Live image of Daspletosaurus torosus
|Upper Cretaceous (Middle to Upper Campanium )|
|80.6 to 72 million years|
|Russell , 1970|
Daspletosaurus ( Greek : "fearfullizard", from daspleto- / δασπλητο- "terrible" and saurus / σαῦρος "lizard") was a theropod dinosaur from the family of Tyrannosauridae , which lived about 80 to 72 million years ago during the Upper Cretaceous ( Campanium ) lived in western North America. The only previously named species ( type species ), Daspletosaurus torosus , was discovered in Alberta ( Canada ); other possible species from Alberta, New Mexico and Montana are still waiting to be described . Including these as yet unnamed species, Daspletosaurus would bethe most species-rich tyrannosaur.
Daspletosaurus was closely related to the much larger and geologically younger Tyrannosaurus . Like all tyrannosaurs, it was a bipedal carnivore with a large skull and small arms, although the arms were proportionally longer than other tyrannosaurs.
As a top predator , Daspletosaurus was at the top of the food chain, possibly hunting large dinosaurs like the Ceratopsia Centrosaurus and the hadrosaurid Hypacrosaurus . In some areas he lived with Gorgosaurus , another tyrannosaurid, although there is evidence that both genera occupied different ecological niches . Although fossils of Daspletosaurus are rarer than those of other tyrannosaurids, the findings allow conclusions to be drawn about the biology of these animals, including social behavior , diet and individual development ( ontogenesis ).
Daspletosaurus was a medium-sized tyrannosaur, adult animals reached a length of eight to nine meters. Weight estimates range from 1.8 tons to 3.8 tons, but most estimates are around 2.5 tons. Like its relatives, this tyrannosaurid had a short, “S” -shaped neck and very small arms with only two fingers, which, however, were relatively longer in Daspletosaurus than in any other tyrannosaurid.
The skull was massive and could reach a length of over a meter. The bones were heavily built overall and some, such as the paired nasal bones , were fused together for stabilization. Large openings in the skull reduced the weight of the skull. Daspletosaurus was equipped with about four dozen very long teeth, which were mostly oval in cross-section. In contrast, the teeth were between jaw bone (premaxilla) -shaped at the front end of the upper jaw in section "D" - an example of the heterodont that is found in every tyrannosaurid. Unique skull features include the rough outer surface of the upper jaw (maxilla) and the ornaments that run around the orbital window (eye socket) on the tearbone (lacrimal), postorbital, and jugal . The orbital window was a large oval and an intermediate piece between the round shape in Gorgosaurus and the "keyhole" shape in Tyrannosaurus .
Daspletosaurus is classified within the Tyrannosauridae family in the subfamily Tyrannosaurinae, along with Tarbosaurus , Tyrannosaurus , Nanotyrannus, and perhaps Alioramus . Members of this subfamily are more closely related to Tyrannosaurus than to Albertosaurus and are characterized by proportionally larger skulls and longer thigh bones than the other subfamily, the Albertosaurinae.
Daspletosaurus is usually considered a close relative of Tyrannosaurus rex or even its direct anagenesis ancestor . Gregory Paul (1988) assigned Daspletosaurus torosus to the genus Tyrannosaurus and put the combination Tyrannosaurus torosus on; however, this did not find general acceptance. Many researchers consider Tarbosaurus and Tyrannosaurus to be sister taxa or even the same genus, with Daspletosaurus as a more basal (primordial) relative. On the other hand, Phil Currie and colleagues believe that Daspletosaurus is more closely related to Tarbosaurus and other Asian tyrannosaurids like Alioramus than it is to the North American tyrannosaurus . The exact relationships of Daspletosaurus may become clearer once all species have been described.
