Dromaeosauridae
Dromaeosauridae is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek , meaning 'running at full speed', 'swift', and , meaning 'lizard'. In informal usage, they are often called raptors, a term popularized by the film Jurassic Park; several genera include the term "raptor" directly in their name, and popular culture has come to emphasize their bird-like appearance and speculated bird-like behavior.
Definitive dromaeosaurid fossils have been found in North America, Europe and Asia. Some paravian fossils found in other continents have been traditionally regarded as dromaeosaurids, but have recently been reinterpreted as a unique family Unenlagiidae outside Dromaeosauridae, with some authors considering them as a separate lineage of Avialae. The earliest body fossils are known from the Early Cretaceous, and they survived until the end of the Cretaceous, existing until the Cretaceous–Paleogene extinction event. The presence of dromaeosaurids as early as the Middle Jurassic has been suggested by the discovery of isolated fossil teeth, though no dromaeosaurid body fossils have been found from this period.
Description
Technical diagnosis
Dromaeosaurids are diagnosed by the following features: short T-shaped frontals that form the rostral boundary of the supratemporal fenestra; a caudolateral overhanging shelf of the squamosal; a lateral process of the quadrate that contacts the quadratojugal; raised, stalked, parapophyses on the dorsal vertebrae, a modified pedal digit II; chevrons and prezygapophysis of the caudal vertebrae elongate and spanning several vertebrae; the presence of a subglenoid fossa on the coracoid.Size and general build
Dromaeosaurids were small to medium-sized dinosaurs, ranging from in length to approaching or over . Large size appears to have evolved at least twice among dromaeosaurids; once among the dromaeosaurines Utahraptor and Achillobator, and again among the unenlagiines. A possible third lineage of giant dromaeosaurids is represented by isolated teeth found on the Isle of Wight, England. The teeth belong to an animal the size of the dromaeosaurine Utahraptor, but they appear to belong to velociraptorines, judging by the shape of the teeth.The distinctive dromaeosaurid body plan helped to rekindle theories that dinosaurs may have been active, fast, and closely related to birds. Robert Bakker's illustration for John Ostrom's 1969 monograph, showing the dromaeosaurid Deinonychus in a fast run, is among the most influential paleontological reconstructions in history. The dromaeosaurid body plan includes a relatively large skull, serrated teeth, narrow snout, and forward-facing eyes which indicate some degree of binocular vision.
File:Pyroraptor olympius reconstruction.png|thumb|Life restoration of Pyroraptor
Dromaeosaurids, like most other theropods, had a moderately long S-curved neck, and their trunk was relatively short and deep. Like other maniraptorans, they had long arms that could be folded against the body in some species, and relatively large hands with three long fingers ending in large claws. The dromaeosaurid hip structure featured a characteristically large pubic boot projecting beneath the base of the tail. Dromaeosaurid feet bore a large, recurved claw on the second toe. Their tails were slender, with long, low, vertebrae lacking transverse process and neural spines after the 14th caudal vertebra. Ossified uncinate processes of ribs have been identified in several dromaeosaurids.
Foot
Like other theropods, dromaeosaurids were bipedal; that is, they walked on their hind legs. However, whereas most theropods walked with three toes contacting the ground, fossilized footprint tracks confirm that many early paravian groups, including the dromaeosaurids, 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, falciform claw, which is thought to have been used in capturing prey and climbing trees. This claw was especially blade-like in the large-bodied predatory eudromaeosaurs. One possible dromaeosaurid species, Balaur bondoc, also 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 also held retracted and bore enlarged, sickle-shaped claws. Similar claws can be found on present day Seriema.Tail
Dromaeosaurids had long tails. Most of the tail vertebrae bore bony, rod-like extensions, as well as bony tendons in some species. In his study of Deinonychus, Ostrom proposed that these features stiffened the tail so that it could only flex at the base, and the whole tail would then move as a single, rigid, lever. However, one well-preserved specimen of Velociraptor mongoliensis has an articulated tail skeleton that is curved horizontally in a long S-shape. This suggests that, in life, the tail could bend from side to side with a substantial degree of flexibility. It has been proposed that this tail was used as a stabilizer or counterweight while running or in the air; in Microraptor, an elongate diamond-shaped fan of feathers is preserved on the end of the tail. This may have been used as an aerodynamic stabilizer and rudder during gliding or powered flight.Feathers
There is a large body of evidence showing that dromaeosaurids were covered in feathers. Some dromaeosaurid fossils preserve long, pennaceous feathers on the hands and arms and tail, as well as shorter, down-like feathers covering the body. Other fossils, which do not preserve actual impressions of feathers, still preserve the associated bumps on the forearm bones where long wing feathers would have attached in life. Overall, this feather pattern looks very much like Archaeopteryx.The first known dromaeosaurid with definitive evidence of feathers was Sinornithosaurus, reported from China by Xu et al. in 1999. Many other dromaeosaurid fossils have been found with feathers covering their bodies, some with fully developed feathered wings. Microraptor even shows evidence of a second pair of wings on the hind legs. While direct feather impressions are only possible in fine-grained sediments, some fossils found in coarser rocks show evidence of feathers by the presence of quill knobs, the attachment points for wing feathers possessed by some birds. The dromaeosaurids Rahonavis and Velociraptor have both been found with quill knobs, showing that these forms had feathers despite no impressions having been found. In light of this, it is most likely that even the larger ground-dwelling dromaeosaurids bore feathers, since even flightless birds today retain most of their plumage, and relatively large dromaeosaurids, like Velociraptor, are known to have retained pennaceous feathers. Though some scientists had suggested that the larger dromaeosaurids lost some or all of their insulatory covering, the discovery of feathers in Velociraptor specimens has been cited as evidence that all members of the family retained feathers.
