Daspletosaurus
Encyclopedia
Daspletosaurus is a genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of tyrannosaurid theropod dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 that lived in western North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 between 77 and 74 million years ago, during the Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period. Fossils of the only named species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 (D. torosus) were found in Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...

, although other possible species from Alberta and Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

 await description. Including these undescribed species
Undescribed taxon
In taxonomy, an undescribed taxon is a taxon that has been discovered, but not yet formally described and named. The various Nomenclature Codes specify the requirements for a new taxon to be validly described and named. Until such a description has been published, the taxon has no formal or...

, Daspletosaurus is the most species-rich genus of tyrannosaur.

Daspletosaurus is closely related to the much larger and more recent Tyrannosaurus
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...

. Like most known tyrannosaurids, it was a multi-ton biped
Biped
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs, or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning "two feet"...

al predator
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

 equipped with dozens of large, sharp teeth. Daspletosaurus had the small forelimbs typical of tyrannosaurids, although they were proportionately longer than in other genera.

As an apex predator
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...

, Daspletosaurus was at the top of the food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

, probably preying on large dinosaurs like the ceratopsid Centrosaurus
Centrosaurus
Centrosaurus is a genus of herbivorous ceratopsian dinosaurs from the late Cretaceous of Canada. Their remains have been found in the Dinosaur Park Formation and uppermost Oldman Formation, dating from 76.5 to 75.5 million years ago....

 and the hadrosaur Hypacrosaurus
Hypacrosaurus
Hypacrosaurus was a genus of duckbill dinosaur similar in appearance to Corythosaurus. Like Corythosaurus, it had a tall, hollow rounded crest, although not as large and straight...

. In some areas, Daspletosaurus coexisted with another tyrannosaurid, Gorgosaurus
Gorgosaurus
Gorgosaurus is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 76.5 and 75 million years ago. Fossil remains have been found in the Canadian province of Alberta and possibly the U.S. state of Montana....

, though there is some evidence of niche differentiation
Niche differentiation
The term niche differentiation , as it applies to the field of ecology, refers to the process by which natural selection drives competing species into different patterns of resource use or different niches...

 between the two. While Daspletosaurus fossils are rarer than other tyrannosaurids, the available specimens allow some analysis of the biology of these animals, including social behavior
Social behavior
In physics, physiology and sociology, social behavior is behavior directed towards society, or taking place between, members of the same species. Behavior such as predation which involves members of different species is not social...

, diet and life history.

Description

While very large by the standard of modern predators, Daspletosaurus was not the largest tyrannosaurid. Adults could reach a length of 8–9 meters (26–30 ft) from snout to tail. Mass estimates have centered around 2.5 tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...

s (2.75 short ton
Short ton
The short ton is a unit of mass equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...

s) but have ranged between 1.8 tonnes (2 tons) and 3.8 tonnes (4.1 tons).

Daspletosaurus had a massive skull that could reach more than 1 meter (3.3 ft) in length. The bones were heavily constructed and some, including the nasal
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....

 bones on top of the snout, were fused for strength. Large fenestrae (openings) in the skull reduced its weight. An adult Daspletosaurus was armed with about six dozen teeth that were very long but oval in cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

 rather than blade-like. Unlike its other teeth, those in the premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

 at the end of the upper jaw had a D-shaped cross section, an example of heterodonty always seen in tyrannosaurids. Unique skull features included the rough outer surface of the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...

 (upper jaw bone) and the pronounced crests around the eyes on the lacrimal
Lacrimal
The term Lacrimal can refer to:*In typography:** A type of Stroke ending *In anatomy** Lacrimal apparatus** Lacrimal artery** Lacrimal bone...

, postorbital
Postorbital
The postorbital is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some vertebrates, the postorbital is fused with the postfrontal...

, and jugal
Jugal
The jugal is a skull bone found in most reptiles, amphibians, and birds. In mammals, the jugal is often called the malar or Zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species....

 bones. The orbit
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...

