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Deinonychus antirrhopus

By Ripley Cook

Etymology: Terrible Claw

First Described By: Ostrom, 1969

Classification: Dinosauromorpha, Dinosauriformes, Dracohors, Dinosauria, Saurischia, Eusaurischia, Theropoda, Neotheropoda, Averostra, Tetanurae, Orionides, Avetheropoda, Coelurosauria, Tyrannoraptora, Maniraptoromorpha, Maniraptoriformes, Maniraptora, Pennaraptora, Paraves, Eumaniraptora, Dromaeosauridae, Eudromaeosauria, Dromaeosaurinae

Status: Extinct

Time and Place: From 115 to 98 million years ago, from the Aptian of the Early Cretaceous until the Cenomanian of the Late Cretaceous

Deinonychus is known from the Little Sheep and Himes Members of the Cloverly Formation of Montana and Wyoming, the Middle Member of the Antlers Formation of Oklahoma, and the Ruby Ranch and Mussentuchit Members of the Cedar Mountain Formation of Utah.

Physical Description: Deinonychus is a Dromaeosaur, or “raptor” dinosaur, and one of the best known of this group. Like other raptors, it looked a lot like a modern bird - fully covered in feathers, with large wings, a fan of feathers on the tail, and a triangular head. However, unlike a living bird, it was unable to fly, did not have a beak, and had a bony tail. Raptors are also characterized by their huge sickle claws on their feet, positioned on the second toe.

By Dinoguy2, CC BY-SA 2.5

Deinonychus specifically was fairly middle of the road in terms of raptor size, reaching up to 3.4 meters in length, and only coming up to about a meter in height. It would have been fairly lightweight, due to the air sacs present in the bones (like living birds). This would have made it about the size of a small wolf or a large dog - still smaller than the raptors in Jurassic Park. It had large openings in the skull to help keep it lightweight, and these were larger than those of other raptors; and it also had a more robust skull roof compared to other raptors. It had a long, narrow snout, which flared rapidly at the eye portion of the skull to give Deinonychus 3D vision.

Deinonychus had a long body, with a relatively short torso and extremely long tail. This tail was stiffened, to keep it very straight and only allowing a little bit of side to side movement. In addition, Deinonychus had a pelvis like a modern bird, giving more room to the gut for digesting food. As for limbs, Deinonychus had particularly long bones in its wing, which would have supported large wing feathers, much like wings of birds today. This is supported by the fact that most raptors found with feather impressions - Microraptor and Zhenyuanlong, notably - have full sized wings. Deinonychus also had a large fan of feathers across the tail. The legs of Deinonychus were fairly short compared to the rest of its body. It would have been warm-blooded, and an extremely active, birdy animal.

By Emily Willoughby, CC BY-SA 4.0

Diet: Deinonychus was a meat-eater, and a predator to boot, associated with animals such as Tenontosaurus and other medium-sized herbivores, though it probably also ate smaller animals such as mammals as well.

Behavior: Deinonychus has been the subject of extensive debate on the behavior of predatory dinosaurs, due to the fact that a large number of Deinonychus were found together with the herbivorous Tenontosaurus. This lead to the natural conclusion that Deinonychus was feeding on Tenontosaurus, and that the group had brought down the prey together - as a pack.

By José Carlos Cortés

This has sparked a lot of controversy, as Deinonychus are significantly smaller than Tenontosaurus, so one taking down this dinosaur alone is rather unlikely. However, most archosaurs don’t actually do cooperative hunting - though crocodilians and living birds might work together on bringing down food, it isn’t usually done in any coordinated fashion, but rather out of convenience. This lead to the idea of pack hunting not being particularly likely in Deinonychus, since living dinosaurs don’t really do much of it either. However, there are birds of prey, as well as crocodilians, that show gregarious pack hunting behavior, though not anything to the complexity seen in mammals. In addition, there are trackways that are parallel, with similar pacing of footprints, indicating that Deinonychus were living in social groups that traveled together. This would have allowed them to bring down larger prey.

In addition to this, Deinonychus probably utilized raptor prey restraint to hold down prey. Raptor Prey Restraint, or RPR, is a behavior seen today in living birds of prey, that would have been possible for Deinonychus and other Dromaeosaurs to do. With this method, Deinonychus would pin down it’s prey with its foot claws and, while holding it down, flap its wings rapidly in order to keep balanced as the prey moved around and tried to escape beneath it. This would have allowed Deinonychus to feed on the prey while it was still alive. This is supported not only by Deinonychus having long arms to support large wings, but also by its foot claws being similar to those of living owls, giving strong foot strength for holding onto prey.

