Daspletosaurus

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Daspletosaurus
Temporal range:
Ma
↓
Cast skeleton mount of D. torosus on display at Milwaukee Public Museum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: †Tyrannosauridae
Clade: †Daspletosaurini
Genus: †Daspletosaurus
Russell, 1970
Type species
†Daspletosaurus torosus
Russell, 1970
Other species
  • †D. horneri
    Carr et al., 2017
  • †D. degrootorum?
    Voris et al., 2020[1]
  • †D. wilsoni?
    Warshaw & Fowler, 2022
Synonyms

Daspletosaurus (

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 only in Montana
. D. wilsoni has been suggested as an intermediate species between D. torosus and D. horneri that evolved through anagenesis, but this theory has been disputed by other researchers.

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

bipedal predator
, with the largest potential specimen measuring around 11 metres (36 ft) long and weighing up to 5 metric tons (5.5 short tons), 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

niche differentiation between the two.[2] 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.[3]

Discovery and naming

The D. torosus holotype specimen mounted at the Canadian Museum of Nature.

The

Ma (million years ago).[6]

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.[7][8] 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.[9]

Assigned species

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.[7][10]

Dinosaur Park specimen (FMNH PR308), mounted at the Field Museum

Along with the

million years ago.[11] Daspletosaurus fossils are known specifically from the middle to upper section of the formation, between 75.6 and 75.0 million years ago.[12] 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.[13] 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. 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.[7]

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.[14] Many later authors have reassigned this specimen, along with a few others from New Mexico, to yet another unnamed species of Daspletosaurus.[7][10][15] 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.[16]

D. horneri holotype skull from Montana in Museum of the Rockies

In 1992,

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.[19] In 2017, the Two Medicine Formation taxon was named as the new species D. horneri.[20]

Isolated tyrannosaurid teeth in the upper portions of the

Triebold Paleontology excavated between 2002 and 2004, known as "Sir William", shows some characteristics of Daspletosaurus suggesting a new earlier species to the genus. However the specimen shows many characteristics typical of early tyrannosaurines such as Teratophoneus and even some of the later Tyrannosaurus, which may suggest an entirely new genus.[21]

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,[22] but this theory was disputed by Scherer and Voiculescu-Holvad in 2024.[23]

Description

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 8.5–9 metres (28–30 ft) from snout to tail,[5] a hip height of 2.2 metres (7.2 ft),[20] and a body mass of 2–3 metric tons (2.2–3.3 short tons).[24][25][26] However there is one potential specimen of D. torosus, CMC VP 15826 (nicknamed "Pete III"), that suggests the genus could reach lengths of nearly 11 metres (36 ft) and weigh 5 metric tons (5.5 short tons).[27][23][28]

Skull

Skull of Daspletosaurus wilsoni

Daspletosaurus had a massive skull that could reach more than 1 metre (3 ft 3 in) in length.

jugal bones. The orbit (eye socket) was a tall oval, somewhere in between the circular shape seen in Gorgosaurus and the 'keyhole' shape of Tyrannosaurus.[13][7][10] Split carinae (edges)[29] have been found on Daspletosaurus teeth.[30]

Postcranial skeleton

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

center of gravity over the hips.[5][10]

Soft tissue reconstruction

From a comparison of the degree of wear of teeth of Daspletosaurus with other extinct and extant animals, it is concluded that Daspletosaurus, as well as other non-avian theropods, had lips that protected the teeth from external influences. Due to this feature, the snout of Daspletosaurus more closely resembled lizards than crocodiles, which lack lips.[31]

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.[32]

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.[10][33] It further belongs to the tribe Daspletosaurini, consisting of it and the taxon Thanatotheristes.[34]

Daspletosaurus is usually considered to be closely related to Tyrannosaurus rex, or even a direct ancestor through

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.[33] The systematics (evolutionary
relationships) of Daspletosaurus have become clearer as new species have been described.