 (eye socket) was a tall oval, somewhere in between the circular shape seen in Gorgosaurus and the 'keyhole' shape of Tyrannosaurus.

Daspletosaurus shared the same body form as other tyrannosaurids, with a short, S-shaped neck supporting the massive skull. It walked on its two thick hindlimbs, which ended in four-toed feet, although the first digit (the hallux
Hallux
In tetrapods, the hallux is the innermost toe of the foot. Despite its name it may not be the longest toe on the foot of some individuals...

) did not contact the ground. In contrast, the forelimbs were extremely small and bore only two digits, although Daspletosaurus had the longest forelimbs in proportion to body size of any tyrannosaurid. A long, heavy tail served as a counterweight
Counterweight
A counterweight is an equivalent counterbalancing weight that balances a load.-Uses:A counterweight is often used in traction lifts , cranes and funfair rides...

 to the head and torso, with the center of gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...

 over the hips.

Classification and systematics


Carr
Thomas Carr (paleontologist)
Thomas D. Carr is a vertebrate paleontologist who received his Ph.D. from the University of Toronto in 2005. He is now a member of the biology faculty at Carthage College in Kenosha, Wisconsin. Much of his work centers on tyrannosauroid dinosaurs...

 et al. 2005


*Note: Carr et al. use the name Tyrannosaurus bataar

Currie
Philip Currie
Philip Henry Wodehouse Currie, 1st Baron Currie GCB , known as Sir Philip Currie between 1885 and 1899, was a British diplomat. He was Ambassador to the Ottoman Empire from 1893 to 1898 and Ambassador to Italy from 1898 to 1902.-Background and education:Currie was the son of Raikes Currie, Member...

 et al. 2003

Daspletosaurus belongs in the subfamily Tyrannosaurinae within the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Tyrannosauridae
Tyrannosauridae
Tyrannosauridae is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to six genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three...

, along with Tarbosaurus
Tarbosaurus
Tarbosaurus is a genus of tyrannosaurid theropod dinosaur that flourished in Asia about 70 million years ago, at the end of the Late Cretaceous Period. Fossils have been recovered in Mongolia, with more fragmentary remains found further afield in parts of China. Although many species have been...

, Tyrannosaurus and Alioramus
Alioramus
Alioramus is a genus of tyrannosaurid theropod dinosaur from the Late Cretaceous period of Asia. The type A. remotus, is known from a partial skull and three metatarsals recovered from Mongolian sediments which were deposited in a humid floodplain between 70 and 65 million years ago. These...

. Animals in this subfamily are more closely related to Tyrannosaurus than to Albertosaurus
Albertosaurus
Albertosaurus is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, more than 70 million years ago. The type species, A. sarcophagus, was apparently restricted in range to the modern-day Canadian province of Alberta, after which...

 and are known — with the exception of Alioramus — for their robust build with proportionally larger skulls and longer femora
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

 than in the other subfamily, the Albertosaurinae.

Daspletosaurus is usually considered to be closely related to Tyrannosaurus rex, or even a direct ancestor through anagenesis
Anagenesis
Anagenesis, also known as "phyletic change," is the evolution of species involving an entire population rather than a branching event, as in cladogenesis. When enough mutations have occurred and become stable in a population so that it is significantly differentiated from an ancestral population,...

. Gregory Paul reassigned D. torosus to the genus Tyrannosaurus, creating the new combination Tyrannosaurus torosus, but this has not been generally accepted. Many researchers believe Tarbosaurus and Tyrannosaurus to be sister taxa or even to be the same genus, with Daspletosaurus a more basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 relative. On the other hand, Phil Currie
Phil Currie
Philip John Currie, AOE 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...

 and colleagues find Daspletosaurus to be more closely related to Tarbosaurus and other Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

n tyrannosaurids like Alioramus than to the North American Tyrannosaurus. The systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...