Cladogram of the Tyrannosauridae according to Holtz, 2004
Alternative cladogram of the Tyrannosauridae according to Currie and others 2003
Discovery history and naming
The type material of Daspletosaurus torosus ( CMN 8506) is a partial skeleton including the skull, the shoulder, an arm, the pelvis, a thigh bone (femur) and all vertebrae of the neck, torso and hip as well as the first eleven caudal vertebrae. It was discovered in 1921 by Charles Mortram Sternberg , who thought it was a new species of Gorgosaurus . It was not until 1970 that Dale Russell fully described the find and established the new genus Daspletosaurus - the name derives from the ancient Greek words δασπλητo- / daspleto- "terrible" and σαῦρος / sauros "lizard". The Artepitheth torosus is Latin and means "muscular". In addition to the type fossil, there is only one other well-known find, a complete skeleton discovered in 2001. Both skeletons were recovered from the layers of the Oldman Formation in the Judith River Group in Alberta, which were deposited during the Middle Campanian 77 to 76 million years ago. Another skeleton from the younger Horseshoe Canyon formation in Alberta has meanwhile been ascribed to the Albertosaurus sarcophagus .
Two or three more species have been assigned to the Daspletosaurus genus over the years , although to date none of these species have received an appropriate description or scientific name. Instead, they were all identified as Daspletosaurus sp. - which does not mean, however, that they all belong to the same species.
In addition to the holotype , Russell identified a find unearthed by Barnum Brown in 1913 as the paratype of Daspletosaurus torosus . This find ( AMNH 5438) consists of parts of the hind legs, the pelvis and some vertebrae and was found in the upper area of the Oldman Formation in Alberta. This upper area was later renamed the Dinosaur Park Formation ; the formation dates to the middle Campanian (80 to 76 million years ago). In 1914 Brown collected a nearly complete skeleton with skull; forty years later, it sold its American Museum of Natural History to the Field Museum of Natural History in Chicago . It was exhibited in Chicago for many years as Albertosaurus libratus - but after it was found that various parts of the skull, including most of the teeth, were modeled with plaster of paris , the skeleton ( FMNH PR308) was reassigned to Daspletosaurus . A total of eight skeletal finds have been collected from the Dinosaur Park Formation over the years, most of them within Dinosaur Provincial Park . Phil Currie believes that the Dinosaur Park finds represent a new species of Daspletosaurus that can be distinguished based on certain skull features. Representations of this new species have been published, but a full description and scientific name are pending.
A new tyrannosaur fossil ( OMNH 10131), including skull fragments, ribs, and parts of the hind legs, was reported from New Mexico in 1990 and assigned to the now invalid genus Aublysodon . Many later authors attributed this fossil, along with several other finds from New Mexico, to another, unnamed Daspletosaurus species. In any case, new, as yet unpublished research results suggest that this species from the Hunter Wash layer member of the Kirtland Formation was actually an early tyrannosaurid, which may have been related to Appalachiosaurus . There is currently disagreement about the age of the Kirtland Formation - while some authors state the late Campanium as the age, others believe that the formation is younger and dates to the early Maastrichtian .
In 1992, Jack Horner and colleagues published a very preliminary report on a tyrannosaurid from the upper sections of the Two Medicine Formation (Campanium) of Montana, which was interpreted as a transition species between Daspletosaurus and the later Tyrannosaurus . Another report from 2001 describes another partial skeleton from the upper Two Medicine Formation, in whose abdominal cavity the remains of a juvenile hadrosaur have been preserved. This find was attributed to Daspletosaurus , but not to a specific species. The remains of at least three other Daspletosaurus were found in a bonebed of the Two Medicine Formation. These finds have yet to be described in detail, but Currie believes that all of the material from the Two Medicine Formation belongs to a third, as yet unnamed, Daspletosaurus species.
Coexistence with Gorgosaurus
In the late Campanium of North America, Daspletosaurus was a contemporary of the Albertosaurine Gorgosaurus . This is one of the few examples of a co-existence of two tyrannosaurid. In modern predator guilds, similarly sized predators occupy various ecological niches that limit competition. Different ecological niches come about primarily through differences in behavior, anatomy or geography. Various studies attempt to clarify which ecological niches Daspletosaurus and Gorgosaurus occupied.
