Albertosaurinae
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Albertosaurines, or dinosaurs of the subfamily Albertosaurinae, lived in the
Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...
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 Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to thirteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...
dinosaurs. It was originally defined as "(''
Albertosaurus ''Albertosaurus'' (; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 71 million years ago. The type species, ''A. sarcophagus'', wa ...
'' + ''
Gorgosaurus ''Gorgosaurus'' ( ; ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (Campanian), between about 76.6 and 75.1 million years ago. Fossil remains have been found in the Ca ...
'')", including only the two genera. The group is sister clade to
Tyrannosaurinae The tyrannosaurines (Tyrannosaurinae) is an extinct subfamily of Tyrannosauridae a family of coelurosaurian theropod dinosaurs that consists of at least three tribes and several genera. All fossils of these genera have been found in the Late Cret ...
. In 2007, it was found that the group also contained '' Maleevosaurus'', often synonymized with ''
Tarbosaurus ''Tarbosaurus'' ( ; meaning "alarming lizard") is a genus of tyrannosaurid dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous Period, considered to contain a single known species, ''Tarbosaurus ba ...
''. However, this classification has not been accepted, and '' Maleevosaurus'' is still considered a juvenile ''
Tarbosaurus ''Tarbosaurus'' ( ; meaning "alarming lizard") is a genus of tyrannosaurid dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous Period, considered to contain a single known species, ''Tarbosaurus ba ...
'' or ''
Tyrannosaurus ''Tyrannosaurus'' is a genus of large theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosaurus'' live ...
''.


Description

Albertosaurines are large, lightly built
tyrannosaurids Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family (biology), family of coelurosaurian Theropoda, theropod dinosaurs that comprises two subfamilies containing up to thirteen genus, genera, including the eponymous ''Tyrannos ...
. Compared to tyrannosaurines, they are lightly built, have shorter, flatter skulls, had shorter ilia, and had proportionally longer
tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
e. 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 ''Tarbosaurus'' ( ; meaning "alarming lizard") is a genus of tyrannosaurid dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous Period, considered to contain a single known species, ''Tarbosaurus ba ...
'' 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 ''Albertosaurus'' (; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 71 million years ago. The type species, ''A. sarcophagus'', wa ...
'' 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 The torso or trunk is an anatomical term for the central part, or the core, of the body of many animals (including humans), from which the head, neck, limbs, tail and other appendages extend. The tetrapod torso — including that of a huma ...
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 Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being '' digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being '' pl ...
, 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 Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to thirteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...
dinosaurs. It was originally defined as "(''
Albertosaurus ''Albertosaurus'' (; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 71 million years ago. The type species, ''A. sarcophagus'', wa ...
'' + ''
Gorgosaurus ''Gorgosaurus'' ( ; ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (Campanian), between about 76.6 and 75.1 million years ago. Fossil remains have been found in the Ca ...
'')", including only the two genera. The group is sister clade to
Tyrannosaurinae The tyrannosaurines (Tyrannosaurinae) is an extinct subfamily of Tyrannosauridae a family of coelurosaurian theropod dinosaurs that consists of at least three tribes and several genera. All fossils of these genera have been found in the Late Cret ...
. In 2007, it was found that the group also contained '' Maleevosaurus'', often synonymized with ''
Tarbosaurus ''Tarbosaurus'' ( ; meaning "alarming lizard") is a genus of tyrannosaurid dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous Period, considered to contain a single known species, ''Tarbosaurus ba ...
''. 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 Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
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 Thomas Richard Holtz Jr. (born September 13, 1965) is an American vertebrate palaeontologist, author, and principal lecturer at the University of Maryland's Department of Geology. He has published extensively on the phylogeny, morphology, ecomorp ...
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 Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to thirteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...
, including genera such as ''
Daspletosaurus ''Daspletosaurus'' ( ; meaning "frightful lizard") is a genus of tyrannosaurid dinosaur that lived in Laramidia between about 79.5 and 74 million years ago, during the Late Cretaceous Period. The genus ''Daspletosaurus'' contains three species ...
'', ''
Tarbosaurus ''Tarbosaurus'' ( ; meaning "alarming lizard") is a genus of tyrannosaurid dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous Period, considered to contain a single known species, ''Tarbosaurus ba ...
'' and ''
Tyrannosaurus ''Tyrannosaurus'' is a genus of large theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosaurus'' live ...
''.


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 William Diller Matthew FRS (February 19, 1871 – September 24, 1930) was a vertebrate paleontologist who worked primarily on mammal fossils, although he also published a few early papers on mineralogy, petrological geology, one on botany, one on ...
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 Dale Alan Russell (27 December 1937 – 21 December 2019) was an American-Canadian geologist and palaeontologist. Throughout his career Russell worked as the Curator of Fossil Vertebrates at the Canadian Museum of Nature, Research Professor a ...
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 Philip John Currie (born March 13, 1949) is a Canadian palaeontologist and museum curator who helped found the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta and is now a professor at the University of Alberta in Edmonton. In the ...
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 Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...
,
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...
and elsewhere in North America may help clarify the situation.
Gregory S. Paul Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dino ...
has suggested that ''Gorgosaurus libratus'' is ancestral to ''Albertosaurus sarcophagus''.