 (evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

ary relationships) of Daspletosaurus may become clearer once all the species have been described.

Discovery and naming

The type specimen of Daspletosaurus torosus (CMN
Canadian Museum of Nature
The Canadian Museum of Nature is a natural history museum in Ottawa, Ontario, Canada. Its collections, which were started by the Geological Survey of Canada in 1856, include all aspects of the intersection of human society and nature, from gardening to gene-splicing...

 8506) is a partial skeleton including the skull, the shoulder, a forelimb, the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

, a femur and all of the vertebrae from the neck, torso and hip, as well as the first eleven tail vertebrae. It was discovered in 1921 by Charles Mortram Sternberg
Charles Mortram Sternberg
Charles M. Sternberg was an American-Canadian fossil collector and paleontologist, son of Charles Hazelius Sternberg.Late in his career, he collected and described Pachyrhinosaurus, Brachylophosaurus, Parksosaurus and Edmontonia...

, who thought it was a new species of Gorgosaurus. It was not until 1970 that the specimen was fully described by Dale Russell
Dale Russell
Dale A. Russell is a Canadian geologist/palaeontologist, currently Research Professor at The Department of Marine Earth and Atmospheric Sciences of North Carolina State University...

, who made it the type of a new genus, Daspletosaurus, from the Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 stems δασπλητo-/daspleto- ('frightful') and σαυρος/sauros ('lizard'). The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

 is D. torosus, which is Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 for 'muscular' or 'brawny.' Aside from the type, there is only one other well-known specimen, a complete skeleton discovered in 2001. Both specimens were recovered from the Oldman
Oldman Formation
The Oldman Formation is the middle member of the Judith River Group, a major geologic unit in southern Alberta. The formation is widely recognized as bearing a great number of well preserved dinosaur skeletons, as well as other fossils.-Age:...

 Formation
Geologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...

 in the Judith River Group
Judith River Group
The Judith River Group is a group of geologic formations in western North America dating from the late Cretaceous and noted as a site for the extensive excavation of dinosaur fossils. The formation is named after the Judith River in Montana. The group is also called the Judith River Wedge...

 of Alberta. A specimen from the younger Horseshoe Canyon Formation
Horseshoe Canyon Formation
The Horseshoe Canyon Formation is part of the Edmonton Group and is up to 230m in thickness. It is Late Campanian to Early Maastrichtian in age and is composed of mudstone, sandstone, and carbonaceous shales...

 in Alberta has been reassigned to Albertosaurus sarcophagus. The Oldman Formation was deposited during the middle Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

 stage
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...

 of the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...

, from about 77 to 76 Ma (million years ago).

Unnamed species

Two or three additional species have been assigned to the genus Daspletosaurus over the years, although as of 2007 none of these species have received a proper description or scientific name
Binomial nomenclature
Binomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages...

. In the meantime, all are assigned to Daspletosaurus sp. although this does not imply that they all are the same species.

Along with the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...

, Russell designated a specimen collected by Barnum Brown
Barnum Brown
Barnum Brown , a paleontologist born in Carbondale, Kansas, and named after the circus showman P.T. Barnum, discovered the second fossil of Tyrannosaurus rex during a career that made him one of the most famous fossil hunters working from the late Victorian era into the early 20th century.Sponsored...

 in 1913 as the paratype
Paratype
Paratype is a technical term used in the scientific naming of species and other taxa of organisms. The exact meaning of the term paratype when it is used in zoology is not the same as the meaning when it is used in botany...

 of D. torosus. This specimen (AMNH
American Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...

 5438) consists of parts of the hindleg, the pelvis and some of its associated vertebrae. It was discovered in the upper part of the Oldman Formation in Alberta. This upper section has since been renamed the Dinosaur Park Formation
Dinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...