Below is a cladogram of Tyrannosaurinae based on the

phylogenetic analysis conducted by Warshaw & Fowler (2022). Here, it is proposed that the three Daspletosaurus species evolved through anagenesis in the Tyrannosaurinae in a line leading to Zhuchengtyrannus, Tarbosaurus, and Tyrannosaurus. Due to their more fragmentary nature, Thanatotheristes and Nanuqsaurus were excluded from this analysis.[22]

Proposed anagenetic transition of Daspletosaurus species
Tyrannosaurinae

Alioramus remotus

Alioramus altai

Qianzhousaurus

Daspletosaurus torosus

Daspletosaurus wilsoni

Daspletosaurus horneri

In 2024, Scherer and Voiculescu-Holvad argued that the stratigraphic ranges of D. torosus, D. wilsoni and an unnamed species from the Dinosaur Park Formation and Oldman Formation show a clear overlap, indicating that anagenesis may not be the predominant factor of speciation within the genus, since all species of Daspletosaurus were contemporaneous with each other at some point during its evolution. Phylogenetic analyzes resolved D. horneri as the most basal species, in spite of the fact that it's stratigraphically the youngest. While the authors did not completely refute the possibility that anagenesis was the main driver of Daspletosaurus evolution based on the intermediate morphological features, they also suggested that D. wilsoni may be a junior synonym of D. torosus, since there is a near lack of autapomorphic characters that can differentiate this species. They also claimed that Daspletosaurus did not evolve from Thanatotheristes, since they found no support on the basis of morphological and stratigraphical data, and that anagenesis will not be supported unequivocally due to the limited sample and nature of the fossil record which doesn't show a great degree of variation in morphology. The cladogram presented for their phylogenetic analysis is shown below.[23]

Tyrannosaurinae
Alioramini
Teratophoneini
Daspletosaurini

Thanatotheristes

Daspletosaurus horneri

Daspletosaurus wilsoni

Daspletosaurus torosus

TMP
2001.36.1)

Tyrannosaurini

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.[36][37][20]

Social behavior

Two D. torosus models in the Canadian Museum of Nature

A young specimen of the Dinosaur Park Daspletosaurus species (TMP 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 and Saurornitholestes. Darren Tanke and Phil Currie hypothesize that the bites are due to intraspecific competition for territory or resources, or for dominance within a social group.[38]

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.[19] Other scientists are skeptical of the evidence for social groups in Daspletosaurus and other large theropods;[39]

Daspletosaurus skull with bite marks from another tyrannosaur

Brian Roach and Daniel Brinkman have suggested that Daspletosaurus social interaction would have more closely resembled the modern

cannibalizing each other in the process.[40]

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.[41][42]

Evidence of cannibalism in Daspletosaurus was published in 2015.[43]

Life history

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 1,800 kilograms (2.0 short 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.[44]

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.[45]

A 2009 study found evidence of Trichomonas gallinae-like infection in the jaws of various specimens of Daspletosaurus.[46]

Paleoecology

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.

Bearpaw Shale.[47][48][49]

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.

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.[48] The Two Medicine Formation was deposited at higher elevations farther inland than the other two formations.[49]

The excellent

birds like Apatornis and Avisaurus flew overhead, while several varieties of mammals coexisted with Daspletosaurus and other types of dinosaurs in the various formations that make up the Judith River wedge.[48]

Skull of a juvenile D. horneri

In the Oldman Formation (the geological equivalent of the Judith River formation), Daspletosaurus torosus could have preyed upon the hadrosaur species

dentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possibly Daspletosaurus.[55]

Coexistence with Gorgosaurus

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.[52] 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.[5] However, a specimen of Daspletosaurus (OTM 200) from the Two Medicine Formation preserves the digested remains of a juvenile hadrosaur in its gut region.[18] 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.[53]

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.

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 stage, tyrannosaurines like Tyrannosaurus rex, hadrosaurines and chasmosaurines like Triceratops were widespread throughout western North America, while albertosaurines and centrosaurines became extinct, and lambeosaurines were very rare.[10]

See also

References

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External links

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