Deinonychus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Dromaeosauridae

Genus: Deinonychus

Species: Deinonychus antirrhopus

Introduction

Deinonychus was a predatory dinosaur that lived in Early Cretaceous North America. Deinonychus was the inspiration for the predatory raptors in the Steven Spielberg film Jurassic Park (1993), though they were called Velociraptors, a related species that was much smaller.

Prior to the mid-Jurassic period, all the landmasses of the world were joined in a single continent known as Pangaea. Tectonic activity eventually broke up Pangaea into two supercontinents, Gondwana in the south and Laurasia in the north. Many dinosaur groups spread across the globe in the Triassic and Jurassic periods, giving rise to a variety of new species as the continents split and populations became geographically isolated.

The discovery of groups of Deinonychus fossils led paleontologists to speculate that the genus was successful and widespread, and it has contributed to the ongoing debate about social behavior among the dinosaurs. Predatory dinosaurs like Deinonychus are also considered the closest dinosaur relatives to the birds.

Classification

Paleontologists use two different methods to classify dinosaurs. Linnaean taxonomy groups animals according to overall physical similarity and has been used to create the modern categories of life that are used today, including grouping organisms into kingdoms, phyla, classes, etc.

An alternate, more dynamic method, called “cladistics,” attempts to place organisms into groups that better reflect the evolutionary relationships between species. Cladistic analysis utilizes key characteristics and attempts to create groups, called “clades,” which match species with related species who inherited the trait in question from a common ancestor.

Using both cladistics and Linnaean taxonomy, paleontologists have placed Deinonychus within the suborder Theropoda, a group that contains all the bipedal, carnivorous dinosaurs. The theropod group contained both smaller predators like Deinonychus and a separate lineage that produced the larger carnivores such as Allosaurus and Tyrannosaurus. Deinonychus was also a member of the family Dromaeosauridae, sometimes called the “raptors.” These dinosaurs are known for their bird-like anatomy and predatory adaptations.

A cladistic analysis places Deinonychus as a member of the coelurosaurs, a clade of the theropods distinguished partially by their rotating ankles and well-developed grasping arms.

In 2017 Matthew Baron and his colleagues posited a common ancestor for therapods, such as Deinonychus, and the ornithischians, such as Triceratops, and proposed they be reclassified into a new clade, Ornithoscelida.

Anatomy

An adult Deinonychus was between 3 and 3.3 meters in length (9 and 11 feet) and weighed 80 to 100 kilograms (176 to 220 pounds). Deinonychus was bipedal, with long back legs adapted for speed. Another key dromaeosaur characteristic is the “killing claw” on the first digit of their feet. These sickle-shaped claws could grow as large as 12.5 centimeters (5 inches). When the large claw was stretched upward, tendons would lock the toe in place. Muscles could then release the claw, which swung downward with force and was presumably used for gouging and slicing prey.

Deinonychus had well-developed arms with grasping hands armed with sharp claws. Deinonychus also had a narrow skull with forward facing eyes and wide jaws with rows of sharp teeth, as well as a long, and presumably stiff, tail that is believed to have functioned to maintain balance.

Early artists’ renderings of Deinonychus depicted the animal with scaly, reptile-like skin, but studies of similar species, like Velociraptor, indicate a covering of simple feathers, suggesting that Deinonychus might also have possessed simple feathers. Unlike flying birds, these early feathers were not used for flight but provided insulation and protection.

Intelligence

University of Chicago paleontologist James A. Hopson used the ratio of brain to body mass, called the encephalization quotient (EQ), to measure the intelligence of several groups of dinosaurs. Hopson found that most dinosaurs fell within the range of 0.1 to 2.0, with a smaller number reaching as high as 5.8. (The scale extends to an upper limit of 8.0, with humans measuring an EQ of 7.44; and dolphins, 5.31.)

The dromaeosaurs, like Deinonychus, Velociraptor and Utahraptor, rank in the range of 5.5 to 5.8, by far the highest EQ ratings of any dinosaur group. While some paleontologists argue that EQ is a questionable measure of intelligence, in general, predators tend to have higher EQ ratings than herbivores, enabling them to outsmart their prey. In addition, some evidence suggests that dromaeosaurs were social animals and tended to be more intelligent than many solitary animals, allowing the animals to negotiate the complex demands of social interactions.

Reproduction and Population

Paleontologists agree that Deinonychus was oviparous, or egg-laying, and some believe that Deinonychus might have brooded its eggs, a behavior common to many birds and some reptiles. Paleontologists know little about the reproductive behavior of Deinonychus, and there is insufficient evidence to determine whether males and females differed in size or behavior.