, which dates back to the middle Campanian, from 76–74 Ma (million years ago). In 1914, Brown collected a nearly complete skeleton and skull; forty years later his American Museum of Natural History
American Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...

 sold this specimen to the Field Museum of Natural History
Field Museum of Natural History
The Field Museum of Natural History is located in Chicago, Illinois, USA. It sits on Lake Shore Drive next to Lake Michigan, part of a scenic complex known as the Museum Campus Chicago...

 in Chicago
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...

. It was mounted for display in Chicago and labeled as Albertosaurus libratus for many years, but after several skull features were later found to be modeled in plaster, including most of the teeth, the specimen (FMNH
Field Museum of Natural History
The Field Museum of Natural History is located in Chicago, Illinois, USA. It sits on Lake Shore Drive next to Lake Michigan, part of a scenic complex known as the Museum Campus Chicago...

 PR308) was reassigned to Daspletosaurus. A total of eight specimens have been collected from the Dinosaur Park Formation over the years since, most of them within the boundaries of Dinosaur Provincial Park
Dinosaur Provincial Park
Dinosaur Provincial Park is a UNESCO World Heritage Site located about two and a half hours drive southeast of Calgary, Alberta, Canada or , about a half hour drive, northeast of Brooks....

. Phil Currie believes that the Dinosaur Park specimens represent a new species of Daspletosaurus, distinguished by certain features of the skull. Pictures of this new species have been published, but it still awaits a name and full description in print.

A new tyrannosaur specimen (OMNH
Sam Noble Oklahoma Museum of Natural History
The Sam Noble Oklahoma Museum of Natural History is a natural history museum in Norman, Oklahoma, operated by the University of Oklahoma. It is currently housed in a building on Chautauqua Avenue that opened on May 1, 2000. The museum's exhibits include a Native American gallery and collections of...

 10131), including skull fragments, ribs and parts of the hindlimb, was reported from New Mexico in 1990 and assigned to the now-defunct genus Aublysodon
Aublysodon
Aublysodon is a name given to a large number of carnivorous dinosaur teeth of a certain form found in numerous late Cretaceous period geological formations...

. Many later authors have reassigned this specimen, along with a few others from New Mexico, to yet another unnamed species of Daspletosaurus. However, research published in 2010 showed that this species, from the Hunter Wash Member of the Kirtland Formation
Kirtland Formation
The Kirtland Formation is a sedimentary geological formation. It is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain that existed on the inland seashore of North America, in the late Cretaceous period. It overlies the Fruitland Formation...

, is actually a more primitive tyrannosauroid, and was classified in the genus Bistahieversor
Bistahieversor
Bistahieversor is a genus of tyrannosauroid dinosaur. Bistahieversor existed around 75 million years ago during the Late Cretaceous....

. There is currently disagreement over the age of the Kirtland Formation, with some workers claiming a late Campanian age, while others suggest a younger age in the early Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

 stage.

In 1992, Jack Horner
Jack Horner (paleontologist)
John "Jack" R. Horner is an American paleontologist who discovered and named Maiasaura, providing the first clear evidence that some dinosaurs cared for their young. He is one of the best-known paleontologists in the United States...

 and colleagues published an extremely preliminary report of a tyrannosaurid from the upper parts of the Campanian Two Medicine Formation
Two Medicine Formation
The Two Medicine Formation is a geologic formation, or rock body, that was deposited between 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma , during Campanian time, and is located in northwestern Montana...

 in Montana, which was interpreted as a transitional species between Daspletosaurus and the later Tyrannosaurus. Another partial skeleton was reported from the Upper Two Medicine in 2001, preserving the remains of a juvenile hadrosaur in its abdominal cavity
Abdominal cavity
The abdominal cavity is the body cavity of the human body that holds the bulk of the viscera. It is located below the thoracic cavity, and above the pelvic cavity. Its dome-shaped roof is the thoracic diaphragm , and its oblique floor is the pelvic inlet...

. This specimen was assigned to Daspletosaurus but not to any particular species. The remains of at least three more Daspletosaurus have also been described in a Two Medicine bonebed. These specimens have not been described in detail, but Currie believes all of the Two Medicine material represents an as-yet-unnamed third species of Daspletosaurus.