By Tas on i-draws-dinosaurs

It’s entirely possible, therefore, that Deinonychus could have grouped up with others of its kind - probably a family grouping - and attacked herbivores much larger than itself, with some members of the group leaping onto the backs of these herbivores and holding on using RPR. The large sickle claws were not quite built for slashing at the prey, but rather for pinning down the prey and stabbing strategically at the animal, allowing it to bleed out rapidly. In addition, Deinonychus could have used the claws on its hands for slashing when it wasn’t using the wings for flight, as well as for grasping food. Like the modern seriema, however, Deinonychus could probably have torn apart smaller prey animals with its feet when eating a solo meal, if necessary.

Though raptors are commonly portrayed as being extremely fast (to quote a movie, “cheetah speed”), this is unlikely given the length of the bones of the leg in Deinonychus and other raptors. It had short bones in its foot as well as its leg, indicating that it was not very fast at running, and probably would have been more of an ambush than a pursuit predator. It might have been very good at leaping, however, and could have used its wings to generate a little lift to take the jump farther, and pounce on its food. Thus, Deinonychus could leap onto large prey such as Tenontosaurus, to get into a position to use RPR and feed upon it. The strong bones on the foot would have aided in handling the stress from such a jump. Deinonychus would have also been able to use its jaws in taking down prey, able to bite through bone if needed with a very high bite force. However, it did not do this habitually, like Tyrannosaurus, and instead used this bite force for defense and the capture of prey, rather than feeding.

By Ashley Patch

Though Deinonychus was too large to fly, it could generate enough lift to utilize another behavior of living birds - wing-assisted incline running, or WAIR. WAIR involves the flapping of wings fast enough that, with a hop and enough speed in the hind limbs, a bird is able to run up a steep surface such as a tree trunk, a wall, or a cliff face. Deinonychus might have been able to use WAIR to run up the surfaces of trees or after prey going up a tall feature such as a mountain and a hill - while Deinonychus was not a fast runner, WAIR could have helped it to gain some lift in these situations and go faster up the mountain or hill to catch its prey. Deinonychus also had a very stiffened tail, which would have helped it to keep its balance as it utilized WAIR and RPR in hunting and movement.

All in all, these traits - family groups, ambush predators, bursts of speed rather than long periods of speed, and pouncing - indicate that Deinonychus and other raptors like it were more like bird-cats, rather than bird-dogs, in terms of similar ecologies and behavior. You know what would have been bird-dogs? Tyrannosaurs. But more on that another time.

Interestingly enough, juvenile - young - Deinonychus are a bit different in proportion than the adults, namely by having longer arm bones and more sharply curved claws. This might mean that juvenile and baby Deinonychus actually went into trees, for safety or to help get food. It is even possible that young Deinonychus could fly, and grew out of it as they got bigger, but that is a hypothesis that has generated quite a bit of controversy and requires more extensive research.

By Kevin on Jurasicore

Deinonychus took care of its young, and eggs have been associated with Deinonychus fossils. Deinonychus would have brooded its eggs like living birds, protecting the nest with its large wings and sitting on top of the nest to aid in keeping the eggs warm and protected. It’s possible that the family groups of Deinonychus would have worked together to protect the nest until the young hatched. We know that Deinonychus laid blue eggs, helping to camouflage them in the foliage and against predators, a trait that evolved in Deinonychus and stuck around through modern birds. Deinonychus young then probably would escape to trees and other high locations whenever the parents weren’t around to protect them; as they got bigger they were probably able to get their own food, but they could have worked together to get large herbivores such as Tenontosaurus.

As we have seen, Deinonychus had a lot of reasons to have the large wings - for hunting, keeping itself and its young warm, and for movement. In addition to this, however, it’s likely that the wings were used for display - and, indeed, this is probably why complex feathers evolved in the first place. These large wings would have been perfect for complicated patterns and colors, which could have then been shown off to other raptors in mating displays. The large tail fan would also have been a good display structure, and could have been lifted and raised in situations of communication between members of the species, as well as in mating displays. By fluffing out its wings and raising up its tail, Deinonychus would have made itself look much bigger, which could have been used to express that it was threatening another animal, or that it was looking for sex. This is, of course, speculative, but based on behavior of living animals with similar structures.

In short, this was a medium-sized, active and agile predator, extremely birdlike in both appearance and behavior. It probably showed fairly complicated social behavior, and utilized its wings for complicated predatory actions. This makes Deinonychus - and other raptors - important keystones in our understanding of the complexity of Mesozoic dinosaurs and the evolution of birds.

By Scott Reid

Ecosystem: Deinonychus is known from the distinctive environments of the “middle” Cretaceous of Western North America, a snapshot between the iconic ecosystems of the late Jurassic and the late Cretaceous - indeed, many of the same environments as Sauropelta, who we learned about earlier this week.

In the Cloverly Formation, Deinonychus lived alongside the Western Interior Seaway as it grew through North America, and the dinosaurs present in this ecosystem such as Deinonychus would have been associated primarily with the floodplain and river system there. The flooding rivers caused mass burials of animals, and then eventually fossil formation. The plains were forested and covered with a variety of conifers and cycads. Deinonychus is known from both the Little Sheep and Himes Members of the Cloverly Formation, and thus was able to persist in this environment even as the Western Interior Seaway grew, and it turned from muddy to more sand based.