Phylogeny

Albertosaurinae is a basal subfamily of
tyrannosaurid Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family of coelurosaurian theropod dinosaurs that comprises two subfamilies containing up to thirteen genera, including the eponymous ''Tyrannosaurus''. The exact number of genera ...
. They were recognized in the 2014 analysis of the new genus '' Nanuqsaurus'', a derived
tyrannosaurine The tyrannosaurines (Tyrannosaurinae) is an extinct subfamily of Tyrannosauridae a family of coelurosaurian theropod dinosaurs that consists of at least three tribes and several genera. All fossils of these genera have been found in the Late Cret ...
, 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 Anthony Ricardo Fiorillo is a senior fellow at the Institute for the Study of Earth and Man at Southern Methodist University, but for many years he was vice president of research & collections and chief curator at the Perot Museum of Nature & S ...
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 niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
s by anatomical, behavioral or geographical differences that limit competition.
Niche differentiation In ecology, niche differentiation (also known as niche segregation, niche separation and niche partitioning) refers to the process by which competing species use the environment differently in a way that helps them to coexist. The competitive excl ...
between the Dinosaur Park tyrannosaurids is not well understood. In 1970, Dale Russell hypothesized the more common ''Gorgosaurus'' actively hunted fleet-footed
hadrosaurs Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which incl ...
, while the rarer and more troublesome ceratopsians and
ankylosauria Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. ...
ns (horned and heavily
armoured Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or f ...
dinosaurs) were left to the more heavy built ''
Daspletosaurus ''Daspletosaurus'' ( ; meaning "frightful lizard") is a genus of tyrannosaurid dinosaur that lived in Laramidia between about 79.5 and 74 million years ago, during the Late Cretaceous Period. The genus ''Daspletosaurus'' contains three species ...
''. However, a specimen of ''Daspletosaurus'' ( OTM 200) from the contemporaneous
Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between and (million years ago), during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky M ...
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 ''Daspletosaurus'' ( ; meaning "frightful lizard") is a genus of tyrannosaurid dinosaur that lived in Laramidia between about 79.5 and 74 million years ago, during the Late Cretaceous Period. The genus ''Daspletosaurus'' contains three species ...
'' 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 Chasmosaurinae is a subfamily of ceratopsid dinosaurs. They were one of the most successful groups of herbivores of their time. Chasmosaurines appeared in the early Campanian, and became extinct, along with all other non- avian dinosaurs, durin ...
and hadrosaurines. At the end of the later Maastrichtian stage, tyrannosaurines like ''
Tyrannosaurus rex ''Tyrannosaurus'' is a genus of large theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosaurus'' live ...
'', hadrosaurines like '' Edmontosaurus'' and
chasmosaurines Chasmosaurinae is a subfamily of ceratopsid dinosaurs. They were one of the most successful groups of herbivores of their time. Chasmosaurines appeared in the early Campanian, and became extinct, along with all other non- avian dinosaurs, durin ...
like ''
Triceratops ''Triceratops'' ( ; ) is a genus of herbivorous chasmosaurine ceratopsid dinosaur that first appeared during the late Maastrichtian stage of the Late Cretaceous period, about 68 million years ago in what is now North America. It is one ...
'' were widespread throughout western North America, while lambeosaurines were rare and albertosaurines and
centrosaurines Centrosaurinae (from the Greek, meaning "pointed lizards") is a subfamily of ceratopsid dinosaurs, a group of large quadrupedal ornithischians. Centrosaurine fossil remains are known primarily from the northern region of Laramidia (modern day Alb ...
had gone extinct.


