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Last updated on April 17, 2014 at 1:21 EDT

Daspletosaurus

Daspletosaurus, meaning “frightful lizard” is a genus of tyrannosaurid theropod dinosaur that resided in western North America between 77 and 74 million years ago, during the Late Cretaceous Period. Fossils of the only named species were found in Alberta, although other possible species from Alberta and Montana wait for description.

Daspletosaurus is closely related to the much larger and more current Tyrannosaurus. Like most of the known tyrannosaurids, it was a multi-ton bipedal predator with dozens of sizable and sharp teeth. Daspletosaurus encompass the small forelimbs that are typical of tyrannosaurids, although they were proportionately longer than in the other genera.

As an apex predator, it was at the top of the food chain, mostly likely preying upon large dinosaurs such as the ceratopsid Centrosaurus and the hadrosaur Hypacrosaurus. In some areas, it coexisted with another tyrannosaurid, Gorgosaurus, although, there is some evidence of niche differentiation between these two. While Daspletosaurus fossils are more infrequent than other tyrannosaurids, the available specimens allow some examination of the biology of these animals, such as their social behavior, their diet, and their life history.

While it is very large by the standard of modern predators, it wasn’t the largest tyrannosaurid. The adults could reach a length of 26 to 30 feet from its snout to its tail. The mass estimates averaged around 2.75 short tons but have ranged between 2.0 and 4.1 short tons.

Daspletosaurus had a gigantic skull that could reach more than 3.3 feet in length. The bones were heavily constructed and some, including the nasal bones, were fused for strength. Large openings in the skull decreased its weight. An adult was armed with about six dozen teeth were very long but oval in cross section rather than like a blade. Unlike its other teeth, those in the premaxilla at the end of its upper jaw had a D-shaped cross section, an example of heterodonty that is always seen in tyrannosaurids. Special skull features included the rough outer surface of the maxilla, which is the upper jaw bone, and the pronounced crests around its eyes on the lacrimal, postorbital, and jugual bones. The orbit, or the eye socket, was a total oval, somewhere in between the circular shape that’s seen in Gorgosaurus and the ‘keyhole’ shape seen in Tyrannosaurus.

It shared the same body form as the other tyrannosaurids, with a short and S-shaped neck supporting the enormous skull. It walked utilizing its two thick hindlimbs, which ended with its four-toed feet, although the hallux, or the first digit, didn’t contact the ground. In contrast, the forelimbs were very small and had only two digits, although Daspletosaurus had the longest forelimbs in proportion to the body size of any tyrannosaurid. A long and heavy tail operated as a counterweight to the head and the torso, with the center of gravity over the hips.

It belongs to the subfamily Tyrannosaurinae within the family Tyrannosauridae, along with Tarbosaurus, Alioramus and Tyrannosaurus. The animals in this subfamily are more closely related to Tyrannosaurus than to Albertosaurus and are known, with the exception of Alioramus, for their strong build with proportionally larger skulls and longer femora than in the other subfamily, Albertosaurinae.

Daspletosaurus is commonly considered to be closely related to Tyrannosaurus rex, or even a direct ancestor through anagenesis. Gregory Paul reassigned D. torosus to the genus Tyrannosaurus, making the new combination Tyrannosaurus torosus, but this has not been formally accepted.

The type specimen of Daspletosaurus torosus (CMN 8506) is an incomplete skeleton including the skull, shoulder, a forelimb, pelvis, a femur and all of the vertebrae from the neck, hip and torso, as the first eleven tail vertebrae. It was discovered in the year 1921 near Steveville, Alberta by Charles Mortram Sternberg, who thought it was a new species of the Gorgosaurus. It wasn’t until the year 1970 when the specimen was fully described by Dale Russell, who made it the type of a new genus, Daspletosaurus. The type species is Daspletosaurus torosus, the specific name torosus being Latin for “muscular” or “strong”. Aside from the type, there is only one other renowned specimen, RTMP 2001.36.1, a relatively complete skeleton discovered in the year 2001. Both of the specimens were recovered from the Oldman Formation in the Judith River Group in Alberta. The Oldman Formation was deposited during the middle of the Campanian stage in the Late Cretaceous, from about 77 to 76 million years ago. A specimen from the younger Horseshoe Canyon Formation in Alberta has been alternately assigned to Albertosaurus sarcophagus.

Along with the holotype, Russell designed a specimen that was collected by Barnum Brown in the year 1913 as the paratype of D. torosus. This specimen (AMNH 5438) consists of parts of the hindleg, pelvis, and some of its associated vertebrae. It was discovered in the upper area of the Oldman Formation of Alberta. This upper section has been renamed since then to Dinosaur Park Formation, which dates back to the middle Campanian. 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 out of Chicago. It was mounted for display and labeled as Albertosaurus libratus for many years, but after several skull traits were later found to be modeled in plaster, including most of its teeth, the specimen (FMNH PR308) was alternately reassigned to Daspletosaurus. Totally, 8 specimens have been collected from the Dinosaur Park Formation over the years since; most of them were within the boundaries of Dinosaur Provincial Park. Phil Curries believes that the Dinosaur Park specimens are representations of a new species of Daspletosaurus, distinguished by certain features of its skull. Pictures of this new species have been published, but it still awaits a name and a full description in print.

