Abelisaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Abelisauridae

Genus:Abelisaurus

Species:Abelisaurus comahuensis

Introduction

First discovered and classified in 1985, Abelisaurus identifies a new kind of dinosaur based on the findings of one partial fossil skull. Like many of its contemporaries and predecessors, its remains were unearthed in the geologically fertile ground of southern Argentina. Scientists have pieced together its physiology from this skull and from the fossils of its close relatives. Over time, Abelisaurus gave its name to many of those seemingly related dinosaurs and became its own family, Abelisauridae, within the suborder Theropoda, a group of bipedal, generally carnivorous dinosaurs. This family includes a varied assemblage of dinosaurs, the fossils of which have been discovered in South America, Asia, and Africa. Those in the Abelisauridae family are closely related to the infamous Tyrannosauridae family of North America.

Classification

Classification of Abelisaurus is based on studies of the one fossil, a partial skull, found for the species, as well as on studies of fossils for its closest relatives, including Carnotaurus. Linnaean classification places Abelisaurus in the order Saurischia, as opposed to Ornithischia, based on its suggested hip structure. Saurischian dinosaurs have pubic bones pointed down and forward, like modern reptiles. They also have larger skulls with large fenestrae, or openings, above the jaw and between the eyes and nostrils.

Saurischian dinosaurs are further divided into Sauropoda and Theropoda suborders. Abelisaurus belongs to the Theropoda suborder of two-legged carnivores. Though only a skull survives of the Abelisaurus, studies of related fossils indicate that Abelisaurus likely walked on two legs, and hunted or scavenged for animal food.

Abelisaurus belongs to the Abelisauridae family, characterized by large heads that were nearly as tall as they were long, with a rounded or blunt snout. Members of this family had stocky but powerful hind legs. Their skulls were also marked by grooves, pits and other seemingly decorative shaping.

Scientists may also use another classification system, called “phylogenetic” or cladistic classification. This system classifies animals in family trees, called “cladograms,” grouped by common ancestry. Each split in the tree creates a new group based on shared physical traits. Under a cladistic analysis, Abelisaurus falls under the large Ceratosauria clade, a group of theropod dinosaurs with hollow bones and similarities in jaw structure. Abelisaurus is further classified under the Abelisauridae clade, which is characterized by size, skull structure, and skull ornamentation.

Anatomy

Little is known about the full anatomy of the Abelisaurus based on its incomplete fossil remains. The skull appears to be large, relative to the anticipated body size of the Abelisaurus (which is inferred from its relative Carnotaurus). The one known skull is 85 centimeters (33 in) long and deep relative to its length. Rows of thick, short, sharp teeth appear smaller than other those of other theropods but are still well-suited for chewing and tearing apart flesh. Unlike its relatives, Abelisaurus lacks distinctive horns, bumps, or ridges on the skull but does have many fenestrae, or openings, as well as other superficial markings, such as dips and grooves, that were likely ornamental. The snout itself is rounded, or blunt, and close examination suggests that the Abelisaurus may have sported a mask or crest made from protein-based keratin.

The remainder of Abelisaurus's anatomy is based on assessments of related fossils. Scientists believe that Abelisaurus had a long body complete with a powerful tail, short but strong hind legs, and incredibly short and almost spindly forearms. The dinosaur most likely walked upright, as did other theropods, and used its tail for balance and support. Likely, its hands and feet both featured three-clawed digits. The torso of Abelisaurus would have been substantial relative to its size; however, the projected weight, height, and length of the dinosaur is based on suppositions that have not been conclusively supported by fossil evidence.

Scientific tradition has long held that dinosaurs were ectothermic, or cold-blooded, as are most modern reptiles. Recent studies, however, challenge this belief and assert that some theropods, including Abelisaurus, may have been endothermic, or warm-blooded—or something in between the two.

Intelligence

Generally, slow-moving, herbivorous sauropods have ranked lowest on the EQ scale, while raptors have ranked highest. Theropods, such as Abelisaurus, tend to have higher EQs, around 1.0, as it is thought that carnivorous dinosaurs required higher intelligences in order to hunt and capture prey. Like other theropods, Abelisaurus likely focused its intelligence on finding food and may have had poor eyesight but acute senses of smell and hearing.

Reproduction and Population

Given that fossil evidence is limited to one skull, few conclusions can be drawn about the population or other demographic characteristics of Abelisaurus. Fossil evidence also provides limited insight into the mating, reproductive, and family habits of Abelisaurus. Inferences regarding the reproduction of Abelisaurus must be made based on studies of more complete theropod skeletons and on existing knowledge regarding the behavior of most dinosaurs.

Existing evidence suggest that most dinosaurs, including theropods such as Abelisaurus, were oviparous. This means that they reproduced by laying eggs. However, fossils proving this circumstance for Abelisaurus have yet to be found.

As with modern reptiles and birds, the eggs of the female dinosaur had to be fertilized first by a male dinosaur. Many scientists have studied modern bird and reptile courtship rituals to propose possible mating habits among the dinosaurs. They believe that male theropods likely attracted mates through displays that highlighted their physical features or through combat with other males vying for female attention.

Once fertilized, most dinosaurs are believed to have laid their eggs (generally several at a time) in nests. Theropods like Abelisaurus produced the most bird-like eggs, which are more elongated than modern reptilian eggs. This elliptical shape likely accommodated an air sac that baby dinosaurs needed to survive during their incubation. Land-bound, Abelisaurus likely made its nests in the earth rather than in trees as raptors might have. Fossils do not provide adequate evidence as to whether those nests were made above or below ground, how they were guarded (if at all), or how long eggs took to hatch.

