Deinonychus
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[Photo] Illustration of Deinonychus antirrhopus, sickle claw at lower right. Deinonychus antirrhopus, a theropod from the Early Cretaceous of North America, pencil drawing. Date May 12, 2007. Author Arthur Weasley http://commons.wikimedia.org/wiki/User:ArthurWeasley

Deinonychus (IPA: GA [da??.??na.ni.k??s], RP [da????n??n??k??s]) (Greek δεινο??, 'terrible' and ονυξ/ονυχο??, 'claw') was a genus of carnivorous dromaeosaurid dinosaur. This 3.4 metre (11 ft) long dinosaur lived during the early Cretaceous Period (Aptian - Albian stages, 121 to 98.9 million years ago). Fossils of the only named species (D. antirrhopus) have been recovered from the U.S. states of Montana, Wyoming and Oklahoma, though teeth that may belong to Deinonychus have been found much farther east in Maryland.

Its name refers to the unusually large, sickle-shaped talon on the second toe of each hind foot, which was probably held retracted while the dinosaur walked on the third and fourth toes. It was commonly thought that Deinonychus would kick with the sickle claw to slash at its prey but recent tests on reconstructions of similar Velociraptor talons suggest that the claw was used to stab, not slash. As in other dromaeosaurids, the tail was stiffened by a series of elongated bones and bone processes. This might have given Deinonychus greater balance and turning ability. In both the Cloverly and Antlers Formation, Deinonychus remains have been found closely associated with those of the ornithopod Tenontosaurus. Teeth discovered associated with Tenontosaurus specimens imply it was hunted or at least scavenged upon by Deinonychus.

Paleontologist John Ostrom's study of Deinonychus in the late 1960s revolutionized the way scientists thought about dinosaurs, igniting the debate on whether or not dinosaurs were warm-blooded. Before this, the popular conception of dinosaurs had been one of plodding, reptilian giants. Ostrom noted lightweight bones and stiffened tendons which revealed an active, agile predator.

Description
Based on the largest known specimens, Deinonychus could reach 3.4 meters (11.1 ft), with a maximum skull length of 410 mm (16.4 in), a hip height of 0.87 meters (2.85 ft), a maximum weight of 73 kilograms (161 lb). Its skull was equipped with powerful jaws lined with around sixty curved, blade-like teeth. Studies of the skull show that the eyes faced mainly to the side. Both the skull and the lower jaw had fenestrae (skull openings) which reduced the weight of the skull. In Deinonychus, the antorbital fenestra, a skull opening between the eye and nostril, was particularly large.

Like all dromaeosaurs, Deinonychus possessed large manus (hands) with three claws on each forelimb. The first digit was shortest and the second one longest. Each hind foot bore a sickle-shaped claw on this second digit, which was probably used during predation; these claws were unusually large for the body size of Deinonychus. Deinonychus had a tail that was highly stiffened primarily by extensions of the tail vertebrae and chevrons; this allowed stability and balance while turning at high speeds.

Classification
Deinonychus is one of the best-known dromaeosaurids, and is a close relative of the smaller Velociraptor, found in younger, Late Cretaceous-age rock formations in Central Asia. The clade they form is called Velociraptorinae. The subfamily name Velociraptorinae was first coined by Rinchen Barsbold in 1983 and originally contained the single genus Velociraptor. Later Phil Currie included most of the dromaeosaurids. Two Late Cretaceous genera, Tsaagan from Mongolia and the North American Saurornitholestes, may also be close relatives, but the latter is poorly known and hard to classify. Velociraptor and its allies are regarded as using their claws more than their skulls as killing tools, as opposed to dromaeosaurids like Dromaeosaurus with stockier skulls. Together with the troodontids, the dromaeosaurids form the Deinonychosauria clade which is a sister taxon of aves. Phylogenetically, the Deinonychosauria represent the group of non-avian dinosaurs the most closely related to birds.

