Albertosaurus

Kingdom: Animalia

Phylum: Chordota

Class: Reptilia

Order: Saurischia

Family: Tyrannosauridae

Genus:Albertosaurus

Species:Albertosaurus sarcophagus

Introduction

Although tyrannosaurids such as Albertosaurus and Tyrannosaurus existed during the Late Cretaceous period (100–66 million years ago), Albertosaurus went extinct millions of years earlier than Tyrannosaurus.

Albertosaurus was a large and formidable predator that roamed the earth some 10 million years before its more famous cousin, Tyrannosaurus. Although it was the largest carnivore to exist when it was alive, it was lighter and faster than the massive Tyrannosaurus. Fossil finds of Albertosaurus have provided very strong evidence to suggest that this species was a pack predator.

Classification

Scientists use two primary methods to organize animals into different groups. Linnaean taxonomy groups organisms in a hierarchical fashion according to overall physical similarity, while an alternate system called “cladistics” attempts to create groups that better reflect the evolutionary relationships between species.

Under the Linnaean system, dinosaurs are divided into one of two orders: Ornithischia (or bird-hipped dinosaurs) and Saurischia (or lizard-hipped dinosaurs). While it may seem logical to assume today's avian dinosaurs (birds) are descendents of the bird-hipped dinosaurs, it is actually from the lizard-hipped dinosaurs that modern-day birds evolved. Scientifically, this is known as convergent evolution, whereby the backward pointing pubis seen in ornithischian dinosaurs and birds actually evolved separately.

The Saurischia order comprises two major groups—the large herbivorous Sauropoda species and the Theropoda species. The bipedal theropods, which include Albertosaurus, are among the most diverse and oldest known dinosaurs, first appearing during the Late Triassic period about 230 million years ago. Although generally considered to be carnivorous, recent evidence has suggested that certain species within this group had a much more varied, even herbivorous, diet.

A cladistic analysis places Albertosaurus into the tetanurae clade. The largest and most advanced theropod clade, it first appeared in the fossil record during the Early or Middle Jurassic period (201.6–161 million years ago). The tetanurae are characterized by their “stiff tails” and are considered more closely related to birds than to ceratosaurs, including dinosaurs within the coelurosauria. Coelurosaurian species are further divided to include the Tyrannosauridae family, in which both Tyrannosaurus and Albertosaurus are located. Albertosaurus is, however, more closely related to Gorgosaurus, which is located in the subfamily albertosaurinae with Albertosaurus. There is significant debate, however, as to whether Gorgosaurus is a species in its own right or whether the specimens found are, in fact, juvenile Albertosaurus.

Anatomy

Albertosaurus weighed up to 2,500 kilograms (5,500 lbs) and reached more than 8 meters (26 feet) in length. Albertosaurus was the largest carnivore of its time, although considerably lighter and swifter than its well-recognized relative, Tyrannosaurus. Despite this smaller stature, it still possessed the large head, impressive jaw, deep-chest, and thick, tapering tail characteristics of the theropod dinosaurs. It had at least fourteen teeth in its lower jaw and seventeen in its upper jaw, which were long, sharp, and serrated and useful for tearing apart its prey.

Like all lizard-hipped, bipedal theropods, Albertosaurus possessed a backwards-facing pubis and extremely long and strong hind limbs that ended with three (or possibly four) clawed toes on each foot. Albertosaurus also possessed the characteristic tyrannosaurid forearms, which were very short and ended with only two claws on each hand. These reduced forearms would have had an impact on its hunting ability and influenced how this dinosaur captured and consumed prey. As with Tyrannosaurus, current theory states that these species possessed a bird-like posture, with their backbones positioned horizontal to the ground and their tails raised during locomotion for balance.

Intelligence

The most often used technique to measure and compare intelligence in animal species is the encephalization quotient (EQ), calculated as a body size to brain size ratio. This figure can reach up to 5.8 in dinosaurs; however, most dinosaurs fall within a range of 0.1 to 2.0 on a scale that extends to an upper limit of 8.0, the value calculated for the human brain. Although all dinosaur species compare unfavorably to animal species today, recent evidence and techniques in neural research suggest that dinosaurs may have been smarter than previously thought.

With an EQ score falling between 1.0–1.9, Albertosaurus was at the higher end of dinosaur intelligence. Many paleontologists theorize that carnivorous species tend to have higher intelligence than their herbivorous counterparts, due to the need for greater cognitive capability and behavioral flexibility to hunt successfully.

