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).

A DYNASTY OF TYRANTS: GORGONOSAURUS

Gorgosaurus meaning "fierce lizard" is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 76.5 and 75 million years ago. Fossil remains have been found in the Canadian province of Alberta and possibly the U.S. state of Montana. Paleontologists recognize only the type species, G. libratus, although other species have been erroneously referred to the genus.Like most known tyrannosaurids, Gorgosaurus was a bipedal predator weighing more than a metric ton as an adult; dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was most closely related to Albertosaurus, and more distantly related to the larger Tyrannosaurus. Gorgosaurus and Albertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts consider G. libratusto be a species of Albertosaurus; this would make Gorgosaurus a junior synonym of that genus.Gorgosaurus lived in a lush floodplain environment along the edge of an inland sea. An apex predator, it was at the top of the food chain, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Though these animals were roughly the same size, there is some evidence of niche differentiation between the two. Gorgosaurus is the best-represented tyrannosaurid in the fossil record, known from dozens of specimens. These plentiful remains have allowed scientists to investigate its ontogeny, life history and other aspects of its biology.Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, closer in size toAlbertosaurus and Daspletosaurus. Adults reached 8 or 9 meters (26 to 30 ft) from snout to tail.Paleontologists have estimated full-grown adults to weigh more than 2.4 tonnes (2.7 short tons). The largest known skull measures 99 centimeters (39 in) long, just slightly smaller than that of Daspletosaurus.As in other tyrannosaurids, the skull was large compared to its body size, although chambers within the skull bones and large openings (fenestrae) between bones reduced its weight. Albertosaurus and Gorgosaurus share proportionally longer and lower skulls than Daspletosaurus and other tyrannosaurids. The end of the snout was blunt, and the nasal and parietal bones were fused along the midline of the skull, as in all other members of the family. The eye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera. A tall crest rose from the lacrimal bone in front of each eye, similar to Albertosaurus and Daspletosaurus. Differences in the shape of bones surrounding the brain set Gorgosaurusapart from Albertosaurus.

A DYNASTY OF TYRANTS: ALECTROSAURUS

Alectrosaurus   is a genus of tyrannosaurid theropod dinosaur from the Late Cretaceous Period of Inner Mongolia. It was a bipedal carnivore with a body shape similar to its much larger relative,Tyrannosaurus rex. Alectrosaurus was much smaller though, most likely less than five meters (17 ft) long.

The generic name Alectrosaurus can also be translated as "alone lizard," and is derived from the Greek words alektros("unmarried") and sauros ("lizard"). At the time of its discovery, it was unlike any other Asian carnivore known. There is one named species (A. olseni), which is named in honor of George Olsen, who discovered the first specimens in 1923 on the thirdAmerican Museum of Natural History expedition to Mongolia. Both genus and species were named by American paleontologist Charles Gilmore in 1933. In a 2001 study conducted by Bruce Rothschild and other paleontologists, 23 foot bones referred to Alectrosaurus were examined for signs of stress fracture, but none were found.The holotype (AMNH 6554), or name-bearing specimen, of Alectrosaurus is a hind limb discovered in the Iren Dabasu Formation of the Inner Mongolia Autonomous Region (Nei Mongol Zizhiqu) of the People's Republic of China. Forelimb material in the type specimen was later found to be misidentified therizinosauroid remains.The age of this geologic formation is not clear, but is commonly cited as the Campanian stage of the Late Cretaceous Period, about 83 to 74 million years ago.

More material, including comparable hind limb material as well as skull and shoulder elements, has been referred to Alectrosaurus. These fossils were found in the Bayan Shireh Formation of Outer Mongolia, a formation which is also of uncertain age. It may possibly extend into the early Campanian, but recent estimates suggest it was deposited from Cenomanian through Santonian times. Iren Dabasu and Bayan Shireh dinosaur faunas are similar, but van Itterbeecka et al. claimed that the Iren Dabasu is probably Campanian-Maastrichtian in age and possibly correlated with the Nemegt Formation, so it is not surprising that a species ofAlectrosaurus would be found there.

Furthermore, several more partial skeletons may have been found in both Inner and Outer Mongolia. These remain undescribed as of early 2007.Alectrosaurus is undoubtedly a tyrannosauroid, but due to its fragmentary nature, there is presently very little confidence in restoring its relationships with other tyrannosauroids and many recent cladistic analyses have omitted it altogether. One study recovered Alectrosaurus at no less than eight equally parsimonious positions in a tyrannosauroid cladogram.Some paleontologists have considered Alectrosaurus olseni to be a species of Albertosaurus.