A DYNASTY OF TYRANTS: DASPLETOSAURUS

Daspletosaurus meaning "frightful lizard) is a genus of tyrannosaurid theropod dinosaur that lived in western North America between 77 and 74 million years ago, during the Late Cretaceous Period. Fossils of the only named species (D. torosus) were found in Alberta, although other possible species from Alberta and Montana await description. Including these undescribed species, Daspletosaurus is the most species-rich genus of tyrannosaur.Daspletosaurus is closely related to the much larger and more recent Tyrannosaurus. Like most known tyrannosaurids, it was a multi-ton bipedal predator 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, aspletosaurus 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 rarer than other tyrannosaurids, the available specimens allow some analysis of the biology of these animals, including social behavior, diet and life history.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 tonnes (2.75 short tons), 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 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 rather than blade-like. Unlike its other teeth, those in the premaxilla 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 (upper jaw bone) and the pronounced crests around the eyes on the lacrimal,postorbital, and jugal bones. The orbit (eye socket) was a tall oval, somewhere in between the circular shape seen in Gorgosaurus and the 'keyhole' shape ofTyrannosaurus.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 by Charles Mortram Sternberg, who thought it was a new species ofGorgosaurus. 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 stems δασπλητo-/daspleto- ('frightful') and σαυρος/sauros('lizard'). The type species is D. torosus, which is Latin 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 Formation in the Judith River Group of Alberta. A specimen from the younger Horseshoe Canyon Formation in Alberta has been reassigned to Albertosaurus sarcophagus. The Oldman Formation was deposited during the middle Campanian stage of the Late Cretaceous, from about 77 to 76 Ma (million years ago).