Utahraptor

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Dromaeosauridae

Genus: Utahraptor

Species: Utahraptor ostrommaysorum

Introduction

Utahraptor was the largest of the predatory dinosaurs informally known as “raptors,” which were one of the dominant groups of terrestrial predators in the Cretaceous period (145–66 million years ago). Utahraptor and its kin descended from a species that first evolved in the mid-Jurassic but did not become a dominant predatory group until the Cretaceous.

In the Triassic period, the continents of the earth were united in a single land mass, known as Pangaea. Dinosaurs first evolved during this period and began to spread across the globe. In the Jurassic, Pangaea began to split as a result of tectonic forces, giving rise to two supercontinents, Gondwana in the south and Laurasia in the north. When the supercontinents divided, species became isolated and developed into a variety of species.

Utahraptor is known from specimens in North and South America, but other dromaeosaurids or “raptors,” have been found in Africa, Europe, and Antarctica. The global scope of the dromaeosaurs leads paleontologists to believe that the group diverged from its ancestor species while Gondwana and Laurasia were still connected by a land bridge.

Classification

Scientists utilize two primary methods to divide organisms into different groups. Traditional Linnaean taxonomy, or “systematics,” creates hierarchical groups based on overall physical similarity, while an alternate system called “cladistics” creates groups called “clades” in an effort to trace the evolutionary relationships between species.

The traditional Linnaean system places Utahraptor in the order Saurischia because of the “lizard-like” orientation of the animal's pelvis. From there, the animal is a member of the suborder Theropoda, or the “beast-footed” dinosaurs, which were bipedal and primarily predatory. The family Dromaeosauridae contains small and mid-sized theropods with bird-like physical adaptations and a modified second digit on each foot.

Cladistics places Utahraptor within the clade Avetheropoda, a division of the theropods on the path toward evolution into birds. The clade Coelurosauria contains avetheropods with enlarged brains and protofeathers. The clade Eumaniraptora contains the already listed important key traits, but with true feathers and brooding behavior. Finally, the dinosaur falls under the clade Dromaeosauridae (enlarged foot claw, skull adaptations) and Dromaeosaurinae, which contains the above traits in addition to enlarged vertebrae and unique skull features.

In 2019, a paper published in The Bulletin of Zoological Nomenclature pointed out that when Utahraptor was initially introduced, its type species name was spelled as ostrommaysi, but that after a suggested correction to ostrommaysorum in the early 2000s, it had commonly been referred to by this name or even, at points, by both. The authors argued that the correction had been erroneous and that the name ostrommaysi should be used.

Anatomy

Reaching lengths of between 6 and 7 meters (19.6–22.9 ft) and standing more than 1.7 meters (5.5 ft) at the hip, Utahraptor was the largest of the dromaeosaurids. Paleontologists estimate that the animal may have weighed between 500 and 700 kilograms (1,102–1,543 lbs).

Though it was as heavy as a grizzly bear, Utahraptor was slender in build compared to other theropod groups. Utahraptor was bipedal with long, muscular rear legs terminating in clawed feet. The second toe of each foot was modified with the addition of a large hooked claw, reaching as long as 9 to 13 inches. Unlike most theropods, dromaeosaurids walked with their second toe held off the ground, using two toes for walking (most dinosaurs walked with three toes on the ground). The animal also had long arms with grasping hands ending in sharp claws.

Utahraptor had forward-facing eyes and a long muzzle. Its jaws were armed with rows of sharp, serrated teeth. Its long tail had vertebrae reinforced with bony, rod-like extensions that stiffened the tail to act like a rudder when running.

It is generally believed that all dromaeosaurids, even large species such as Utahraptor, were covered in feathers. Ancestral species have been found with impressions of feathers, and most paleontologists think it is unlikely that feathers disappeared in descendant species. As a predator, Utahraptor may have been cryptically colored like modern birds of prey, enabling the animal to ambush potential prey.

Intelligence

Utahraptor and the other dromaeosaurs had EQ ratings in the 5 to 5.8 range, making them among the most intelligent of all dinosaurs. Utahraptor may have been as intelligent as a small dog, capable of evaluating its environment and making relatively complex decisions. This adaptation likely allowed the dromaeosaurs to become one of the dominant predatory groups, utilizing intelligence to compete with larger, more physically dominant species.

Reproduction and Population

Scientists are uncertain about the breeding behavior of Utahraptor. Eggs have been found for at least one member of the oviraptors, Late-Cretaceous theropod dinosaurs that were closely related to the dromaeosaurs. These findings indicate that oviraptor males brooded the eggs, and at least some species laid nests with ten or more eggs.

