Dromaeosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Dromaeosauridae

Genus: Dromaeosaurus

Species: Dromaeosaurus albertensis

Introduction

Dromaeosaurus was a small, predatory dinosaur from the dinosaur group informally called the “raptors.” The species is known from scattered fossil remains but may have been relatively common in North America. On a larger scale, Dromaeosaurus was representative of a successful group of predators that ranged across North America, Europe, South America, and much of Asia.

During the Triassic period (between 251 and 201 million years ago), there existed a supercontinent called Pangaea. During this time, the dinosaurs evolved and spread across the world, populating various regions. During the subsequent Jurassic and Cretaceous periods, the supercontinent of Pangaea broke apart, giving rise to two supercontinents: Laurasia, containing most of what would become the Northern Hemisphere continents, and Gondwana, containing much of the Southern Hemisphere continents. Dinosaur species that evolved and radiated during the Triassic period became geographically isolated when Pangaea began to split, evolving into a variety of unique species.

The dromaeosaurs, or “raptors,” were successful predators in North America, Asia, and South America, and were among the groups of dinosaurs most closely related to modern birds. Discoveries of the bird-like fossils of species like Dromaeosaurus indicate that the species evolved from a common ancestor that lived in Laurasia after the breakup of Pangaea.

Classification

According to Linnaean taxonomy, Dromaeosaurus belongs to the order Saurischia, or “lizard-hipped” dinosaurs, which shared a common arrangement of their pelvic girdle inherited from reptile ancestors. From there, Dromaeosaurus is included in the suborder Theropoda, which contains other bipedal, predatory dinosaurs, and then into the family Dromaeosauridae, the members of which are united by features of the feet, tail, and skull.

Cladistic analysis also places Dromaeosaurus into the clade Theropoda, and then into the clade Coelurosauria, a group with complex chambers in their vertebra and holes called “fenestrae” in the front of their skulls, as well as common traits of the feet and jaw. Further, the animal is a member of the Maniraptora clade, based largely on hand structure, and then the Deinonychosauria, the members of which share the characteristic claws of the raptors, as well as other skull and limb developments.

As with any dynamic study in the field of science, cladistic analyses serve as hypotheses and can change over time as new specimens are discovered and as the science advances. In 2017 Matthew Baron and his colleagues posited a common ancestor for theropods like Velociraptor, and ornithischians like Stegasaurus, and proposed they be reclassified into a new clade, Ornithoscelida.

Anatomy

Dromaeosaurus was a relatively small dinosaur, ranging from 1.8 to 1.9 meters (6 to 6.5 feet) as an adult, nearly half of which was the animal's tail. Dromaeosaurus stood about 0.8 meters (2.5 feet) tall and weighed between 15 and 35 kilograms (30 to 77 pounds), which is about the weight of a medium to large dog.

Dromaeosaurus had longer hind limbs than forelimbs, with four toes on each hind foot. The animal walked on two of its rear toes, while a third toe was equipped with a larger, sickle-shaped claw that was held above the ground as the animal moved. The forelimbs were thinner than the hind limbs, with long, grasping hands that featured sharp claws. The animal had a short, compact torso, with a large pelvic area and a long, stiffened tail.

The skull of Dromaeosaurus was equipped with fenestrae, or open spaces, which both increased strength and reduced weight. The animal's jaws were relatively thick and short, containing rows of sharp teeth. Dromaeosaurus had large, forward-facing eyes. Paleontologists now believe that many of the dromaeosaurs were partially covered by feathers or “protofeathers” in life. Because the animal was a hunter, it may have been cryptically colored, increasing its ability to stalk and ambush prey.

Intelligence

Paleontologist James Allen Hopson found that most dinosaurs' encephalization quotient (EQ) fell into the range of 0.1 to 2.0, while a few, including the dromaeosaurs, had EQ ratings as high as 5.8. The scale extends to 8.0, the approximate value calculated for the human brain.

An EQ rating in the range of 5.5 to 6.0 places dromaeosaurs in a similar category with many predatory mammals, like cats and dogs. Dromaeosaurus may have evolved advanced cognitive abilities as a way to increase success in hunting. Most of the herbivores living in the Late Cretaceous had EQ ratings below 1.0, making them significantly less intelligent than the dromaeosaurs. Advanced intelligence may have enabled these animals to outthink their prey and thereby helped the relatively small dromaeosaurs to tackle prey many times their size.

