Albertosaurus

***Albertosaurus*** (/ælˌbɜːrtəˈsɔːrəs/; meaning "Alberta lizard") is a genus of tyrannosaurid theropod dinosaurs that lived in western North America during the Late Cretaceous Period, about 70 million years ago. The type species, *A. sarcophagus*, was apparently restricted in range to the modern-day Canadian province of Alberta, after which the genus is named. Scientists disagree on the content of the genus, with some recognizing *Gorgosaurus libratus* as a second species.

As a tyrannosaurid, *Albertosaurus* was a bipedal predator with tiny, two-fingered hands and a massive head that had dozens of large, sharp teeth. It may have been at the top of the food chain in its local ecosystem. While *Albertosaurus* was very large for a theropod, it was much smaller than its larger and more famous relative *Tyrannosaurus*, growing to nine metres long and weighing less than possibly 2 metric tons.

Since the first discovery in 1884, fossils of more than 30 individuals have been recovered, providing scientists with a more detailed knowledge of *Albertosaurus* anatomy than is available for most other tyrannosaurids. The discovery of 26 individuals at one site provides evidence of pack behaviour and allows studies of ontogeny and population biology, which are impossible with lesser-known dinosaurs.

## Description

*Albertosaurus* was smaller than some other tyrannosaurids, such as *Tarbosaurus* and *Tyrannosaurus*. Typical *Albertosaurus* adults measured up to 9 m (30 ft) long, while rare individuals of great age could grow to be over 10 metres (33 feet) long. Several independent mass estimates, obtained by different methods, suggest that an adult *Albertosaurus* weighed between 1.3 tonnes and 1.7 tonnes (1.9 tons).

*Albertosaurus* shared a similar body appearance with all other tyrannosaurids. Typically for a theropod, *Albertosaurus* was bipedal and balanced the heavy head and torso with a long tail. However, tyrannosaurid forelimbs were extremely small for their body size and retained only two digits. The hind limbs were long and ended in a four-toed foot on which the first digit, called the hallux, was short and did not reach the ground. The third digit was longer than the rest. *Albertosaurus* may have been able to reach walking speeds of 14−21 km/hour (8−13 mi/hour). At least for the younger individuals, a high running speed is plausible.

### **Skull and teeth**

The massive skull of *Albertosaurus*, which was perched on a short, S-shaped neck, was about 1 metre (3.3 feet) long in the largest adults. Wide openings in the skull (fenestrae) reduced the weight of the head while also providing space for muscle attachment and sensory organs. Its long jaws contained, both sides combined, 58 or more banana-shaped teeth; larger tyrannosaurids possessed fewer teeth, *Gorgosaurus* at least 62. Unlike most theropods, *Albertosaurus* and other tyrannosaurids were heterodont, with teeth of different forms depending on their position in the mouth. The premaxillary teeth at the tip of the upper jaw, four per side, were much smaller than the rest, more closely packed, and D-shaped in cross section. Like with *Tyrannosaurus*, the maxillary (cheek) teeth of *Albertosaurus* were adapted in general form to resist lateral forces exerted by a struggling prey. The bite force of *Albertosaurus* was less formidable, however, with the maximum force, by the hind teeth, reaching 3,413 Newtons. Above the eyes were short bony crests that may have been brightly coloured in life and used in courtship to attract a mate.

William Abler observed in 2001 that *Albertosaurus* tooth serrations resemble a crack in the tooth ending in a round void called an ampulla. Tyrannosaurid teeth were used as holdfasts for pulling flesh off a body, so when a tyrannosaur pulled back on a piece of meat, the tension could cause a purely crack-like serration to spread through the tooth. However, the presence of the ampulla distributed these forces over a larger surface area, and lessened the risk of damage to the tooth under strain. The presence of incisions ending in voids has parallels in human engineering. Guitar makers use incisions ending in voids to, as Abler describes, "impart alternating regions of flexibility and rigidity" to wood they work. The use of a drill to create an "ampulla" of sorts and prevent the propagation of cracks through material is also used to protect aircraft surfaces. Abler demonstrated that a plexiglass bar with incisions called "kerfs" and drilled holes was more than 25% stronger than one with only regularly placed incisions. Unlike tyrannosaurs, ancient predators like phytosaurs and *Dimetrodon* had no adaptations to prevent the crack-like serrations of their teeth from spreading when subjected to the forces of feeding.

## Classification and systematics

*Albertosaurus* is a member of the theropod family Tyrannosauridae, in the subfamily Albertosaurinae. Its closest relative is the slightly older *Gorgosaurus libratus* (sometimes called *Albertosaurus libratus*; see below). These two species are the only described albertosaurines; other undescribed species may exist. Thomas Holtz found *Appalachiosaurus* to be an albertosaurine in 2004, but his more recent unpublished work locates it just outside Tyrannosauridae, in agreement with other authors.

The other major subfamily of tyrannosaurids is the Tyrannosaurinae, which includes *Daspletosaurus*, *Tarbosaurus* and *Tyrannosaurus*. Compared with these robust tyrannosaurines, albertosaurines had slender builds, with proportionately smaller skulls and longer bones of the lower leg (tibia) and feet (metatarsals and phalanges).