Scientists are uncertain whether Utahraptor provided any care for its young after hatching, but the remains of some oviraptors, as well as other closely related species, indicate that parental care may have been necessary as the hatchlings were not fully able to feed themselves. Some paleontologists have speculated that dromaeosaurs may have remained with their young for a short time after hatching for protection, similar to modern crocodiles.

Utahraptor is known only from a few isolated specimens, so there is little evidence to suggest population size. It may have existed only in small localized populations.

Diet

Utahraptor was most likely an opportunistic predator. An opportunistic predator was a predatory animal with a varied diet based on the availability of prey, and not concentrating on hunting a specific type of prey. It is likely that Utahraptor preyed on dinosaurs, reptiles, amphibians, small mammals, and any other reasonably sized prey item. Areas where Utahraptor fossils have been found are rich with potential prey, ranging from small animals to large herbivorous dinosaurs.

Paleontologists have found groups of dromaeosaur fossils in close association with large herbivore species, and some believe that this provides evidence that dromaeosaurs hunted in packs, perhaps working cooperatively to bring down larger prey. Other paleontologists have suggested that dromaeosaurs may have chaotically mobbed a large prey animal, similar to behavior seen in crocodiles and Komodo dragons (the world's largest lizards).

Behavior

Paleontologists have debated whether dromaeosaurs lived in groups. Evidence of group feeding on large animals has been found and paleontologists have also discovered, from fossil sites in Asia, footprints of dromaeosaurs traveling in groups while not actively hunting. Though evidence was not conclusive, many paleontologists began to assume that many dromaeosaurs displayed at least limited social behavior.

The function of the enlarged second toe has been an area of intense debate among paleontologists. Some believe that the claw was used to slash at prey; others have suggested that the claw allowed dromaeosaurs to climb onto the sides and/or backs of large dinosaurs, inflicting wounds while “riding” the prey animal.

Alternatively, the claw may have been used for grooming, especially if dromaeosaurs were covered in feathers, a behavior seen in some modern birds. Some paleontologists have hypothesized that smaller dromaeosaurs may have used their claws for tree climbing, suggesting that the animals lived partially arboreal lifestyles. It is possible that Utahraptor may have used its claw for a variety of functions, from hunting to locomotion.

Habitat and Other Life Forms

Early Cretaceous North America had a diverse variety of habitats, from open, arid plains to dense, tropical forests. Conifer trees and cycadophytes were the most abundant types of vegetation, though the first flowering plants had begun to appear in some parts of the continent. There were a wide variety of small mammals, reptiles, and amphibians living alongside the dinosaurs, and the first birds appeared at this time. Flying reptiles called pterosaurs were the most common flying vertebrates of the era.

Utahraptor was one of the largest predators in its North American environment. It lived alongside a number of smaller predators, like the theropod Nedcolbertia.

By the Early Cretaceous, most of the large sauropod herbivores were gone in North America, but a few species, including Cedarosaurus, were still present and may have fallen prey to Utahraptors. The early horned dinosaur Zuniceratops was also present in North America, as well as the armored dinosaur Polocanthus, one of the ankylosaurs. In South America, Utahraptor may also have encountered the sail-backed sauropod Amargasaurus.

Research

Paleontologists Robert Gaston, James Kirkland, and Donald Burge discovered the first specimen of Utahraptor from the Cedar Mountain Formation in Utah in 1991. The dinosaur was named for famed paleontologist John Ostrom, an early proponent of the dinosaur-bird connection, and Chris Mays. Utahraptor is similar in size to the “velociraptors” depicted in the film Jurassic Park (1993), though the dinosaur had not yet been described when the film was made. The actual genus Velociraptor, which lived in Asia, was far smaller, about the size of a small dog.

After a leg bone was dicovered in part of the Cedar Mountain Formation in 2001, paleontologists worked diligently to remove a block of stone that they had surmised was once quicksand that had trapped a number of dinosaurs attempting to hunt. Analysis had indicated that the skeletons likely belonged, in part, to more than one Utahraptor. While the large slab of stone, weighing several tons, was eventually removed from the formation by the mid-2010s, paleontologists affiliated with the effort were unable to secure the kind of funding needed to be able to extract the bones from the slab for more detailed investigation. Hoping to gain more insight into Utahraptor, a crowdfunding campaign was started in 2016.

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