Discovery and naming
Fossilized remains of Deinonychus have been recovered from the Cloverly Formation of Montana and Wyoming and in the Antlers Formation of Oklahoma, in North America. Additionally, teeth found in the Arundel Clay Facies (Aptian), of the Potomac Formation on the Atlantic Coastal Plain of Maryland may be assigned to the genus.

The first remains were uncovered in 1931 in southern Montana near the town of Bridger. The team leader, paleontologist Barnum Brown, was primarily concerned with excavating and preparing the remains of the ornithopod dinosaur Tenontosaurus, but in his field report from the dig site to the American Museum of Natural History, he reported the discovery of a small carnivorous dinosaur close to a Tenontosaurus skeleton, "but encased in lime difficult to prepare." He informally called the animal "Daptosaurus" and made preparations for describing it and having the skeleton put on display, but never finished this work. Brown brought back from the Cloverly Formation the skeleton of a smaller theropod with seemingly oversized teeth that he informally named "Megadontosaurus". John Ostrom, reviewing this material decades later, realized that the teeth came from Deinonychus, but the skeleton came from a completely different animal. He named this skeleton Microvenator.

A little more than thirty years later, in August of 1964, paleontologists John Ostrom and Grant E. Meyer analyzed Brown's "Daptosaurus" in detail and published their findings in 1969, giving the remains the new name of Deinonychus antirrhopus. "antirrhopus" means "counterbalancing" and refers to the animal's stiff tail's likely purpose. Expeditions during the following two summers uncovered more than 1000 bones, among which were at least three individuals. Since the association between the various recovered bones was weak, making the exact number of individual animals represented impossible to determine properly, the type specimen of Deinonychus was restricted to the complete left foot and partial right foot that definitely belonged to the same individual. The remaining specimens were catalogued in fifty separate entries at Yale's Peabody Museum of Natural History.

A skeleton of Deinonychus including bones from the original (and most complete) specimen can be seen on display at the American Museum of Natural History, with another specimen on display at the Museum of Comparative Zoology at Harvard University. The American Museum and Harvard specimens are from a different locality than the Yale specimens.

Further findings
Following Ostrom's description of the original specimen uncovered by Brown, several small blocks of lime-encased material remained unprepared in storage at the American Museum. These consisted mostly of isolated bones and bone fragments, including the original matrix, or surrounding rock in which the specimens were initially buried. An examination of these unprepared blocks by Gerald Grellet-Tinner and Peter Makovicky in 2000 revealed an interesting, overlooked feature. Several long, thin bones identified on the blocks as ossified tendons (structures which helped stiffen the tail of Deinonychus) turned out to actually represent gastralia (abdominal ribs). More significantly, a large number of previously unnoticed fossilized eggshells were discovered in the rock matrix which had surrounded the original Deinonychus specimen.

In a subsequent, more detailed report on the eggshells, Grellet-Tinner and Makovicky concluded that the egg almost certainly belonged to Deinonychus, representing the first dromaeosaurid egg to be identified. Moreover, the external surface of one eggshell was found in close contact with the gastralia suggesting that Deinonychus might have brooded its eggs. This implies that Deinonychus used body heat transfer as a mechanism for egg incubation, and indicates an endothermy similar to modern birds. Further study by Gregory Erickson and colleagues finds that this individual was 13 or 14 years old at death and its growth had plateaued. Unlike other theropods in their study of specimens found associated with eggs or nests, it had finished growing at the time of its death.

Implications
The description in 1969 by Ostrom of Deinonychus has been described as the most important single discovery of dinosaur paleontology in the mid 20th century. The discovery of this clearly active, agile predator did much to change the scientific (and popular) conception of dinosaurs and opened the door to speculation that dinosaurs may have been warm-blooded. This development has been termed the Dinosaur renaissance. Several years later, Ostrom noted similarities between the forefeet of Deinonychus and that of birds, which observation led him to revive the hypothesis that birds are descended from dinosaurs. Thirty years later, this idea is almost universally accepted.