A new tyrannosaurid specimen (OMNH 10131) that included skull fragments, parts of the hindlimb and ribs, was documented from New Mexico in the year 1990 and assigned to the now-defunct genus Aublysodon. Many authors have reassigned this specimen since then, along with a few others from New Mexico, to yet another unnamed species of Daspletosaurus.

A young specimen of the Dinosaur Park Daspletosaurus species (TMP 94. 143. 1) shows bite marks on its face that were inflicted by another tyrannosaur. The bite marks are healed over, showing that the animal endured the bite. A fully grown Dinosaur Park Daspletosaurus also shows tyrannosaur marks from a bite, showing that attacks to their faces weren’t limited to the younger dinosaurs. While it’s possibly that the bites were attributable to other species, aggression between species, including facial biting, is common amongst predators.

Some evidence stating that Daspletosaurus lived in social groups comes from a bonebed found in the Two Medicine Formation of Montana. It includes the remains of three Daspletosaurus, including a large adult, a small juvenile, and another individual of an intermediate size. At least five hadrosaurs are preserved at the same location. Geologic evidence shows that the remains were not brought together by currents of a river but that all of the animals were buried simultaneously in the same location. The hadrosaur remains are scattered and have numerous marks from tyrannosaur teeth, showing that the Daspletosaurus were feeding on the hadrosaurs at the time of death, although the cause of death isn’t known. Currie guesses that the daspletosaurs formed a pack, although it can’t be stated with certainty. Other scientists are skeptical of the evidence for social groups; Brian Roach and Daniel Brinkman have proposed that Daspletosaurs social interaction would have more closely looked like the modern Komodo Dragon, where non-cooperative individuals mob carcasses, often attacking and even cannibalizing each other in the process.

Paleontologist Gregory Erickson and colleagues have performed studies on the growth and life history of tyrannosaurids. Analysis of bone histology is able to determine the age of a specimen when it passes on. Growth rates can be examined when the age of various individuals are mapped with their size on a graph. Erickson has shown that after a long time, as juveniles, tyrannosaurs underwent extreme growth spurts for the span of about four years midway through their lives. After the quick growth phase ending with sexual maturity, growth slowed down significantly in adult animals.

By charting the number of specimens of each age group, Erickson and his colleagues could draw conclusions about the life history in a population of Albertosaurus. Their analysis depicted that while juveniles were rare in the fossil record, subadults in the quick growth phase and the adults were far more common. While this could be because of preservation or collection biases, Erickson hypothesized that the difference was because of low mortality among the juveniles over a specific size, which is also seen in some modern sizable mammas such as elephants. This low mortality might have resulted from a lack of predation, since tyrannosaurs surpassed all contemporaneous predators in size by the age of two. Paleontologists haven’t found enough Daspletosaurus remains for a similar analysis, but Erickson notes that the same usual tend seems to apply.

All of the known Daspletosaurus fossils have been recovered from 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 Cretacous, North America had been divided by the Western Interior Seaway, with much of Montana and Alberta below the surface. However, the uplift of the Rocky Mountains in the Laramide Orogeny to the west, which started during the time of Daspletosaurus, forced the seaway to move eastwards and southwards. Rivers flowed down from the mountains and drained into the seaway, carrying some sediment along with them that formed the Two Medicine Formation, the Judith River Group, and other sedimentary formations in that region. About 73 million years ago, the seaway started to advance towards the west and north again, and the entire region was covered by the Bearpaw Sea, represented throughout the western United States and Canada by the huge Bearpaw Shale.

They lived in an enormous floodplain along the western shore of the interior seaway. Sizable rivers watered the land, occasionally flooding and covering the region with new sediment. When the water was plentiful, the region could support a lot of plant and animal life, but periodic droughts also struck the region, ensuing in mass mortality as preserved in the many bonebed deposits found in Two Medicine and Judith River sediments, including the Daspletosaurus bonebed. Conditions much like them exist today in East Africa. Volcanic eruptions from the west periodically blanket the region with ash, also ensuing in large-scale mortality, while at the same time, enriching the soil for future plant growth. It’s these ash beds that enable precise radiometric dating, too. Fluctuating sea levels also resulted in a variety of other environments at different times and locations within the Judith River Group, including nearshore and offshore marine habitats, deltas and lagoons, and coastal wetlands, in addition to the inland floodplains.

Image Caption: Daspletosaurus mount at the Field Museum of Natural History, Chicago. Credit: ScottRobertAnselmo/Wikipedia (CC BY-SA 3.0)

Daspletosaurus