Deinonychus

Deinonychus is a genus of dromaeosaurid theropod dinosaur, with one described species, Deinonychus antirrhopus. Its name comes from Ancient Greek words meaning "terrible" and "claw." Not to be confused with Deinocheirus or Deinosuchus, this dinosaur is also the subject of a doom metal band, Deinonychus (band), which shares its name.

This species could grow up to 3.4 meters (11 ft) long. It lived during the Early and Late Cretaceous Period, about 115–98 million years ago, from the mid-Aptian to Cenomanian stages. Fossils have been found in the U.S. states of Montana, Utah, Wyoming, and Oklahoma, in rocks of the Cloverly Formation and Antlers Formation. Teeth that may belong to Deinonychus have also been found much farther east, in Maryland.

Paleontologist John Ostrom studied Deinonychus in the late 1960s, and his work changed how scientists thought about dinosaurs. It led to the "dinosaur renaissance" and started a debate about whether dinosaurs were warm-blooded or cold-blooded. Before this, many people pictured dinosaurs as slow, reptilian giants. Ostrom noted the dinosaur's small body, sleek posture, ratite-like spine, and especially its enlarged claws on the feet, which suggested an active, agile predator.

The "terrible claw" is the unusually large, sickle-shaped talon on the second toe of each hind foot. Ostrom reconstructed the claw of the fossil YPM 5205 as over 120 millimetres (4.7 in) long. The species name antirrhopus means "counter balance," which refers to Ostrom's idea about the tail. A fossil of the closely related Velociraptor mongoliensis suggests that the tail could actually bend to the sides with a high degree of flexibility.

Deinonychus remains have been found closely linked with those of the ornithopod Tenontosaurus. Teeth found with Tenontosaurus remains suggest that Deinonychus hunted them, or at least scavenged them. This has led to a debate about whether Deinonychus lived in groups or alone, with recent evidence suggesting it may have been social, although not in mammal-like packs.

Discovery and naming

Fossils of Deinonychus have been found in the Cloverly Formation of Montana and Wyoming, and in the similar-aged Antlers Formation of Oklahoma, in North America. The Cloverly Formation dates from the late Aptian to the early Albian stages of the early Cretaceous, about 115 to 93 million years ago. Teeth that may belong to this genus were also found in the Arundel Clay Facies of the Potomac Formation on the Atlantic Coastal Plain of Maryland. The first remains were uncovered in 1931 in southern Montana near Billings by paleontologist Barnum Brown, who was mainly digging up an ornithopod dinosaur called Tenontosaurus. Brown's notes described a small meat-eating dinosaur, which he informally called "Daptosaurus agilis," but he never finished describing it. Later, John Ostrom realized that some teeth Brown had named "Megadontosaurus" belonged to Deinonychus, while the skeleton came from a different animal, which he named Microvenator.

In 1964, Ostrom led an expedition from Yale's Peabody Museum of Natural History that found more bones near Bridger. In the following summers, more than 1,000 bones were uncovered. Because it was hard to tell which bones came from which animal, the type specimen was limited to a foot that clearly belonged to one individual. In 1969, Ostrom and Grant E. Meyer found that Brown's material and their own belonged to the same species. Ostrom named it Deinonychus antirrhopus, and the specific name means "counterbalancing," which refers to the likely purpose of its stiff tail. Ostrom later published a very detailed monograph on the animal in 1969. Many important bones were still missing or hard to understand, so Ostrom's early picture of its skeleton changed over time. In 1974, he realized that a bone he had first described as a pelvic bone was actually part of the shoulder. Another specimen found that year in Montana added new bones, which helped Ostrom improve his reconstruction.

Skeletons of Deinonychus are on display at the American Museum of Natural History and at the Museum of Comparative Zoology at Harvard University, though both are still missing some parts. In 2000, Gerald Grellet-Tinner and Peter Makovicky examined rock blocks that had been stored unprepared at the American Museum. They found bones that had been wrongly identified as tendons, which were actually gastralia (abdominal ribs). They also found many fossil eggs in the rock around the original specimen, and concluded that they almost certainly belonged to Deinonychus. This was the first dromaeosaurid egg ever identified. One eggshell was close to the gastralia, which suggested that Deinonychus might have brooded its eggs. Ostrom's 1969 description is often called the most important single discovery in dinosaur science in the mid-20th century. It helped change how scientists and the public saw dinosaurs, and it led some people to think that dinosaurs may have been warm-blooded. This shift is called the dinosaur renaissance. Ostrom also noticed that the forefeet of Deinonychus looked like those of birds, which led him to support the idea that birds come from dinosaurs. Today, this idea is almost universally accepted. Because Deinonychus was so bird-like and closely related to other dromaeosaurids, scientists think it was probably covered in feathers, though no direct evidence of feathers has been found for it yet. Feathers, including wing feathers, have been found on several related dromaeosaurs.

Description

Based on the few fully grown specimens, Paul estimated that Deinonychus could reach 3.3 to 3.4 meters (10 ft 10 in to 11 ft 2 in) in length, with a body mass of 60 to 73 kilograms (132 to 161 lb). Campione and his colleagues proposed a higher mass estimate of 100 kilograms (220 lb). Its skull had powerful jaws lined with around seventy curved, blade-like teeth. Scientists' understanding of the skull has changed over the decades. Ostrom first described it as triangular, broad, and fairly similar to Allosaurus. Later finds showed that the roof of the mouth was more arched, making the snout much narrower, while the jugals flared out widely. The skull also differed from Velociraptor, having a sturdier skull roof like that of Dromaeosaurus and lacking the sunken nasal bones of Velociraptor. Both the skull and lower jaw had fenestrae, which are openings that made them lighter. The antorbital fenestra, an opening between the eye and nostril, was particularly large.