Coexistence with Gorgosaurus

In the late Campanian of North America, Daspletosaurus was a contemporary of the albertosaurine tyrannosaurid Gorgosaurus. This is one of the few examples of two tyrannosaur genera coexisting. In modern predator guilds
Guild (ecology)
A guild is any group of species that exploit the same resources, often in related ways. As can be seen from the list of examples below, it does not follow that the species within a guild occupy the same, or even similar, ecological niches...

, similar-sized predators are separated into different ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

s by anatomical, behavioral or geographical differences that limit competition. Several studies have attempted to explain niche differentiation in Daspletosaurus and Gorgosaurus.

Dale Russell hypothesized that the more lightly built and more common Gorgosaurus may have preyed on the abundant hadrosaurs of the time, while the more robust and less common Daspletosaurus may have specialized on the less prevalent but better-defended ceratopsids, which may have been more difficult to hunt. However, a specimen of Daspletosaurus (OTM 200) from the Two Medicine Formation preserves the digested remains of a juvenile hadrosaur in its gut region. The higher and broader muzzles of tyrannosaurines like Daspletosaurus are mechanically stronger than the lower snouts of albertosaurines like Gorgosaurus, although tooth strengths are similar between the two groups. This may indicate a difference in feeding mechanics or diet.

Other authors have suggested that competition was limited by geographical separation. Unlike some other groups of dinosaurs, there appears to be no correlation with distance from the sea. Neither Daspletosaurus nor Gorgosaurus was more common at higher or lower elevations than the other. However, while there is some overlap, Gorgosaurus appears to be more common at northern latitudes, 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 and in southwestern North America during the Campanian. Thomas Holtz has suggested that this pattern indicates shared ecological preferences between tyrannosaurines, chasmosaurines and hadrosaurines. Holtz notes that, at the end of the later Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...

 stage, tyrannosaurines like Tyrannosaurus rex, hadrosaurines and chasmosaurines like Triceratops
Triceratops
Triceratops is a genus of herbivorous ceratopsid dinosaur which lived during the late Maastrichtian stage of the Late Cretaceous Period, around 68 to 65 million years ago in what is now North America. It was one of the last dinosaur genera to appear before the great Cretaceous–Paleogene...

 were widespread throughout western North America, while albertosaurines and centrosaurines went extinct, and lambeosaurines were very rare.

Social behavior

A young specimen of the Dinosaur Park Daspletosaurus species (TMP
Royal Tyrrell Museum of Palaeontology
The Royal Tyrrell Museum is a popular Canadian tourist attraction and a leading centre of palaeontological research noted for its collection of more than 130,000 fossils....

 94.143.1) shows bite marks on the face that were inflicted by another tyrannosaur. The bite marks are healed over, indicating that the animal survived the bite. A full-grown Dinosaur Park Daspletosaurus (TMP 85.62.1) also exhibits tyrannosaur bite marks, showing that attacks to the face were not limited to younger animals. While it is possible that the bites were attributable to other species, intraspecific aggression, including facial biting, is very common among predators. Facial bites are seen in other tyrannosaurs like Gorgosaurus and Tyrannosaurus, as well as in other theropod genera like Sinraptor
Sinraptor
Sinraptor is a genus of theropod dinosaur from the Late Jurassic. The name Sinraptor comes from the Latin prefix "Sino", meaning Chinese, and "Raptor" meaning thief. The specific name dongi honours Dong Zhiming...

 and Saurornitholestes
Saurornitholestes
Saurornitholestes is a genus of carnivorous dromaeosaurid theropod dinosaur from the late Cretaceous of Alberta, Montana and New Mexico....