Dale Russell hypothesized that the lighter built and more common Gorgosaurus might have hunted the hadrosaurs that were abundant at the time, while the more robust and less common Daspletosaurus might have specialized in the ceratopsians , who were perhaps less common and harder to hunt. In any case, the digested remains of a juvenile hadrosaur were found in the intestinal region in a Daspletosaurus skeleton ( OTM 200) from the Two Medicine Formation. The higher and wider snouts of tyrannosaurines like Daspletosaurus are mechanically stronger than the flatter snouts of Albertosaurines like Gorgosaurus , although the teeth in both groups are similarly well defined. This could indicate differences in diet or in the feeding mechanisms.
Other authors suggested that the competition between the two genera was restricted by geographical separation. Unlike some other groups of dinosaurs, there does not appear to be a relationship with the distance to the sea - neither Daspletosaurus nor Gorgosaurus was more common at higher or lower altitudes. In any case, Gorgosaurus seems to have been more common in more northern latitudes, while Daspletosaurus was more common in 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 Campanium. Thomas Holtz therefore suspects that tyrannosaurines, chasmosaurines and hadrosaurines preferred the same habitats. He further notes that at the end of the Maastrichtian period tyrannosaurines like Tyrannosaurus rex , chasmosaurines like Triceratops and hadrosaurines were widespread in western North America, while alberto and centrosaurines became extinct and lambeosaurines became very rare.
A juvenile specimen of the Dinosaur Park species ( TMP 94.143.1) shows bite marks on the face from another tyrannosaur. The bite marks have healed, which shows that the animal survived the bite. An adult Daspletosaurus from Dinosaur Park also shows such bite marks, indicating that facial attacks were not limited to young animals. While the bites could be from other genera, aggression within species, including face biting, is very common among predators. Face bites can be seen in other tyrannosaurs such as Gorgosaurus and Tyrannosaurus, as well as other theropods such as Sinraptor and Saurornitholestes . Darren Tanke and Phil Currie believe the bites were caused by intraspecific competition for territory, resources, or dominance within a group.
Evidence of group life in Daspletosaurus comes from a bonebed from the Two Medicine Formation in Montana. This bonebed contains the remains of three Daspletosaurus , including a large adult, a small cub and another medium-sized individual. The remains of at least five hadrosaurs were also found. Geological studies show that the remains were not washed up in this place, but that all animals were buried in the same place at the same time. The hadrosaur remains are scattered and show numerous tyrannosaur bite marks, indicating that the Daspletosaurus had been feeding on the hadrosaur carcasses before they died. The cause of death is unknown. Currie speculates that the Daspletosaurus lived in a group, although that cannot be said with certainty. Other scientists are skeptical of evidence that Daspletosaurus and other large theropods lived in groups; Brian Roach and Danial Brinkman suspect that the social interactions of Daspletosaurus resemble those of the recent Komodo dragons, in which uncooperative individuals attack cadavers and attack their own species.
Ontogeny and Population Biology
Paleontologists around Gregory Erickson have studied the individual development ( ontogenesis ) of tyrannosaurids on the basis of skeletal finds. Analysis of the bone tissue ( histology ) suggests the age of the animal when it died; growth rates can also be determined by comparing the age of different individuals with their respective sizes. Erickson showed that tyrannosaurids lived a long time as hatchlings and then grew rapidly in the middle of their lives. After this rapid growth period, which probably ended with sexual maturity , growth in adult animals slowed down considerably. Although Erickson only examined Daspletosaurus from the Dinosaur Park formation, the results show the same pattern as with other tyrannosaurids. Compared to Albertosaurinen, Daspletosaurus showed faster growth during the rapid growth period due to its higher adult weight. The researchers determined the greatest growth rate for Daspletosaurus to be 180 kilograms per year - they assumed a weight of 1,800 kilograms for adults. Other authors stated that Daspletosaurus was heavier; however, a higher weight would only change the rate of growth, not the general pattern.