Research indicates that the animal may have reached sexual maturity in 3 to 4 years, the approximate age at which the dinosaurs reached their full adult size. Discoveries of groups of Deinonychus fossils indicate that the animal may have been fairly common throughout its range, though there is insufficient evidence to estimate population density.

Diet

Deinonychus had the teeth and claws of a predator, so paleontologists believe they were primarily hunters and not scavengers or omnivores. Some fossils of Deinonychus have been found close to fossils of Tenontosaurus tilletti, an herbivorous dinosaur common in the Early Cretaceous. Some paleontologists believe that the Deinonychus specimens were in the process of hunting the Tenontosaurus when they perished.

Deinonychus was most likely an opportunistic predator, taking any prey it encountered, large or small. Discoveries of Deinonychus in close association with larger dinosaurs has led some paleontologists to theorize that Deinonychus's claws may have allowed it to hunt larger prey.

Behavior

One of the primary debates regarding the behavior of Deinonychus and other dromaeosaurs is whether or not they lived and hunted in packs. When paleontologists discovered Deinonychus specimens of different ages in close association with fossils of herbivores, it opened a debate about possible cooperative hunting.

Alternatively, Deinonychus may have gathered around prey in chaotic groups, like modern crocodilians or large lizards such as Komodo dragons (the world's heaviest living lizards), which do not form social units. In 2006, paleontologists found the first preserved tracks of dromaeosaurs, representing two species of “raptor” from Asia. The tracks indicate individuals of several different sizes traveling as a group. Another five dromaeosaur trackways were together in Niger in 2011. Paleontologists have interpreted the tracks as strong evidence that some dromaeosaurs lived and traveled in packs or at least exhibited social behavior at times.

Habitat and Other Life Forms

Early Cretaceous North America supported a variety of habitats from moist forests to wide-open plains. A large inland sea, known as the Interior Seaway, covered much of central and southern North America. The gymnosperms, which include conifers and cycads, remained dominant during the Early Cretaceous, but the emergence of angiosperms, or flowering plants, began to change the landscape during this period.

In addition to dinosaurs, the Early Cretaceous saw many reptile and amphibian species similar to extant species in North and South America. The skies were dominated by flying reptiles called pterosaurs, which lived throughout the Mesozoic.

There were many varieties of dinosaurs living alongside Deinonychus. One of the most common herbivore groups was the ornithopods, herbivores that included the hadrosaurs, or “duck billed” dinosaurs, which became the most dominant group during the Late Cretaceous. In addition, there were several varieties of ankylosaurs, large heavily armored dinosaurs characterized by their clubbed and spiked tails. Deinonychus lived alongside a number of other dinosaur predators, including other dromaeosaurs and members of the larger carnosaur group.

Research

Deinonychus was first discovered by paleontologists John Ostrom and Grant E. Meyer in 1964, in a fossil bed near the town of Bridger in south-central Montana. Ostrom was the first to describe the species and developed some of the early theories on the dinosaur's appearance and behavior. Specimens have since been found throughout North America.

Scientists have also utilized research on Deinonychus skeletons in the debate over whether dinosaurs were endothermic (warm-blooded), like mammals and birds, or ectothermic (cold-blooded), like amphibians and reptiles. Examinations of Deinonychus and other dromaeosaur fossils indicate that these animals grew relatively quickly, may have possessed four-chambered hearts, and had similar bones to endothermic animals, supporting the argument that at least some dinosaurs may have been endothermic. Spectroscopic analyses of fossilized proteins conducted in the late 2010s has suggested that Deinonychus may have had the faster metabolism associated with warm-blooded animals.

Several studies undertaken in the 2010s found that great similarity between Deinonychus claws, particularly on the second toe, and modern birds of prey, suggesting that like modern predatory birds, Deinonychus likely used its claws to immobilize prey smaller than itself and to consume it live. Its strong feet may have been better suited for grasping than running, and dromaeosaurids might have evolved flapping to maintain stability while holding prey. Moreover, musculoskeletal models have suggested a crouched hindlimb posture suitable for prey immobilization, although near-range stabbing or cutting might have been possible as well.

Isotopic analysis of Deinonychus teeth revealed a transition in diet with age, which suggests parental feeding was unlikely. Researchers also argued this countered the idea of pack hunting, as modern crocodilians and Komodo dragons hunt singly and consume a diverse diet over time, unlike modern pack hunters.

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