Reproduction and Population

Like most dinosaurs, Dromaeosaurus was likely oviparous, meaning it reproduced by laying eggs. No nests or eggs have been found for the genus specifically, but eggs of other closely related species have been found. Some members of the Maniraptora group appeared to lay eggs two at a time and to fill the nest with several sets of eggs. In addition, evidence from other genera suggests that some male maniraptorids brooded their eggs, like modern birds.

Dromaeosaurus is known only from a few fossils, leading to the speculation that the animal may have been rare. There are insufficient fossils to determine whether males and females of the species displayed any physical differences, a phenomenon known as "sexual dimorphism."

Diet

Dromaeosaurus had the pointed teeth and sharp claws of a predator, and most paleontologists believe that it hunted a variety of prey, ranging from small animals to larger herbivorous dinosaurs. The claws of both the feet and hands suggest that the animal could also use its claws to grasp and wound its prey in addition to its teeth.

Behavior

Paleontologists have found numerous fossil beds containing adult dromaeosaurs in close proximity to herbivorous dinosaurs. Some paleontologists have theorized that dromaeosaurs may have hunted in packs, though an alternate theory suggests that Dromaeosaurus and other raptors gathered in chaotic groups around prey.

Paleontologists have also found fossilized tracks of dromaeosaurs in Asia, indicating small groups traveling together, though the footprints indicate that the animals were not in the process of hunting. This provides evidence for dromaeosaurs displaying some degree of social behavior. More evidence is required to determine whether Dromaeosaurus or its relatives lived in groups or displayed other social behaviors.

The enlarged claw found in Dromaeosaurus and its relatives has also been a source of debate among paleontologists. Anatomical analyses indicate that dromaeosaurs used the claw to slash or puncture their prey, causing deep wounds. Alternatively, the claws may have allowed the dromaeosaurs to climb onto their prey or pin them down in order to inflict wounds. Another theory holds that dromaeosaurs used their claws to climb trees, perhaps sleeping in trees for safety or using trees to ambush or search for prey. The predation and climbing theories are not seen as mutually exclusive.

Habitat and Other Life Forms

In the Late Cretaceous, the middle section of North America was covered in a shallow inland sea. Dromaeosaurus may have lived in an environment that included shallow flood plains, open scrub, and various types of forest. Gymnosperms like conifers and cycads were the dominant plants, though the first angiosperms evolved during this period and were beginning to spread.

There were a wide variety of reptiles, amphibians, and small mammals living in North America, many closely resembling extant species. While the first birds appeared in this period, the skies were dominated by pterosaurs, flying reptiles that evolved alongside the dinosaurs.

Dromaeosaurus was one of several dromaeosaur species living in North America, including Saurornitholestes. There were also a variety of other theropods, including “ostrich mimic” dinosaurs like Struthiomimus and large predators like Gorgosaurus, which is related to the giant tyrannosaurid predators. Herbivores in the environment included several varieties of ceratopsians, or “horned dinosaurs” like Pachyrhinosaurus, and hadrosaurs, or “duck-billed” dinosaurs like Edmontosaurus.

Research

The first skeletons of Dromaeosaurus were found by American paleontologist Barnum Brown in 1914 in fossil beds in Alberta, Canada. The fossil beds of what became Dinosaur Provincial Park in Alberta revealed a wide variety of fossils, including both herbivorous species and predators. Further specimens were uncovered in Alberta and then later in Montana, indicating that the species was widespread.

Studies of Dromaeosaurus and other closely related predators helped paleontologists to demonstrate a strong evolutionary connection between dinosaurs and birds. It is thought that ancestors of Dromaeosaurus gave rise to the forerunners of modern birds and both groups shared a variety of features.

Research on dromaeosaur teeth has improved scientific understanding of their feeding behaviors as well as identification. Examination of the wear patterns on indicate Dromaeosaurus and Gorgosaurus had small rectangularly serrated teeth while Saurornitholestes pointed serrations. A lack of pitting suggests the teeth either did not contact many bones or hard materials or were replaced frequently. Based on simulation of bite-force angles, research Angelica Torices and her team concluded dromaeosaurs ate using a "puncture-and-pull" motion, in which they bit into the prey and then tugged. This would likely correspond with selective flesh removal, ingestion whole, or a softer diet. The stronger bite force of Dromaeosaurus also suggests it could withstand more tooth pressure and thus feed easily on struggling prey. Another study by Derek Larson and Philip Currie found variations in serration size and crown size and shape even within a single species, however, making them less suited to taxonomic identification than other traits.

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