By comparing the number of finds in each age group, the researchers were able to draw conclusions about the population biology of Albertosaurus . As their analyzes showed, young animals are seldom passed down as fossils; Animals within the rapid growth phase and adult animals are found much more frequently. This could be because the bones of pups petrify less often or are more difficult to find and collect - however, Erickson suspects a low death rate among pups above a certain size, as can be seen in some modern large mammals like the elephant . This low death rate could result from the lack of predators, as tyrannosaurids outnumbered any contemporary predator two years old in size. It is true that paleontologists have not yet found enough remains for a similar analysis in Daspletosaurus ; However, Erickson suggests that the same pattern seems to apply.
All known Daspletosaurus fossils were found in rock formations deposited during the Middle and Late Campanian (about 80 to 72 Mya ago). Since the Early Upper Cretaceous North America was divided in half by a sea called the Western Interior Seaway , which also covered most of Montana and Alberta. At the time of Daspletosaurus , however, the rise of the Rocky Mountains began in the west as a result of the Laramian mountain formation , which displaced the arm of the sea to the east and south. Rivers poured from the mountains into the Western Interior Seaway , bringing with them sediments that formed the Two Medicine Formation, Judith River Group, and other fossil sites in the area. 73 million years ago the sea expanded again to the west and north, and the entire region was covered by the Bearpaw Sea, the marine deposits of which now form the Bearpaw Formation in western North America.
Daspletosaurus lived in a formidable flood plain along the western coast of the Western Interior Seaway . Large rivers irrigated the land, occasionally flooding the region and covering it with new sediment. When there was a lot of water, there was a diverse animal and plant life in the region - but periodic droughts repeatedly led to mass deaths, as can also be seen in many bonebeds in the Two Medicine Formation and the Judith River Formation, including the Daspletosaurus -Bonebed. Similar circumstances exist today in East Africa. Periodic volcanic eruptions in the west covered the region with ashes and led to further mass extinctions, but enriched the soil with nutrients for future plant growth. These ashes also allow researchers a precise radiometric dating of the rock layers. Changing sea levels in the Judith River Formation led to a variety of habitats at different times and places - in addition to the flood plains, there were marine habitats near the coast, coastal swamp areas, as well as deltas and lagoons. The Two Medicine Formation was deposited at higher elevation inland than the other two formations.
The fossilized vertebrate fauna of the Two Medicine and Judith River Formation is very well known due to the abundance of wildlife at the time of the depositions, periodic natural disasters resulting in high death rates, and large amounts of sediment being deposited. Many forms of freshwater and estuarine fish have survived, including sharks , rays , sturgeon , bonefish and others. The Judith River Group has preserved the remains of many aquatic amphibians and reptiles , including frogs, salamanders, turtles, champsosaurs, and crocodiles . Terrestrial lizards such as rail lizards , skinks , iguanas, and sneaks have also been spotted. Pterosaurs (Pterosauria) from the family of azhdarchidae and modern birds as Apatornis dominated the skies during the Enantiornithe bird Avisaurus and various mammals from the groups of Multituberculaten , marsupials and higher mammals between the legs of Daspletosaurus and other dinosaurs were running around.
In the Oldman Formation, various dinosaurs could have belonged to the prey spectrum of Daspletosaurus torosus , including hadrosaurs such as Brachylophosaurus and Hypacrosaurus , small ornithopods such as Orodromeus , ceratopsians such as Centrosaurus , pachycephalosaurs , ornithomimosaurs , thericinosaurs and possibly ankylosaurs . Other predators of the formation include Troodontids , Oviraptorosaurs , the Dromaeosaurids Saurornitholestes and perhaps Albertosaurine Tyrannosaurs (genus currently unknown). The Dinosaur Park Formation and Two Medicine Formation have a fauna comparable to the Oldman, with a huge range of different dinosaurs discovered in particular in the Dinosaur Park Formation. The Albertosaurine Gorgosaurus was found along with Daspletosaurus in the Dinosaur Park Formation and the Upper Two Medicine Formation. Young tyrannosaurs may have filled the niche between adult tyrannosaurs and smaller theropods.
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