Deinonychus had large "hands" (manus) with three claws on each arm. The first finger was the shortest and the second was the longest. Each hind foot had a sickle-shaped claw on the second toe, which was probably used to catch prey. No skin impressions have been found with Deinonychus fossils, but the evidence suggests that the Dromaeosauridae, including Deinonychus, had feathers. The genus Microraptor is older and more primitive than Deinonychus, and it belongs to the same family. Many of its fossils show feathers like those of modern birds on the arms, legs, and tail. Velociraptor is younger than Deinonychus but even more closely related. One Velociraptor specimen has bumps on the ulna where feather attachments were, which is a sign of feathers like those of modern birds.

Classification

Deinonychus antirrhopus is one of the best-known dromaeosaurid species. It is a close relative of the smaller Velociraptor, which lived in younger rocks in Central Asia. The clade they form is called Velociraptorinae. Rinchen Barsbold first named this group in 1983, when it held only the genus Velociraptor. Later, Phil Currie included most of the dromaeosaurids in it. Two other Late Cretaceous animals, Tsaagan from Mongolia and the North American Saurornitholestes, may also be close relatives, though Saurornitholestes is poorly known. Velociraptor and its relatives seem to have relied more on their claws than their skulls to kill, unlike Dromaeosaurus, which had stockier skulls. Based on family relationships, dromaeosaurids are one of the groups of non-bird dinosaurs most closely related to birds. A 2015 study places Deinonychus in the Dromaeosaurinae group.

A 2021 study of the dromaeosaurid Kansaignathus placed Deinonychus in the Velociraptorinae group instead. In that study, Kansaignathus was an in-between form, more advanced than Deinonychus but more primitive than Velociraptor. A study in 2022, however, moved Deinonychus back into Dromaeosaurinae as an early member of that group.

Paleobiology

Scientists have studied how Deinonychus hunted and moved. In 2011, Denver Fowler and colleagues proposed the "raptor prey restraint" model. In this model, Deinonychus leapt onto its prey, pinned it under its body weight, and gripped it with its large, sickle-shaped claws. Its feet and legs were most similar to those of eagles and hawks. Studies of its bite force have given different estimates, and a 2010 study found that it could bite through bone. Early tests on a robotic model suggested that its claws made only shallow punctures. Researchers also found that Deinonychus was not especially fast compared with other dinosaurs, and was certainly not as fast as modern flightless birds like the ostrich. A probable Deinonychus egg was found with one of the original fossils, and a 2018 study suggested that it laid blue eggs.

Scientists have debated whether Deinonychus lived in groups. Many of its teeth have been found alongside fossils of Tenontosaurus, and one quarry contained several Deinonychus skeletons. Ostrom and Maxwell suggested that it might have hunted in packs. A 2007 study by Roach and Brinkman questioned this idea, comparing Deinonychus to Komodo dragons, which often hunt alone. Other researchers have argued that it was likely social.

Paleoenvironment

Geological evidence suggests that Deinonychus lived in a floodplain or swamplike habitat. The places where its remains were found, the upper Cloverly Formation and the Antlers Formation, had tropical or sub-tropical forests, deltas, and lagoons, perhaps like modern-day Louisiana. Other animals in its world included plant-eating dinosaurs such as the nodosaurid Sauropelta and the ornithopods Zephyrosaurus and Tenontosaurus. In Oklahoma, its ecosystem also included the large theropod Acrocanthosaurus, the huge sauropod Sauroposeidon, the crocodilians Goniopholis and Paluxysuchus, and the gar Lepisosteus. If the teeth found in Maryland belong to Deinonychus, its contemporaries would include the sauropod Astrodon and the poorly known nodosaur Priconodon.

Deinonychus lived in humid places with lots of forest. During the Albian, much of North America was covered by vast forests of many kinds of plants. The tree canopy was mostly made up of pines, redwoods, araucarians, cheirolepidiaceans, ginkgos, yews, taxodioids, athrotaxidoids, podocarps, plane trees (sycamores), magnoliales, laurales, fabids, malvids, Tempskyaceae ferns, and Cyatheales ferns. The forest floor had palms, cycads, cycadeoids, horsetails, forked ferns, spleenworts, and Osmundaceae ferns growing beneath the trees. More open areas were home to the fern known as Ruffordia. These plants would have supported the many plant-eating animals Deinonychus hunted, such as Tenontosaurus.

Cultural significance

Deinonychus were featured in Harry Adam Knight's novel Carnosaur and its film adaption. They also appeared in Michael Crichton's novels Jurassic Park and The Lost World, and in the franchise adapted from the novels. Crichton chose to use the name Velociraptor instead of Deinonychus. He met with John Ostrom several times while writing, to talk about how these dinosaurs might have behaved and looked. Crichton told Ostrom that he had picked the name Velociraptor because he felt it was "more dramatic." Ostrom said that Crichton had told him the novel's Velociraptor was based on Deinonychus in almost every detail, and that only the name had been changed.

The makers of the Jurassic Park film did the same. They designed the film's dinosaur models almost entirely on Deinonychus instead of the real Velociraptor. They also reportedly asked for all of Ostrom's published papers on Deinonychus during production. As a result, the film's dinosaurs had the body proportions and snout shape of a large Deinonychus. The six-metre-long (20-foot) Utahraptor is often seen as a closer match to the film's dinosaurs in size, because it was larger than the known Deinonychus specimens were in life.