. Darren Tanke
Darren Tanke
Darren H. Tanke is a Canadian fossil preparation technician of the Dinosaur Research Program at the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta. Born in Calgary, Tanke became interested in natural history at an early age. In 1979, Tanke began working for Philip J...

 and Phil Currie hypothesize that the bites are due to intraspecific competition
Intraspecific competition
Intraspecific competition is a particular form of competition in which members of the same species vie for the same resource in an ecosystem...

 for territory or resources, or for dominance within a social group.

Evidence that Daspletosaurus lived in social groups comes from a bonebed found in the Two Medicine Formation of Montana. The bonebed includes the remains of three Daspletosaurus, including a large adult, a small juvenile, and another individual of intermediate size. At least five hadrosaurs are preserved at the same location. Geologic evidence indicates that the remains were not brought together by river currents
Current (stream)
A current, in a river or stream, is the flow of water influenced by gravity as the water moves downhill to reduce its potential energy. The current varies spatially as well as temporally within the stream, dependent upon the flow volume of water, stream gradient, and channel geometrics...

 but that all of the animals were buried simultaneously at the same location. The hadrosaur remains are scattered and bear numerous marks from tyrannosaur teeth, indicating that the Daspletosaurus were feeding on the hadrosaurs at the time of death. The cause of death is unknown. Currie speculates that the daspletosaurs formed a pack
Pack hunter
A pack hunter is a predator belonging to the animal kingdom, which has evolved to hunt its prey by working together with other members of its species. Normally, such animals are closely related. The most commonly known pack hunter is the Gray Wolf, the ancestor of all breeds of domesticated dogs...

, although this cannot be stated with certainty. Other scientists are skeptical of the evidence for social groups in Daspletosaurus and other large theropods; Brian Roach and Daniel Brinkman have suggested that Daspletosaurus social interaction would have more closely resembled the modern Komodo dragon
Komodo dragon
The Komodo dragon , also known as the Komodo monitor, is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. A member of the monitor lizard family , it is the largest living species of lizard, growing to a maximum length of in rare cases...

, where non-cooperative individuals mob carcasses, frequently attacking and even cannibalizing
Cannibalism (zoology)
In zoology, cannibalism is the act of one individual of a species consuming all or part of another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded for more than 1500 species...

 each other in the process.

Life history

Paleontologist Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids. Analysis of bone histology
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...

 can determine the age of a specimen when it died. Growth rates can be examined when the age of various individuals are plotted against their size on a graph. Erickson has shown that after a long time as juveniles, tyrannosaurs underwent tremendous growth spurts for about four years midway through their lives. After the rapid growth phase ended with sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...

, growth slowed down considerably in adult animals. Erickson only examined Daspletosaurus from the Dinosaur Park Formation, but these specimens show the same pattern. Compared to albertosaurines, Daspletosaurus showed a faster growth rate during the rapid growth period due to its higher adult weight. The maximum growth rate in Daspletosaurus was 180 kilograms (400 lb
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...

) per year, based on a mass estimate of 1800 kilograms (2 tons) in adults. Other authors have suggested higher adult weights for Daspletosaurus; this would change the magnitude of the growth rate but not the overall pattern.

By tabulating the number of specimens of each age group, Erickson and his colleagues were able to draw conclusions about life history in a population of Albertosaurus. Their analysis showed that while juveniles were rare in the fossil record, subadults in the rapid growth phase and adults were far more common. While this could be due to preservation or collection biases
Biased sample
In statistics, sampling bias is when a sample is collected in such a way that some members of the intended population are less likely to be included than others. It results in a biased sample, a non-random sample of a population in which all individuals, or instances, were not equally likely to...

, Erickson hypothesized that the difference was due to low mortality among juveniles over a certain size, which is also seen in some modern large mammals like elephant
Elephant
Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...

s. This low mortality may have resulted from a lack of predation, since tyrannosaurs surpassed all contemporaneous predators in size by the age of two. Paleontologists have not found enough Daspletosaurus remains for a similar analysis, but Erickson notes that the same general trend seems to apply.