The discoveries of closely related dinosaurs from China bearing the remains of feather-like structures, Sinornithosaurus and Microraptor, indicate that Deinonychus may have borne feathers as well. The features of these two dinosaurs suggest they were more primitive than Deinonychus on the Dromaeosaurid evolutionary tree, which, coupled with their greater age (both dinosaurs predate Deinonychus by around ten million years), strongly suggest that Deinonychus may also have been feathered. Feathers impression have also been found with theropods even more distantly related to birds than Sinornithosaurus and Microraptor, including Caudipteryx, Shuvuuia, Beipiaosaurus and Dilong, giving further credence according to the phylogenetic bracketing principle, that all dromaesaurids were indeed feathered.

Paleoecology
Geological evidence suggests that Deinonychus inhabited a floodplain or swamplike habitat. The paleoenvironment of both the Cloverly Formation and the Antlers Formation, in which remains of Deinonychus have been found, consisted of forests, deltas and lagoons, not unlike today's Louisiana. Other dinosaurs it shared its world with include herbivorous dinosaurs such as the armoured Sauropelta and two ornithopods, Zephyrosaurus and the larger Tenontosaurus. In Oklahoma, the ecosystem of Deinonychus also includes the large theropod Acrocanthosaurus, the huge sauropod Sauroposeidon, the crocodilian genus Goniopholis and the gar genus Lepisosteus.

Paleobiology

Predatory behavior
Based on the association of a number of Deinonychus skeletons in a single quarry, and the fact that shed teeth of Deinonychus have been found alongside skeletons of the ornithopod dinosaur Tenontosaurus, Deinonychus may have fed on that animal, and perhaps hunted it. Ostrom and Maxwell have even used this information to speculate that Deinonychus might have lived and hunted in packs.

However, a recent study by Roach and Brinkman has put into question the cooperative pack hunting behavior of Deinonychus, based on what is known of modern carnivore hunting and the taphonomy of tenontosaur sites. Modern archosaurs (birds and crocodiles) and komodo dragons display little cooperative hunting; instead, they are usually either solitary hunters, or are drawn to previously-killed carcasses, where much conflict occurs between individuals of the same species. For example, in situations where groups of komodo dragons are eating together, the largest individuals eat first and will attack smaller komodos that attempt to feed; if the smaller animal is killed, it is cannibalized. When this information is applied to the tenontosaur sites, it appears that what is found is consistent with Deinonychus having a komodo- or crocodile-like feeding strategy. Deinonychus skeletal remains found at these sites are from subadults, with missing parts consistent with having been eaten by other Deinonychus.

Despite being the most distinctive feature of Deinonychus, the shape and curvature of the sickle-claw varies between specimens. The type specimen described by Ostrom in 1969 has a strongly curved sickle claw, while a newer specimen described in 1976 had a claw with much weaker curvature, more similar in profile with the 'normal' claws on the remaining toes. Ostrom suggested that this difference in the size and shape of the sickle claws could be due to individual, sexual, or age-related variation.

Ostrom originally speculated that Deinonychus gripped its prey with its talons while delivering disemboweling slashes with its sickle claws. Later studies, however, have shown that the sickle claws were not used to slash but rather to deliver small stabs to the victim. Biomechanical studies by Ken Carpenter in 2002 confirmed that the most likely function of the forelimbs in predation was grasping, as their great lengths would have permitted longer reach than for most other theropods. The rather large and elongated coracoid, hinting for powerful muscles in the forelimbs, further strengthen this interpretation. Carpenter's biomechanical studies using bones casts also showed that Deinonychus could not fold its arms against its body like a bird ("avian folding"), contrary to what was inferred from the earlier 1985 descriptions by Gauthier and Paul in 1988. Studies by Senter in 2006 indicate that Deinonychus forelimbs can be used not only for grasping but also for clutching objects towards the chest.

Parsons has shown that juvenile and sub-adult specimens of Deinonychus display some morphological differences with the adults. For instance, the arms of the younger specimens were proportionally longer than those of the adults, a possible indication of difference in behavior between young and adults.

