi Maleev 1955 was renamed by Bryn Mader and Robert Bradley as Albertosaurus novojilovi, although today it is considered a synonym of Tarbosaurus
On two occasions, species based on valid Albertosaurus material were reassigned to a different genus. In 1922 William Diller Matthew renamed A. sarcophagus as Deinodon sarcophagus and in 1939 the German paleontologist Oskar Kuhn renamed A. arctunguis as Deinodon arctunguis.
Classification
Albertosaurus is a member of the theropod family Tyrannosauridae. Within this family, Albertosaurus sarcophagus is usually classified with the slightly older Gorgosaurus libratus, sometimes called Albertosaurus libratus, in the subfamily Albertosaurinae. These two species are the only Albertosaurines described, although other undescribed species may exist. Appalachiosaurus has been placed as an Albertosaurine in at least one study by Thomas Holtz in 2004, although this is disputed.. In more recent unpublished work, Holtz locates it just outside the Tyrannosauridae, in agreement with other authors. Albertosaurs were more slender than stocky tyrannosaurids, such as Tarbosaurus, Tyrannosaurus and Daspletosaurus, with proportionally smaller skulls and longer leg, tibia and foot bones, metatarsals and phalanges.
Phylogeny
Below is the cladogram of the Tyrannosauridae based on phylogenetic analysis carried out by Loewen et al. in 2013.
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Paleobiology
Like many other tyrannosaurids, Albertosaurus has been extensively studied across many different specimens, allowing for detailed studies of its life and providing evidence of its behavior.
Growth Patterns
Almost all ages of Albertosaurus are represented in the fossil record. By means of histological studies of the bones, the age of an individual at the time of death can almost always be determined, thus allowing growth rates to be estimated and compared with those of other species. A two-year-old Albertosaurus fossil, the youngest found so far, has recently been found in the reopened Dry Island bone bed, which must have weighed about fifty kilograms and measure two meters long. Another specimen from the same quarry is the oldest, around twenty-eight years of age and about thirty feet long. A twenty-four-year-old specimen from the collection of the Tyrrell Museum of Paleontology would have weighed approximately 1.14 tons. However, another 1.28-ton individual, in the collection of the American Museum of Natural History, was twenty-two years old when he died. When specimens of intermediate age and size are studied, plotting their growth curve, we obtain an S shaped graph, where the highest growth rate occurs over a period of four years, ending around age sixteen, as seen in other tyrannosaurids. The growth rate during this phase is estimated at an increase of 122 kg per year. Other tyrannosaurids of similar size show a similar growth rate, but the rate is much slower than that of Tyrannosaurus, which grew almost six times faster, at about 601 kg per year, during this phase of growth. growth.
Albertosaurus appears to reach skeletal maturity at sixteen years of age; it is marked by the end of the rapid growth phase, which apparently was also the age at which it reached its sexual maturity. However, growth continued at a slow rate throughout the animals' lives. Reaching sexual maturity when growth is still active appears to be a shared characteristic between small and large dinosaurs, as well as large mammals such as such as humans and elephants. This pattern of relatively early sexual maturation differs strikingly from the pattern in birds, which delay sexual maturity until after they have finished growing.
During growth, the thickening in the morphology of the teeth causes them to change so much that, had there not been an association of young and adult skeletons in the Dry Island bone bed that would prove their membership in the same taxon, it is It is likely that the teeth of the juveniles would have been identified by statistical analysis as those of a different species.
Developmental Biology
Most known Albertosaurus were around fourteen years of age or older at the time of death. Juvenile animals are rarely found fossilized, mainly because of a case of biased sampling, where the smaller bones of younger animals are less likely to be preserved by fossilization than the larger bones of adults, and that small bones are more difficult for collectors to notice in the field. Even so, young Albertosaurus are relatively large for juvenile animals, but their remains are rare in the fossil record compared to adults. It has been suggested that this phenomenon is a consequence of developmental biology and that fossils of young Albertosaurus are rare because not as many died as adults.