Pathology

Split carinae
Carinae
Carinae was an area of ancient Rome. It was one of its most exclusive neighborhoods.The Carinae was the northern tip of the Oppian Hill on its western side; it extended between the Velian Hill and the Clivus Pullius. Its outlook was southwestern, across the swamps of the Palus Ceroliae toward the...

 have been found on Daspletosaurus teeth.

Paleoecology

All known Daspletosaurus fossils have been found in formations dating to the middle to late Campanian stage of the Late Cretaceous Period, between 77 and 74 million years ago. Since the middle of the Cretaceous, North America had been divided in half by the Western Interior Seaway
Western Interior Seaway
The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, was a huge inland sea that split the continent of North America into two halves, Laramidia and Appalachia, during most of the mid- and late-Cretaceous Period...

, with much of Montana and Alberta below the surface. However, the uplift of the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

 in the Laramide Orogeny
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...

 to the west, which began during the time of Daspletosaurus, forced the seaway to retreat eastwards and southwards. Rivers flowed down from the mountains and drained into the seaway, carrying sediment along with them that formed the Two Medicine Formation, the Judith River Group, and other sedimentary
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

 formations in the region. About 73 million years ago, the seaway began to advance westwards and northwards again, and the entire region was covered by the Bearpaw Sea, represented throughout the western United States and Canada by the massive Bearpaw Shale.

Daspletosaurus lived in a vast floodplain along the western shore of the interior seaway. Large rivers watered the land, occasionally flooding and blanketing the region with new sediment. When water was plentiful, the region could support a great deal of plant and animal life, but periodic droughts also struck the region, resulting in mass mortality as preserved in the many bonebed deposits found in Two Medicine and Judith River sediments, including the Daspletosaurus bonebed. Similar conditions exist today in East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...

. Volcanic eruptions from the west periodically blanketed the region with ash, also resulting in large-scale mortality, while simultaneously enriching the soil for future plant growth. It is these ash beds that allow precise radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...

 as well. Fluctuating sea levels also resulted in a variety of other environments at different times and places within the Judith River Group, including offshore and nearshore marine habitats, coastal wetlands, deltas and lagoons, in addition to the inland floodplains. The Two Medicine Formation was deposited at higher elevations farther inland than the other two formations.
The excellent vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 fossil record of Two Medicine and Judith River rocks resulted from a combination of abundant animal life, periodic natural disasters, and the deposition of large amounts of sediment. Many types of freshwater and estuarine fish are represented, including shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

, sturgeon
Sturgeon
Sturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genera Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus. The term includes over 20 species commonly referred to as sturgeon and several closely related species that have distinct common...

s, gar
Gar
In American English the name gar is strictly applied to members of the Lepisosteidae, a family including seven living species of fish in two genera that inhabit fresh, brackish, and occasionally marine, waters of eastern North America, Central America, and the Caribbean islands.-Etymology:In...

s and others. The Judith River Group preserves the remains of many aquatic amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s and reptiles, including frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...

s, salamander
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

s, turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, Champsosaurus
Champsosaurus
Champsosaurus is an extinct genus of diapsid reptile belonging to the order Choristodera. It grew to about 1.50 m long....

 and crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...

ns. Terrestrial lizards, including whiptails
Teiidae
Teiidae is a family of lizards native to the Americas, generally known as whiptails. The group includes the parthenogenic genera Cnemidophorus and Aspidoscelis, and the non-parthenogenic Tupinambis. It has over 230 member species in ten genera...

