Daspletosaurus ( ; meaning "frightful lizard") is a genus of tyrannosaurid dinosaur that lived in Laramidia between about 77 and 74.4 million years ago, during the Late Cretaceous Period. The genus Daspletosaurus contains three named species. Fossils of the earlier type species, D. torosus, have been found in Alberta, while fossils of a later species, D. horneri, have been found in Montana and Alberta. D. wilsoni has been suggested as an intermediate species between D. torosus and D. horneri that evolved through anagenesis, though further research may be required to definitively support the theory.
There are also multiple specimens which may represent new species of Daspletosaurus from Alberta and Montana, but these have not been formally described. The taxon Thanatotheristes has been suggested to represent a species of Daspletosaurus, D. degrootorum, but this has not been widely supported. Daspletosaurus is closely related to the much larger and more recent tyrannosaurid Tyrannosaurus. Like most tyrannosaurids, Daspletosaurus was a large bipedal predator, with the average adult measuring and weighing . Daspletosaurus had the small forelimbs typical of tyrannosaurids, although they were proportionately longer than in other genera.
As an apex predator equipped with dozens of large, sharp teeth, Daspletosaurus was at the top of the food chain, probably preying on large dinosaurs like the ceratopsid Centrosaurus and the hadrosaur Hypacrosaurus. In some areas, Daspletosaurus coexisted with another tyrannosaurid, Gorgosaurus, though there is some evidence of niche differentiation between the two. While Daspletosaurus fossils are not as common as other tyrannosaurid fossils, the available specimens allow some analysis of the biology of these animals, including social behavior, diet, and life history.
Discovery and naming
alt=|left|thumb|The D. torosus holotype specimen mounted at the [[Canadian Museum of Nature. ]]
The type specimen of Daspletosaurus torosus (CMN 8506) is a partial skeleton including the skull, the shoulder, a forelimb, the pelvis, 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 near Steveville, Alberta, by Charles Mortram Sternberg, who thought it was a new species of Gorgosaurus. It was not until 1970 that the specimen was fully described by Dale Russell, who made it the type of a new genus, Daspletosaurus, from the Greek δασπλής (', stem and connective vowel resulting in '-) ("frightful") and () ("lizard"). The type species is Daspletosaurus torosus, the specific name torosus being Latin for 'muscular' or 'brawny'.
Dale Russell also suggested that a specimen of an immature Albertosaurus (CMN 11315) from the younger Horseshoe Canyon Formation in Alberta actually belonged to a third specimen of Daspletosaurus as D. cf. torosus, extending the temporal range of the genus by approximately 3.5 million years into the Maastrichtian. He based this referral on features of its limb and pelvic girdle, as well as the curvature of the hand claws, which he interpreted as traits matching Daspletosaurus. This reassignment was not universally accepted, and thorough re-examination of the specimen favored its initial referral to Albertosaurus sarcophagus, despite lacking many of the diagnostic skeletal traits used to identify mature tyrannosaurids. An additional maxilla and various teeth from an Edmontosaurus-dominated bonebed in the Horseshoe Canyon Formation was also mistakenly referred to Daspletosaurus, but all the tyrannosaurid material has all since been confirmed to belong to Albertosaurus.
Assigned species
thumb|left|Originally believed to be a specimen of [[Gorgosaurus, this skull was later sold to the Field Museum and is now reassigned to Daspletosaurus torosus]]
Over the years, various additional species have been assigned to the genus Daspletosaurus. Though some have been designated as Daspletosaurus spp, this does not imply that they all represent the same species. Daspletosaurus fossils are known specifically from the middle to upper section of the formation, between 75.6 and 75.0 million years ago. In 1914, Brown collected a nearly complete skeleton and skull; forty years later his American Museum of Natural History sold this specimen to the Field Museum of Natural History in Chicago. 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 PR308) was reassigned to Daspletosaurus torosus by Thomas Carr in 1999. In 2025, Coppock and colleagues referred the Dinosaur Park Formation specimen CMN 350, a partial skull and skeleton with left dentary (TMP 2010.121.0001), to D. horneri.
A new tyrannosaurid specimen (OMNH 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. 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, is actually a more primitive tyrannosauroid, and was classified in the genus Bistahieversor.
thumb|D. horneri holotype skull from Montana in [[Museum of the Rockies]]
In 1992, Jack Horner and colleagues published an extremely preliminary report of a tyrannosaurid from the upper parts of the Campanian Two Medicine Formation in Montana, which was interpreted as a transitional species between Daspletosaurus and the later Tyrannosaurus. The remains of at least three more Daspletosaurus have also been described in a Two Medicine bonebed by Currie et al. (2005); the authors stated that this fossil material likely represents then-unnamed species mentioned by Horner et al. (1992), but cautioned that further study and description of Daspletosaurus would be necessary before the species can be determined with certainty. In 2017, the Two Medicine Formation taxon was named as the new species D. horneri.