One hypothesis postulates that after mass hatching small albertosaurs died in large numbers, but it has not been preserved in the fossil record due to its small size and fragility. After just two years, the young were larger than any other predator in the region apart from adult Albertosaurus and faster than most of their prey. This resulted in a dramatic decrease in their mortality rate corresponding to the rarity of the fossils. Mortality rates double by age twelve, perhaps as a result of the physiological demands of the rapid growth phase, and then double again with the onset of sexual maturity between the ages of fourteen and sixteen.. This high mortality rate continues through adulthood, perhaps due to the high physiological demands, stress, and injuries received during intraspecific competition for mates and resources, and ultimately the increasing effects of senescence. The higher mortality rate in adults may explain its more common preservation. Very large animals were rare because few individuals survived long enough to achieve such sizes. High infant mortality rates, followed by reduced mortality among the young and a sudden increase in mortality after sexual maturity, with very few animals reaching maximum size, is a pattern observed in many large modern mammals, including elephants, African buffalo, and rhinoceros. The same pattern is also seen in other tyrannosaurids. In conclusion and comparing with modern animals and what is known of other tyrannosaurids, it seems to support this previously detailed developmental biology hypothesis, but bias in the fossil record may still play an important role, especially since more than two-thirds of all Albertosaur specimens are known from one location.
Behavior in packs
The Albertosaurus bone layer discovered by Barnum Brown and his team contains the remains of between ten and twenty-six individuals; the largest number of individuals found in one location of any large Cretaceous theropod, and the second largest number of any large theropod dinosaur behind Allosaurus found at the Cleveland-Lloyd Dinosaur Quarry, in the Utah state. The group was made up of a single older adult, eight adults between seventeen and twenty-three years old, seven sub-adults that were in the rapid growth phase between twelve and sixteen years old, and six juveniles between two and eleven years old, which they had not yet entered this phase.
The absence of herbivore remains in the immediate vicinity and the similar state of preservation among the many individuals in the Albertosaurus Bone Layer Quarry led Currie to the conclusion that the locality was not a "predator trap" such as Rancho La Brea in California, and that all the animals found died at the same time, providing evidence of herd behavior. Other scientists are skeptical, pointing out that the animals may have grouped together due to flooding or other causes.
There is extensive evidence of gregarious behavior among herbivorous dinosaurs, including ceratopsians and hadrosaurs. In contrast, multiple predatory dinosaurs have only rarely been found in a single site. Small theropods such as Coelophysis bauri, Deinonychus antirrhopus and Megapnosaurus rhodesiensis have been found in groups, when there are larger predators such as Allosaurus fragilis or Mapusaurus roseae. There is also some evidence of gregarious behavior in other tyrannosaurids. Fragmentary remains of smaller individuals were found alongside Sue, a mounted Tyrannosaurus rex specimen at the Field Museum of Natural History in Chicago. A layer of bones in the Two Medicine Formation of Montana contained at least three specimens of an unnamed species of Daspletosaurus, preserved alongside several hadrosaurs. These finds may corroborate evidence for social behavior in Albertosaurus, although some or all of the aforementioned locations may represent temporary or unnatural aggregations. Others have speculated that rather than social groups, at least some of these finds represent a case of feeding frenzy similar to those of the Komodo dragons on carcasses, where the carnivores compete aggressively, some of which may die in the fights, on which the rest practice cannibalism.
Currie also speculated about the group hunting habits of Albertosaurus. The lower limb proportions of the smaller individuals are comparable to those of ornithomimids, which were probably among the fastest dinosaurs. The younger Albertosaurus was probably as fast as its prey, which could include ceratopsians and hadrosaurs. Currie suggested that the younger members of the pack might be responsible for driving the prey towards the adults, which were larger and stronger, but also slower. The young must have had a different way of life than the adults., occupying the space as a predator between the huge adults and the smaller contemporary theropods; the largest of them was half the mass of Albertosaurus. A similar situation is observed in modern Komodo dragons, with the young beginning life as small insectivores later becoming the dominant predators in their islands. However, as preservation of behavior in the fossil record is extremely rare, this idea cannot be easily tested. In 2010, Currie, while still supporting the hunting pack hypothesis, conceded that the concentration could have been caused by other causes, such as a slowly rising water level during a prolonged flood.