Speed
Dromaeosaurids, especially Deinonychus, are often depicted as unusually fast-running animals in the popular media, and Ostrom himself speculated that Deinonychus was fleet-footed in his original description. However, when first described, a complete leg of Deinonychus had not been found, and Ostrom's speculation about the length of the femur (upper leg bone) later proved to have been an overestimate. In a later study, Ostrom noted that the ratio of the femur to the tibia (lower leg bone) is not as important in determining speed as the relative length of the foot and lower leg. In modern, fleet-footed birds like the ostrich, the foot-tibia ratio is .95. In unusually fast-running dinosaurs like Struthiomimus, the ratio is .68, but in Deinonychus, the ratio is .48. Ostrom stated that the "only reasonable conclusion" is that Deinonychus was not particularly fast compared to other dinosaurs, and certainly not as fast as modern flightless birds.

The low foot to lower leg ratio in Deinonychus is due partly to an unusually short metatarsus (upper foot bones). The ratio is actually larger in smaller individual than in larger ones. Ostrom suggested that the short metatarsus may be related to the function of the sickle claw, and used the fact that it appears to get shorter as individuals aged as support for this. He interpreted all these features ??? the short second toe with enlarged claw, short metatarsus, etc. ??? as support for the use of the hind leg as an offensive weapon, where the sickle claw would strike downwards and backwards, and the leg pulled back and down at the same time, slashing and tearing at the prey. Ostrom suggested that the short metatarsus reduced overall stress on the leg bones during such an attack, and interpreted the unusual arrangement of muscle attachments in the Deinonychus leg as support for his idea that a different set of muscles were used in the predatory stroke than in walking or running. Therefore, Ostrom concluded that the legs of Deinonychus represented a balance between running adaptations needed for an agile predator, and stress-reducing features to compensate for its unique foot weapon.

Eggs
The identification in 2000 of a probable Deinonychus egg associated with one of the original specimens allowed comparison with other theropod dinosaurs in terms of egg structure, nesting, and reproduction. In their 2006 examination of the specimen, Grellet-Tinner and Makovicky examined the possibility that the dromaeosaurid had been feeding on the egg, or that the egg fragments had been associated with the Deinonychus skeleton by coincidence. They dismissed the idea that the egg had been a meal for the theropod, noting that the fragments were sandwiched between the belly ribs and forelimb bones, making it impossible that they represented contents of the animals stomach. In addition, the manner in which the egg had been crushed and fragmented indicated that it had been intact at the time of burial, and was broken by the fossilization process. The idea that the egg was randomly associated with the dinosaur were also found to be unlikely; the bones surrounding the egg had not been scattered or disarticulated, but remained fairly intact relative to their positions in life, indicating that the area around and including the egg was not disturbed during preservation. The fact that these bones were belly ribs (gastralia), which are very rarely found articulated, supported this interpretation. All the evidence, according to Grellet-Tinner and Makovicky, indicates that the egg was intact beneath the body of the Deinonychus when it was buried. It is possible that this represents brooding or nesting behavior in Deinonychus similar to that seen in the related troodontids and oviraptorids, or that the egg was in fact inside the oviduct when the animal died.

Examination of the Deinonychus egg's microstructure confirms that it belonged to a theropod, since it shares characteristics with other known theropod eggs and shows dissimilarities with ornithischian and sauropod eggs. Compared to other maniraptoran theropods, the egg of Deinonychus is more similar to those of oviraptorids than to those of troodontids, despite studies which show the later are more closely related to dromaeosaurids like Deinonychus. While the egg was too badly crushed to accurately determine its size, Grellet-Tinner and Makovicky estimated a diameter of about 7 cm (2.7 in) based on the width of the pelvic canal through which the egg had to have passed. This size is similar to the 7.2 cm diameter of the largest Citipati (an oviraptorid) eggs; Citipati and Deinonychus also shared the same overall body size, supporting this estimate. Additionally, the thicknesses of Citipati and Deinonychus eggshells are almost identical, and since shell thickness correlates with egg volume, this further supports the idea that the eggs of these two animals were about the same size.

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