Temporal range: Middle Jurassic-Late Cretaceous, 167–65.5Ma
|Mounted skeleton of Deinonychus antirrhopus|
Colbert & Russell, 1969
The Deinonychosauria ("fearsome claw lizards") were a clade of coelurosaurian theropod dinosaurs from the Late Jurassic and Cretaceous periods. These omnivores and carnivores are known for their sickle-shaped second toe claws and for displaying numerous bird-like characteristics. The clade has been divided into two further groups — Dromaeosauridae and Troodontidae. The largest known of these was Utahraptor (7 m or 23 ft), and Anchiornis was the smallest (34 cm or 13 in). By modern classifications, birds are closely related to this group of dinosaurs.
Like other theropods, deinonychosaurs were bipedal; that is, they walked on their two hind legs. However, whereas most theropods walked with three toes contacting the ground, fossilized footprint tracks confirm that most deinonychosaurs held the second toe off the ground in a hyperextended position, with only the third and fourth toes bearing the weight of the animal. This is called functional didactyly. The enlarged second toe bore an unusually large, curved sickle-shaped claw (held off the ground or 'retracted' when walking). This claw was especially large and flattened from side to side in the large-bodied predatory eudromaeosaurs. The first toe (hallux) was relatively small and angled inward toward the center of the body, but was not fully reversed as in modern birds. One eudromaeosaur species, Balaur bondoc, possessed a first toe which was highly modified in parallel with the second. Both the first and second toes on each foot of B. bondoc were held retracted and bore enlarged, sickle-shaped claws.
The teeth of deinonychosaurs were curved and serrated, but not blade-like except in some advanced species such as Dromaeosaurus albertensis. The serrations on the front edge of deinonychosaur teeth were very small and fine, while the back edge had serrations which were very large and hooked. Deinonychosaurs generally had long, winged forelimbs, though these were smaller in some troodontids. The wings usually bore three large, flexible claws.
Claw function 
One of the best-known features of deinonychosaurs is the presence of an enlarged and strongly curved "sickle claw" on a hyper-extendible second toe, modified to hold the sickle claw clear of the ground when walking. While this characteristic claw and its associated modifications to the anatomy of the foot (such as a shortened metatarsus in eudromaeosaurs) had been known since the mid 20th Century, their possible functions were the subject mainly of speculation, and few actual studies were published. Initial speculation regarded the claws as slashing implements used to disembowel large prey. In this scenario, the shortened upper foot would serve as an anchor point for powerful tendons to improve kicking ability. However, subsequent studies of the actual claw shape showed that the underside of the claw was only weakly keeled and would not have been an effective cutting instrument. Instead, it appeared to be more of a hooking implement. Manning et al. suggested in 2006 that the claws were similar to crampons and were used for climbing, and in the case of larger species or individuals, climbing up the flanks of very large prey.
A larger study of deinonychosaur claw function, published in 2011 by Fowler and colleagues, concluded that the earlier study by Manning and colleagues was correct and that the "sickle claws" of deinonychusaurs would have been ineffective as cutting weapons. They compared the claw and overall foot anatomy of various deinonychosaurs with modern birds to shed light on their actual function. Fowler and colleagues showed that many modern predatory birds also have enlarged claws on the second toes. In modern raptors, these claws are used to help grip and hold prey of sizes smaller than or equal to the predator, while the birds use their body weight to pin their prey to the ground and eat it alive. Folwer and colleagues suggested that this behavior is entirely consistent with the anatomy of advanced deinonychosaurs like Deinonychus, which had slightly opposing first toes and strong tendons in the toes and foot. This makes it likely that advanced dromaeosaurids also used their claws to puncture and grip their prey to aid in pinning it to the ground, while using shallow wing beats and tail movements to stabilize themselves. Other lines of evidence for this behavior include teeth which had large, hooked serrations only on the back edge (useful in pulling flesh upward rather than slicing it) and large claws on the wings (for greater maneuvering of prey while mantling it with the wings).
In more primitive dromaeosaurids and in troodontids, the feet were not as specialized and the claws were not as large or as hooked. Additionally, the toe joints allowed more range of motion than the simple up-down movements of advanced dromaeosaurids. This makes it likely that these species specialized in smaller prey that could be pinned using only the inner toes, not requiring the feet to be as strong or sturdy.
The dromaeosaur subgroup is very diverse, and includes the long-snouted unenlagiines, the tiny microraptorines, the svelte velociraptorines, and the stout dromaeosaurines. Dromaeosauridae is named for Dromaeosaurus, the first-discovered member of this clan. Other especially well-known members of Dromaeosauridae include Velociraptor and Deinonychus, both mid-sized velociraptorines, and Microraptor, a gliding, four-winged microraptorine. The group is colloquially referred to as the "raptors". Dromaeosaurids are principally differentiated from their sister group Troodontidae by anatomical features of the skull and jaw joint, as well as by their larger retractable toe claws and rod-like tail ossifications, which would have made the tail more rigid and inflexible. Various aspects of dromaeosaurid morphology, such as the enlarged retractable toe-claws and shorter legs, are also possibly indicative of ecological separation from the troodontids, and suggest that dromaeosaurids may have been ambush predators. There is evidence of parallel dromaeosaurid trackways which suggests that certain species may have cooperated gregariously in groups.
Troodontidae is somewhat less well-known. Its eponymous representative Troodon was named only from teeth and for many years the phylogenetic position of the group was uncertain. Nowadays the clade is recognized as a sister taxon to Dromaeosauridae. Other well-known troodontids include Mei long, the dinosaur famously preserved in a sleeping position, and Anchiornis, the first dinosaur for which a full color pattern was determined (though Anchiornis may be an archaeopterygid). Troodontids are generally quite small, and one popular feature that distinguishes them from dromaeosaurs is a larger braincase, with particularly enlarged areas for sight and hearing. Other distinguishing features include a forward-pointing pubis, smaller (and in some cases less-retractable) "sickle" claws, relatively longer legs, a pinched middle metatarsal bone, and bump-ridged or serration-less teeth. The smaller second toe-claws and relatively longer legs of troodontids may indicate that they evolved into a more cursorial niche in contrast to their slower, more powerful dromaeosaurid brethren.
- Infraorder Deinonychosauria
- Family Archaeopterygidae?
- Superfamily Dromaeosauroidea
- Superfamily Troodontoidea
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