, skink
Skink
Skinks are lizards belonging to the family Scincidae. Together with several other lizard families, including Lacertidae , they comprise the superfamily or infraorder Scincomorpha...

s, monitors and alligator lizards
Anguidae
The Anguidae is a large and diverse family of lizards native to the northern hemisphere. The group includes the slowworms, glass lizards, and alligator lizards, among others. Anguidae is divided into three subfamilies and contains 94 species in eight genera. Their closest living relatives are the...

 have also been discovered. Azhdarchid pterosaur
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...

s, and neornithean birds like Apatornis
Apatornis
Apatornis is a prehistoric terrestrial bird genus endemic to North America during the Late Cretaceous living about 83.5 mya. It is known from a single fossil specimen: a synsacrum, the fused series of vertebrae over the hips.-Taxonomy:...

 flew overhead, while the enantiornithiform bird
Enantiornithes
Enantiornithes is an extinct group of primitive birds. They were the most abundant and diverse avialans of the Mesozoic. Almost all retained teeth and clawed fingers on each wing, but otherwise looked much like modern birds externally. Over 50 species of Enantiornithines have been named, but some...

 Avisaurus
Avisaurus
Avisaurus is a genus of enantiornithine bird from the Late Cretaceous of North America.Two species are known; the type species A. archibaldi and A. gloriae...

 and several varieties of multituberculate, marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...

 and placental mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s scurried beneath the feet of Daspletosaurus and other dinosaurs.

In the Oldman Formation, Daspletosaurus torosus could have preyed upon hadrosaurs like Brachylophosaurus
Brachylophosaurus
Brachylophosaurus was a mid-sized member of the hadrosaurid family of dinosaurs. It is known from several skeletons and bonebed material from the Judith River Formation of Montana and the Oldman Formation of Alberta, living about 76.5 million years ago....

 and Hypacrosaurus
Hypacrosaurus
Hypacrosaurus was a genus of duckbill dinosaur similar in appearance to Corythosaurus. Like Corythosaurus, it had a tall, hollow rounded crest, although not as large and straight...

, small ornithopod
Ornithopod
Ornithopods or members of the clade Ornithopoda are a group of ornithischian dinosaurs that started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American...

s like Orodromeus
Orodromeus
Orodromeus is a genus of herbivorous ornithopod dinosaur from the Late Cretaceous of North America.-Discovery and naming:...

, ceratopsians like Centrosaurus
Centrosaurus
Centrosaurus is a genus of herbivorous ceratopsian dinosaurs from the late Cretaceous of Canada. Their remains have been found in the Dinosaur Park Formation and uppermost Oldman Formation, dating from 76.5 to 75.5 million years ago....

, pachycephalosaurs, ornithomimids, therizinosaur
Therizinosaur
Therizinosaurs are theropod dinosaurs belonging to the clade Therizinosauria. Therizinosaur fossils have been found in Early through Late Cretaceous deposits in Mongolia, the People's Republic of China and Western North America...

s and possibly ankylosaurs. Other predators included troodonts, oviraptorosaurs, the dromaeosaur Saurornitholestes
Saurornitholestes
Saurornitholestes is a genus of carnivorous dromaeosaurid theropod dinosaur from the late Cretaceous of Alberta, Montana and New Mexico....

 and possibly an albertosaurine tyrannosaur (genus currently unknown). The Dinosaur Park and Two Medicine Formations have fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...

s comparable to the Oldman, with the Dinosaur Park in particular preserving an unrivaled array of dinosaurs. The albertosaurine Gorgosaurus lived alongside species of Daspletosaurus in the Dinosaur Park and Upper Two Medicine environments. Young tyrannosaurs may have filled the niches in between adult tyrannosaurs and smaller theropods, which were separated by two orders of magnitude in mass. A Saurornitholestes
Saurornitholestes
Saurornitholestes is a genus of carnivorous dromaeosaurid theropod dinosaur from the late Cretaceous of Alberta, Montana and New Mexico....

 dentary has been discovered in the Dinosaur Park Formation
Dinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...

 that bore tooth marks left by the bite of a young tyrannosaur, possibly Daspletosaurus.

External links

  • Discussion and specimen list at The Theropod Database.
  • Skull image of the Dinosaur Park Formation
    Dinosaur Park Formation
    The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...

    Daspletosaurus at The Graveyard.
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