Reproduction and Population

Population numbers of particular dinosaur species are difficult to determine from fossilized remains. However, Albertosaurus appeared to be quite widespread throughout western North America and is considered to be the most common carnivore to have roamed that area during the Late Cretaceous period.

As with all dinosaurs, Albertosaurus was an egg-laying animal (oviparous), and would have laid a number of eggs per clutch. While fossilization destroys soft tissue, making it almost impossible to determine sex, some species show signs of sexual dimorphism, which provides clues to mating and reproductive behavior.

Diet

Albertosaurus was the largest carnivore for its time. It roamed what is now western North America and hunted other dinosaur species for food. Like other tyrannosaurids, with its large head, strong jaw, and sharp teeth, Albertosaurus most likely used its teeth to “grip-and-rip” prey, known as the “puncture-pull” feeding hypothesis. Albertosaurus would most likely have hunted the Late Cretaceous herbivore species, including hadrosaurs, horned ceratopsians, and the armored dinosaurs, the ankylosaurs.

As with Tyrannosaurus, there has been debate for many years as to whether Albertosaurus was an active hunter only or a scavenging feeder. The majority of researchers currently believe, however, that it is more likely that Albertosaurus was in fact both, hunting for the most part and scavenging when the opportunity arose.

Behavior

Behavior is difficult to determine for prehistoric animals. Fossil records are often incomplete, with some species named from only one, often partial, skeleton. With so little evidence, it is almost impossible to elucidate social behavior. Such difficulties often lead to scientific hypotheses based on the behavior of extant animals that share a similar ecological niche to extinct species.

For Albertosaurus, however, the discovery of one particular bone bed has led to interesting theories regarding its behavior. One of the most contentious issues is the possibility that Albertosaurus was a pack animal, or at the very least, may have hunted in packs.

The discovery of twenty-six individual dinosaurs of varying ages, from juvenile to adult, within the one site suggests that this species formed non-segregated hunting packs. It is theorized that the faster juveniles were more likely to match pace with prey species and may have assisted in the hunt by chasing prey towards waiting adults.

While this theory is supported by current bone-bed evidence, different theories are also possible based on the same evidence. Other researchers claim that the location of so many individuals in the one bone bed may not be related to pack hunting at all but, rather, may be due to the convergence of dinosaurs for some other reason, such as mass opportunistic scavenging of a carcass or enforced close proximity due to a natural disaster.

Habitat and Other Life Forms

The continents continued to drift during the Late Cretaceous period (100–66 million years ago), and the world's major land masses became increasingly isolated. Dinosaur species underwent their final radiation and geographical differentiation during this time, before their complete extinction at the end of the Cretaceous period some 66 million years ago, known as the K/T extinction event.

The tyrannosaur species lived during the Late Cretaceous, which was characterized by a tropical, although cooling, climate. The polar ice caps were non-existent and little change occurred in the thermal gradient from the equator to the poles. This mild climate allowed dinosaur species such as Albertosaurus to migrate between the warm-temperate areas of America to the far north, cool-temperate areas of Canada. While gymnosperms still dominated many areas in the Northern Hemisphere, the Late Cretaceous was also characterized by the rapid radiation and diversification of angiosperms. Much like the later Tyrannosaurus, Albertosaurus favored the wide forested river valleys of western North America, which were comprised of tree species such as firs, sequoia, sycamore, and magnolia.

Most likely Albertosaurus was at the top of the food chain within its environment. It would have lived alongside other carnivore species and hunted the Late Cretaceous herbivore species, including the horned ceratopsians, the armored dinosaurs, the ankylosaurs, and the duck-billed dinosaurs.

Research

The first Albertosaurus fossil was discovered by Joseph B. Tyrrell in the Red Deer River region of Alberta, Canada, in 1884. Although the species was originally named Laelaps and then Dryptosaurus, it was reclassified as Albertosaurus sarcophagus by Henry Fairfield Osborn from the American Museum of Natural History in 1905.

Perhaps the most famous Albertosaurus discovery was in 1910, when twenty-six individuals were found in the Dry Island bonebed in Alberta. Later research on these remains in 2006 demonstrated that juvenile mortality for this species was relatively low. In 2010, the health of these specimens was reported on, showing that the species had a relatively low number of abnormalities.

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