In the earlier, muddier Little Sheep ecosystem, Deinonychus lived alongside the ankylosaur Sauropelta, the ornithopod Tenontosaurus, the fast-moving Zephyrosaurus, the oviraptor Microvenator, and an unnamed large sauropod. There were also lungfish, sharks, and many bony fish, frogs, salamanders, and turtles. Mammals were also extremely common in this ecosystem and were probably a good source of prey for Deinonychus - animals such as Atokatheridium, Gobiconodon, Oklatheridium, and Paracimexomys. There were also plenty of crocodilians and lizards.

By Fred Wierum, CC BY-SA 4.0

In the later, sandier Himes ecosystem, Deinonychus still lived with Microvenator, Sauropelta, and Tenontosaurus, but also the large sauropod Sauroposeidon and the poorly known sauropod Rugocaudia, the nodosaur Tatankacephalus, and the larger predator Acrocanthosaurus which would have been a major threat to Deinonychus. There was also potentially a proto-bird in the ecosystem. There were many different kinds of lizards, frogs, salamanders, crocodylomorphs, turtles, fish, and sharks, as well as many mammals for Deinonychus to eat in addition to Tenontosaurus.

Deinonychus is also known from the Antlers Formation, from slightly more east in North America. It is known from the middle member of this ecosystem, and from about the same time as the Cloverly Formation to the west. This was also a floodplain ecosystem, which was periodically flooded with sources of water. Here, there was the tall sauropod Sauroposeidon, the ornithopod Tenontosaurus, a variety of mammals such as Astroconodon and Atokatheridium and Oklatheridium and Pappotherium, sharks and bony fish, and many types of Crocodylomorphs. There was also the large predator Acrocanthosaurus which, again, probably would have been a major predator of Deinonychus. Unfortunately, no plant fossils are known from the Antlers Formation to better piece together details as to the foliage Deinonychus would have lived among.

Finally, Deinonychus is also known from the Cedar Mountain Formation, which was an ecosystem that immediately followed the famous Morrison Formation of the Late Jurassic. Deinonychus is known from later time periods in the Cedar Mountain Formation than right after the Morrison, of course, but this ecosystem does represent a transitory time period between the Late Jurassic and Late Cretaceous. Here, Deinonychus is known from the Ruby Ranch and Mussentuchit members, which were the two latest occuring ecosystems in the formation. During the Ruby Ranch period - the earlier one - there was an extensive amount of mud, created by a large number of river channels forming due to the growing Western Interior Seaway. Here there were a lot of ferns, magnolias, conifers, and cycads, indicating a mainly forested ecosystem surrounding these rivers. Deinonychus was accompanied by the ankylosaur Gastonia, the ornithopod Tenontosaurus, the sauropods Astrodon and Brontomerus, the large predator Acrocanthosaurus, and a variety of crocodilians.

By Emily Willoughby, CC BY-SA 3.0

The later Mussentuchit Member was still covered in ferns, magnolias, conifers, and cycads, so the actual foliage didn’t seem to change much. However, there does appear to have been a volcanic explosion, leading to more detailed preservation of the animals and plants present - and most fossils from Cedar Mountain come, as such, from this ecosystem. Here, Deinonychus was accompanied by the ankylosaurs Animantarx, Cedarpelta, and Peloroplites - so, unfortunately, Gastonia went extinct. There were other ornithopods besides Tenontosaurus, though Tenontosaurus was present - such as Eolambia and the fast moving Zephyrosaurus. The only known sauropod from the Mussentuchit is Abydosaurus, indicating a drop in sauropod diversity that would continue in North America throughout the Cretaceous. The larger predator Siats was present, as well as the mysterious theropod Richardoestesia and a potential aquatic birdie dinosaur. There was a variety of sharks, bony fish, and lungfish; salamanders and frogs; turtles, snakes, lizards, and crocodilians; and many kinds of mammals present in this ecosystem as well.

Other: Deinonychus was of vital importance to the understanding of the evolution of birds. As a definitively active and bird-like dinosaur, Deinonychus showed researchers in the middle of the twentieth century that dinosaurs were actually the most likely candidates for the ancestors to modern birds. As research went on and more birdie dinosaurs were discovered, this became more and more certain - a path started by Deinonychus. In addition, Deinonychus in conjunction with Maiasaura helped to transition our understanding of dinosaurs from “slow, stupid, big lizards” to “active almost-birds”, a transition in perception that was cemented by the Jurassic Park Franchise. Deinonychus was a fairly derived raptor dinosaur, closely related to some of the largest members of the group such as Achillobator and Utahraptor.

~ By Meig Dickson

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