Some dinosaurs are believed to have stayed with their nests throughout incubation. Others are thought to have shared nesting responsibilities with other members of their herd. Still others likely hid and then left their nests. Scientists must find more detailed fossil evidence for Abelisaurus or related theropods before they can determine their nesting habits. Fossil evidence also fails to indicate whether or how long theropods raised their young once they had hatched. Evidence regarding other dinosaur babies suggests that hatchlings remained in their nests for a while and that a parent or other adult dinosaur brought them food or supervised their own search for food.

Diet

Abelisaurus falls under the theropod group, which is known to be carnivorous and largely considered predatory. However, scientific study debates whether meat-eating theropods were primarily hunters or scavengers. Like other theropods, Abelisaurus possessed rows of sharp, pointed teeth designed to consume flesh. However, Abelisaurus's teeth were smaller than other theropods of its size. Fossil evidence is inconclusive as to whether Abelisaurus or its relatives, including the mighty Tyrannosaurus, stalked and killed their own prey or consumed carcasses left by other dinosaurs or both.

The Late Cretaceous period in what is now South America is believed to have provided fertile ground for an abundance of life. Not only did many dinosaurs live in what is now Patagonia (the southern region of South America) during the time of Abelisaurus, but it was during this time that mammalian life began to diversify. For this reason, most scientists believe that Abelisaurus and other theropods fed on herbivorous dinosaurs and on other animals. The large, slow sauropods may have been an important food source.

Like modern carnivores, theropods likely spent much of their time at rest, saving their energy for short bursts in which they would pursue food. When available, they likely gorged themselves on a kill or a scavenged meal.

Behavior

Like other theropods, Abelisaurus likely walked upright on two powerful legs. This physiology enabled the dinosaur to move more quickly than the larger, four-legged sauropods. This may have been an advantage in hunting and likely supported a more solitary lifestyle. Current research suggests that most theropods were solitary, not herd, animals; however, debate exists about whether some carnivores may have socialized and sought the company or protection of their fellow dinosaurs. This type of pack hunting would have been more likely among smaller carnivores, such as Abelisaurus, than among larger ones, who were better able to fend for themselves and therefore less inclined to share a meal.

Thus far, no evidence suggests that the Abelisaurus had armor, horns, or other physiological weapons, aside from teeth and claws. Abelisaurus likely defended itself by running, and like modern reptiles, most dinosaurs probably used their unique markings as camouflage. More fossil evidence needs to be uncovered in order to form specific conclusions about Abelisaurus behavior.

Habitat and Other Life Forms

By the Late Cretaceous, the great supercontinents of Laurasia and Gondwana had split apart, and the continents that we know today had begun to take shape. Though the climate remained warmer than today, global shifts in climate and sea levels were changing the land and the life that lived there. Areas farther from the equator, such as the Patagonian region where Abelisaurus thrived, began to experience more seasonal shifts in weather. New vegetation grew, and animal life diversified.

During the Late Cretaceous, dinosaurs continued to live on each of the new continents. In what is now South America, theropods flourished as did sauropods, which had once been thought to have existed only in the Jurassic period. The Saltasaurus was one sauropod contemporary with Abelisaurus in Argentina. Other contemporaries include Aeolosaurus, Antarctosaurus, Laplatasaurus, Secernosaurus and Velocisaurus.

During the Cretaceous period, angiosperms, or flowering plants, began to grow and spread, significantly altering habitats and food chains. These flowering plants began to replace other plant types, such as ferns and cycads, which had once dominated the landscape. Large flowering trees such as oaks and maples began to thrive. New kinds of insects also emerged, and small mammals made their first appearance, as did snakes.

During this time, ocean life, including fish, marine reptiles, and the ancestors of modern turtles, also diversified. However, no evidence suggests that Abelisaurus interacted with sea life. In the air, pterosaurs had begun to decline and were giving way to growing species of birds.

Research

Research into Abelisaurus is limited to the discovery of one partial skull in southwestern Argentina, in the region of Patagonia, and to inferences made from fossils of related theropods, including Carnotaurus.

Abelisaurus is named for Roberto Abel, one-time director of the Cipolletti Museum in Argentina, who discovered the skull in 1985. Abel uncovered the skull in a Late Cretaceous rock formation known as the Anacleto Formation in the Neuquén Group, a large area of rock formations that range in age from 100 million to 80 million years old. Formative research, including a detailed description of the fossil, was conducted and published by Argentinean paleontologists Joseph F. Bonaparte and Fernando E. Novas. Bonaparte and Novas published their definitive study of the skull under the title “Abelisaurus comahuensis from the Late Cretaceous of Patagoni” in 1985. The holotype, or first known fossil for Abelisaurus, resides at the Cipolletti Museum in Argentina.

Additional research has been conducted into related theropod fossils, such as those of Carnotaurus, also discovered in Argentina in 1985; Noasaurus, discovered in Argentina in 1980; Majungasaurus, uncovered in Madagascar in 1955; and more recently, Aucasaurus, discovered in a nearby formation in Argentina in 2002. Most recently, in 2006, paleontologist Rodolfo A. Coria uncovered the skull of an older relative, Mapusaurus, in the Neuquén Group. Larger than the famed Tyrannosaurus rex, a more distant relative of Abelisaurus, this new fossil provides additional insight into the Abelisauridae family and its relations.

In 2016, an Abelisaurus footprint measuring about 115 centimeters—almost four feet—was found in Bolivia. The footprint became the largest discovered to date.

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