Growth

Gregory Erickson and colleagues have studied the growth and life history of
tyrannosaurids Tyrannosauridae (or tyrannosaurids, meaning "tyrant lizards") is a family (biology), family of coelurosaurian Theropoda, theropod dinosaurs that comprises two subfamilies containing up to thirteen genus, genera, including the eponymous ''Tyrannos ...
using bone
histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vi ...
, 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 Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definit ...
may have ended this rapid growth phase, after which growth slowed down considerably in adult animals. Examining five ''
Gorgosaurus ''Gorgosaurus'' ( ; ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (Campanian), between about 76.6 and 75.1 million years ago. Fossil remains have been found in the Ca ...
'' 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 ''Daspletosaurus'' ( ; meaning "frightful lizard") is a genus of tyrannosaurid dinosaur that lived in Laramidia between about 79.5 and 74 million years ago, during the Late Cretaceous Period. The genus ''Daspletosaurus'' contains three species ...
'' and ''
Tyrannosaurus ''Tyrannosaurus'' is a genus of large theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosaurus'' live ...
'', but comparable to ''
Albertosaurus ''Albertosaurus'' (; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 71 million years ago. The type species, ''A. sarcophagus'', wa ...
''. ''
Gorgosaurus ''Gorgosaurus'' ( ; ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (Campanian), between about 76.6 and 75.1 million years ago. Fossil remains have been found in the Ca ...
'' 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 dragon The Komodo dragon (''Varanus komodoensis''), also known as the Komodo monitor, is a member of the monitor lizard family Varanidae that is endemic to the Indonesian islands of Komodo, Rinca, Flores, and Gili Motang. It is the largest extant ...
s, whose hatchlings start off as tree-dwelling insectivores and slowly mature into massive
apex predator An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic lev ...
s 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 A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
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 ''Albertosaurus'' (; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 71 million years ago. The type species, ''A. sarcophagus'', wa ...
'' 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
hatchling In oviparous biology, a hatchling is a newly hatched fish, amphibian, reptile, or bird. A group of mammals called monotremes lay eggs, and their young are hatchlings as well. Fish Fish hatchlings generally do not receive parental care, similar to ...
s 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 A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species ...
. The same pattern is also seen in other tyrannosaurids. The comparison with modern animals and other tyrannosaurids lends support to this life history
hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...
, 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 ''Allosaurus'' () is a genus of large carnosaurian theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic epoch ( Kimmeridgian to late Tithonian). The name "''Allosaurus''" means "different lizard" alludin ...
'' assemblage at the
Cleveland-Lloyd Dinosaur Quarry Jurassic National Monument, at the site of the Cleveland-Lloyd Dinosaur Quarry, well known for containing the densest concentration of Jurassic dinosaur fossils ever found, is a paleontological site located near Cleveland, Utah, in the San Raf ...
in
Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...
. 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 California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
, 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.(published abstract only) There is plentiful evidence for gregarious behaviour among herbivorous dinosaurs, including ceratopsians and
hadrosaur Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which incl ...
s. However, only rarely are so many dinosaurian predators found at the same site. Small theropods like ''
Deinonychus ''Deinonychus'' ( ; ) is a genus of dromaeosaurid theropod dinosaur with one described species, ''Deinonychus antirrhopus''. This species, which could grow up to long, lived during the early Cretaceous Period, about 115–108 million y ...
'', ''
Coelophysis ''Coelophysis'' ( traditionally; or , as heard more commonly in recent decades) is an extinct genus of coelophysid theropod dinosaur that lived approximately 228 to 201.3 million years ago during the latter part of the Triassic Period fro ...
'' and ''
Megapnosaurus ''Megapnosaurus'' (meaning "big dead lizard", from Greek μεγα = "big", 'απνοος = "not breathing", "dead", σαυρος = "lizard") is an extinct genus of coelophysid theropod dinosaur that lived approximately 188 million years ago dur ...
'' (''Syntarsus rhodesiensis'') have been found in aggregations, as have larger predators like ''Allosaurus'' and ''
Mapusaurus ''Mapusaurus'' () was a giant carcharodontosaurid carnosaurian dinosaur from the early Late Cretaceous (early Turonian stage), approximately 93.9 to 89.6 million years ago, of what is now Argentina. Discovery ''Mapusaurus'' was excavated b ...
''. There is some evidence of
gregarious behaviour Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies. Sociality is a survival response to evolutionary pressures. For example, when a mother wasp ...
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 (''City in a Garden''); I Will , image_map = , map_caption = Interactive Map of Chicago , coordinates = , coordinates_footnotes = , subdivision_type = Country , subdivision_name ...
, and a bonebed in the
Two Medicine Formation The Two Medicine Formation is a geological formation, or rock body, in northwestern Montana and southern Alberta that was deposited between and (million years ago), during Campanian (Late Cretaceous) time. It crops out to the east of the Rocky M ...
of Montana contains at least three specimens of ''
Daspletosaurus ''Daspletosaurus'' ( ; meaning "frightful lizard") is a genus of tyrannosaurid dinosaur that lived in Laramidia between about 79.5 and 74 million years ago, during the Late Cretaceous Period. The genus ''Daspletosaurus'' contains three species ...
'', 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 The Komodo dragon (''Varanus komodoensis''), also known as the Komodo monitor, is a member of the monitor lizard family Varanidae that is endemic to the Indonesian islands of Komodo, Rinca, Flores, and Gili Motang. It is the largest extant ...
-like mobbing of carcasses, where aggressive competition leads to some of the predators being killed and
cannibal Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, bo ...
ized. Even though ''Albertosaurus'' are commonly found in packs, there is no evidence of group behavior in the close relative ''Gorgosaurus''.


See also

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Timeline of tyrannosaur research This timeline of tyrannosaur research is a chronological listing of events in the history of paleontology focused on the tyrannosaurs, a group of predatory theropod dinosaurs that began as small, long-armed bird-like creatures with elaborate cr ...


References


Footnotes


Bibliography

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External links

* {{Taxonbar, from=Q2044915 Fossil taxa described in 2003 Tyrannosaurids Dinosaurs of North America Cretaceous dinosaurs