Albertosaurus

Description

Albertosaurus walked on two legs and reached up to 10 m in length — about as long as a school bus. It weighed between 1.3 and 2.5 tonnes. Like other famous members of the Tyrannosauridae family, this animal had powerful hindlimbs and short forelimbs. Each of their hands had only two fingers. At the end of their short necks hung a large, robust skull that reached up to 1 m in length. Albertosaurus and its close relative Gorgosaurus are members of the subfamily Albertosaurinae. Members of this subfamily were more lightly-built and had relatively longer legs than tyrannosaurids belonging to the subfamily Tyrannosaurinae (e.g., Tyrannosaurus rex, Daspletosaurus). Because of these differences, some paleontologists have suggested that Albertosaurus may have been a swift predator.

Albertosaurus had 34–38 teeth in its upper jaw and 28–32 teeth in its lower jaw. Their teeth were serrated, like a steak knife, for slicing through flesh.

Although several theropods were covered with hair-like feathers, there is so far no evidence that Albertosaurus, or any other tyrannosaurid, had feathers. Fossilized skin impressions reveal that Albertosaurus had scales, but these fossils come from only a small portion of the Albertosaurus body. It is theoretically possible that Albertosaurus could have had feathers in places, and used them for courtship or individual identification. Albertosaurus had bony projections, like a small horns, in front of each eye. These horns likely played a role in courtship displays or individual identification and, as such, may have been covered by colourful scales.

Range and Habitat

Fossil remains of Albertosaurus are found in exposures of the Horseshoe Canyon Formation in southcentral Alberta. They are found as far north as Edmonton and as far south as Drumheller, and date to between 73.1 and 69.6 million years ago. Tyrannosaurid teeth, bones, and footprints have also been discovered farther north (Wapiti Formation), west (Brazeau Formation), and south (St. Mary River Formation in Alberta and Montana). However, it is unclear if these fossils belong to Albertosaurus or a different tyrannosaurid species.

Albertosaurus lived on the subtropical coastal plain on the western shore of the Western Interior Seaway, an inland sea that connected the modern-day Arctic Ocean to the Gulf of Mexico, dividing North America in two. In response to dropping sea levels, changes in drainage conditions, and climatic fluctuations over nearly 4 million years, Albertosaurus lived in a number of different habitats. These ranged from warm and humid wetlands to dry and cool forested landscapes. Conifers and gingkoes dominated the vegetation and formed a canopy. Ferns and flowering plants formed the understory.

Reproduction and Development

Although presumed to have laid eggs, no eggs or eggshell fragments have been discovered for Albertosaurus or any tyrannosaurid. This could be because tyrannosaurs nested in environments where it was unlikely for their eggs to be buried by river sediment and eventually fossilized. However, a recent study suggested that tyrannosaurs may have laid soft-shelled eggs, like many lizards and turtles. Soft-shelled eggs are less likely to fossilize than hard-shelled eggs.

Diet

Albertosaurus and all tyrannosaurids were carnivores. Tooth marks on bones left by the predator indicate that mature individuals commonly fed on dinosaurian megaherbivores, such as duckbilled dinosaurs and horned dinosaurs. Large tyrannosaurids like Tyrannosaurus rex and Daspletosaurus are known to have pulverized and ingested bones while feeding. However, different jaw adaptations in Albertosaurus and Gorgosaurus suggest that bone-crushing may have been less common in these animals.

Behaviour

The strong jaws of Albertosaurus allowed it to capture and hold onto struggling prey. By comparison, most other theropods only delivered slashing bites, much like Komodo dragons today. The small forelimbs of tyrannosaurids were too short to be used to capture prey.

The social behaviour of Albertosaurus is a subject of debate. A bonebed discovered in Dry Island Buffalo Jump Provincial Park contains the remains of numerous Albertosaurus individuals (between 12 and 26 depending on the estimation methods). The age of these individuals ranges from 2 to 28 years old. Some have interpreted this fossil accumulation as evidence of gregarious behaviour in Albertosaurus. Others, however, have cautioned that the bone accumulation could be the result of a catastrophic event. This event could have wiped out a group of individuals that had gathered together during a drought. Therefore, the fossil assemblage does not necessarily reflect typical Albertosaurus behaviour.

Discovery

Albertosaurus was first discovered in 1884 along the Red Deer River, near what would become the town of Drumheller, Alberta. Joseph Burr Tyrrell, a geologist employed by the Geological Survey of Canada, made the discovery. Represented by an incomplete skull with teeth, Tyrrell’s specimen was the first meat-eating dinosaur discovered in Canada and the first good dinosaur skeleton discovered in Alberta. The skull was initially believed to belong to a previously known theropod named Laelaps incrassatus. It wasn’t until the discovery and description of Tyrannosaurus rex in 1905 that Tyrrell’s specimen was recognized as a distinct theropod species. It was named Albertosaurus in honor of the province of Alberta, which had been formally established that same year.

In 1910, an expedition of the American Museum of Natural History, led by Barnum Brown, discovered a bonebed containing the remains of several Albertosaurus individuals. Over the span of about two weeks, the expedition collected the skeletal remains of at least nine individuals, predominantly leg and foot bones. For decades, the material was casually mentioned in publications, but never the subject of detailed study. In 1997, Philip J. Currie, at the time working for the Royal Tyrrell Museum of Palaeontology, rediscovered Brown’s Albertosaurus bonebed based on 1910 field photographs and notes. It was located in what is today Dry Island Buffalo Jump Provincial Park. From 1998 to 2010, the Royal Tyrrell Museum and the University of Alberta renewed excavation efforts. Their work led to the recovery of hundreds of bones representing several individuals (see Behaviour, above). To this day, paleontologists have discovered approximately 13 skeletons and at least one bonebed of Albertosaurus.