Paleopathology
In 2009, researchers hypothesized that the smooth-edged holes found in the fossil jaws of tyrannosaurids such as Albertosaurus were caused by a parasite similar to Trichomonas gallinae i> which infects birds. They suggested that tyrannosaurids transmitted the infection by biting each other and that the infection impaired their ability to eat.
In 2001, Bruce Rothschild et al. published a study on the evidence for stress fractures and tendon avulsion in theropod dinosaurs and their implications for development. They found that only one of the 319 Albertosaurus foot bones studied had stress fractures and none of the hand bones. The scientists found that stress caused significantly fewer injuries in Albertosaurus than in the carnosaur Allosaurus. ROM 807, the holotype of A. arctunguis, now considered A. sarcophagus, has a 2.5 by 3.5 centimeter deep hole in the ilium, although the describer of the fossils did not consider this pathological. The specimen also has some exostosis on the left fourth metatarsal. Two of the five Albertosaurus sarcophagus specimens with a humerus in 1970 were reported by Dale Russel to have some pathological damage to them.
In 2010, a report was made on the health of Albertosaurus on Dry Island. Most of the samples did not show any signs of disease. In three phalanges of the feet, strange bone spurs appeared, consisting of abnormal ossification of the tendons, the so-called enthesophytes were present; its cause unknown. Two ribs and a belly segment showed signs of fracture and healing. One adult specimen had a puncture wound and two healed and unhealed bite marks on the lower left jaw. The low number of anomalies compares favorably with the health status of the Majungasaurus population, in which in 2007 it was established that 19% of individuals had bone pathologies.
Paleoecology
All fossils identifiable as A. sarcophagus are native to the Horseshoe Canyon Formation in Alberta. This geological formation dated from the early Maastrichtian to the Late Cretaceous period, approximately 73 to 70 million years ago. Immediately below this formation is the Bearpaw Shale, a formation of marine origin that represents a section of the Western Interior Seaway. The seaway receded as the average temperature for the year receded into the late Cretaceous and sea levels dropped, exposing land that had previously been underwater. It was not an even process, and the seaway would periodically ingress to cover parts of the region through Horseshoe Canyon before fully retreating afterwards. Due to changes in sea levels, many different environments are represented in the Horseshoe Canyon Formation, including offshore and nearshore marine habitats and habitats such as lagoons, estuaries, and tidal zones. Numerous coal seams represent ancient peat swamps. Like most other vertebrate fossils from the formation, the remains of Albertosaurus are found in deposits established in the deltas and floodplains of large rivers during the second half of the period covered by the geological formation.
The fauna of the Horseshoe Canyon Formation is well known: it contains fossil vertebrates, commonly including dinosaurs. Sharks, rays, sturgeons, Amiiformes, Lepisosteiformes and the simile Lepisosteiformes Aspidorhynchus forming part of the fish fauna. Mammals include multituberculates and the marsupial Didelphodon. The marine plesiosaur Leurospondylus has been found in rocks of the Horseshoe Canyon Formation, with freshwater environments abounding in turtles, Champsosaurus and crocodiles such as Leidyosuchus and Stangerochampsa. Dinosaurs dominated the fauna, especially hadrosaurids, which make up half of all known dinosaurs, including the genera Edmontosaurus, Saurolophus, and Hypacrosaurus. Ceratopsians or ornithomimids were also common, making up another third of the fauna. Along with the much rarer ankylosaurids and pachycephalosaurids, all of these animals would have been prey for a diverse variety of carnivorous theropods, including troodontids, dromaeosaurids, and cenagnathids. Mixed with the remains of Albertosaurus from the bed of bones from Dry Island, the bones of the small theropod Albertonykus were found. Adults Albertosaurus are the main predators in this environment, with juveniles possibly occupying intermediate niches.