Isolated tyrannosaurid teeth in the upper portions of the Judith River Formation are likely from Gorgosaurus as well as some species of Daspletosaurus, probably D. torosus. In 2009, preliminary preparation of a Daspletosaurus specimen from the Coal Ridge Member of the Judith River Formation measuring about long was reported. Some researchers assigned this specimen to D. torosus, while others considered it to be a distinct species yet to be named, referred to as D. sp.
thumb|left|Skeletal reconstruction of Daspletosaurus sp. (specimen CMC VP15826)
In 2017, John Wilson discovered the bones of a tyrannosaurid, including a partial disarticulated skull, cervical, sacral, and caudal vertebrae, and a rib, chevron, and first metatarsal, from the "Jack's B2" site of the Judith River Formation. Elías A. Warshaw and Denver W. Fowler described these remains (BDM 107) in 2022 as belonging to a new species of Daspletosaurus, D. wilsoni. It represents a transitional species between D. torosus and D. horneri, as it existed between them in time. It has been suggested that the three species may have evolved directly through anagenesis, but this theory was disputed by Scherer and Voiculescu-Holvad (2024) who suggested that D. wilsoni may be a junior synonym of D. torosus,
Description
thumb|left|Size of D. wilsoni, D. torosus, and D. horneri compared to a human
While very large by the standard of modern predators, Daspletosaurus was not the largest tyrannosaurid. Adults could reach a length of from snout to tail, a hip height of , However, one large specimen of Daspletosaurus sp. (CMC VP 15826) suggests that the genus could reach lengths of over , potentially reaching sizes of in length and in weight.
Skull
thumb|Skull of Daspletosaurus wilsoni
Daspletosaurus had a massive skull that could reach more than in length. Split carinae (edges) have been found on Daspletosaurus teeth.
Postcranial skeleton
thumb|Restoration of D. torosus
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) 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 to the head and torso, with the center of gravity over the hips.
A skin impression from Daspletosaurus torosus has been described, showing small polygonal scales measuring 3 mm in diameter. The placement of the scales on the body is not known.
Classification and systematics
Daspletosaurus belongs in the subfamily Tyrannosaurinae within the family Tyrannosauridae, along with Tarbosaurus, Tyrannosaurus, and Alioramini. Animals in this subfamily are more closely related to Tyrannosaurus than to Albertosaurus and are known – with the exception of Alioramus – for their robust build with proportionally larger skulls and longer femora than in the other subfamily, the Albertosaurinae. It further belongs to the tribe Daspletosaurini, consisting of it and the taxon Thanatotheristes.
Daspletosaurus is usually considered to be closely related to Tyrannosaurus, or even a direct ancestor through anagenesis. Gregory Paul reassigned D. torosus to the genus Tyrannosaurus, creating the new combination Tyrannosaurus torosus, On the other hand, Phil Currie and colleagues find Daspletosaurus to be more closely related to Tarbosaurus and other Asian tyrannosaurids like Alioramus than to the North American Tyrannosaurus.
Paleobiology
Senses
There are indications of D. horneri possessing integumentary sensory organs, possibly used in touch, modulation of precise jaw movements, temperature reading, and prey detection. The large flat scales may have further protected the snout during prey capture and intra-specific combat. A young specimen of the tyrannosaurid (TMP 1994.143.1), initially identified as the Dinosaur Park Daspletosaurus but subsequently referred to Gorgosaurus libratus, also 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.
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 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, although this cannot be stated with certainty.
thumb|Juvenile Gorgosaurus libratus skull TMP 1994.143.1 from the Dinosaur Park Formation, previously assigned to Daspletosaurus sp., with several bite marks by other tyrannosaurs
Brian Roach and Daniel Brinkman have suggested that Daspletosaurus social interaction would have more closely resembled the modern Komodo dragon, where non-cooperative individuals mob carcasses, frequently attacking and even cannibalizing each other in the process.
Fossils of other tyrannosaurids like Teratophoneus and Albertosaurus among other genera suggest that gregarious behavior may have been widespread in tyrannosaurs and thus may vindicate the hypothesis of Daspletosaurus being a social animal, as bonebeds of these genera containing multiple specimens in a wide range of ages have been excavated and described from these different genera.
Evidence of cannibalism in Daspletosaurus was published in 2015.
Life history
thumb|300px|left|A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids. Daspletosaurus is shown in green, based on Erickson et al., 2004
Paleontologist Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids. Analysis of bone histology can determine the age of a specimen when it died. Growth rates can be examined when the ages 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, 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) per year, based on a mass estimate of 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.
thumb|Growth series of D. horneri
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, 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 elephants. 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.
A 2009 study found evidence of Trichomonas gallinae-like infection in the jaws of various specimens of Daspletosaurus.
Paleoecology
thumb|left|D. torosus skeleton cast in the [[Rocky Mountain Dinosaur Resource Center based on a nearly complete specimen from Montana's Judith River Formation.]]
All known Daspletosaurus fossils have been found in formations dating to the middle to late Campanian stage of the Late Cretaceous Period, between 78 and 74.4 million years ago.
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. 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 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. A Saurornitholestes dentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possibly Daspletosaurus.
Coexistence with Gorgosaurus
thumb|Restoration of Daspletosaurus torosus feeding on a juvenile [[ceratopsian]]
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, similar-sized predators are separated into different ecological niches 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.
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.