More recently, the discovery of Zhenyuanlong established the presence of a full feathered coat in relatively large dromaeosaurids. Additionally, the animal displays proportionally large, aerodynamic wing feathers, as well as a tail-spanning fan, both of which are unexpected traits that may offer an understanding of the integument of large dromaeosaurids. Dakotaraptor is an even larger dromaeosaurid species with evidence of feathers, albeit indirect in the form of quill knobs, though the taxon is considered as chimeara by other researchers as even the dinosaurian elements with supposed traits diagnostic for dromaeosaurs also referrable to caenagnathids and ornithomimosaurians.
Classification
Relationship with birds
Dromaeosaurids share many features with early birds. The precise nature of their relationship to birds has undergone a great deal of study, and hypotheses about that relationship have changed as large amounts of new evidence became available. As late as 2001, Mark Norell and colleagues analyzed a large survey of coelurosaur fossils and produced the tentative result that dromaeosaurids were most closely related to birds, with troodontids as a more distant outgroup. They even suggested that Dromaeosauridae could be paraphyletic relative to Avialae. In 2002, Hwang and colleagues utilized the work of Norell et al., including new characters and better fossil evidence, to determine that birds were better thought of as cousins to the dromaeosaurids and troodontids. The consensus of paleontologists is that there is not yet enough evidence to determine whether any dromaeosaurids could fly or glide, or whether they evolved from ancestors that could.Alternative theories and flightlessness
Dromaeosaurids are so bird-like that they have led some researchers to argue that they would be better classified as birds. First, since they had feathers, dromaeosaurids are "birds" under traditional definitions of the word "bird", or "Aves", that are based on the possession of feathers. However, other scientists, such as Lawrence Witmer, have argued that calling a theropod like Caudipteryx a bird because it has feathers may stretch the word past any useful meaning.At least two schools of researchers have proposed that dromaeosaurids may actually be descended from flying ancestors. Hypotheses involving a flying ancestor for dromaeosaurids are sometimes called "Birds Came First". George Olshevsky is usually credited as the first author of BCF. In his own work, Gregory S. Paul pointed out numerous features of the dromaeosaurid skeleton that he interpreted as evidence that the entire group had evolved from flying, dinosaurian ancestors, perhaps an animal like Archaeopteryx. In that case, the larger dromaeosaurids were secondarily flightless, like the modern ostrich. In 1988, Paul suggested that dromaeosaurids may actually be more closely related to modern birds than to Archaeopteryx. By 2002, however, Paul placed dromaeosaurids and Archaeopteryx as the closest relatives to one another.
In 2002, Hwang et al. found that Microraptor was the most primitive dromaeosaurid. Xu and colleagues in 2003 cited the basal position of Microraptor, along with feather and wing features, as evidence that the ancestral dromaeosaurid could glide. In that case the larger dromaeosaurids would be secondarily terrestrial—having lost the ability to glide later in their evolutionary history.
Also in 2002, Steven Czerkas described Cryptovolans, though it is a probable junior synonym of Microraptor. He reconstructed the fossil inaccurately with only two wings and thus argued that dromaeosaurids were powered fliers, rather than passive gliders. He later issued a revised reconstruction in agreement with that of Microraptor
Other researchers, like Larry Martin, have proposed that dromaeosaurids, along with all maniraptorans, were not dinosaurs at all. Martin asserted for decades that birds were unrelated to maniraptorans, but in 2004 he changed his position, agreeing that the two were close relatives. However, Martin believed that maniraptorans were secondarily flightless birds, and that birds did not evolve from dinosaurs, but rather from non-dinosaurian archosaurs.
In 2005, Mayr and Peters described the anatomy of a very well preserved specimen of Archaeopteryx, and determined that its anatomy was more like non-avian theropods than previously understood. Specifically, they found that Archaeopteryx had a primitive palatine, unreversed hallux, and hyper-extendable second toe. Their phylogenetic analysis produced the controversial result that Confuciusornis was closer to Microraptor than to Archaeopteryx, making the Avialae a paraphyletic taxon. They also suggested that the ancestral paravian was able to fly or glide, and that the dromaeosaurids and troodontids were secondarily flightless. Corfe and Butler criticized this work on methodological grounds.
A challenge to all of these alternative scenarios came when Turner and colleagues in 2007 described a new dromaeosaurid, Mahakala, which they found to be the most basal and most primitive member of the Dromaeosauridae, more primitive than Microraptor. Mahakala had short arms and no ability to glide. Turner et al. also inferred that flight evolved only in the Avialae, and these two points suggested that the ancestral dromaeosaurid could not glide or fly. Based on this cladistic analysis, Mahakala suggests that the ancestral condition for dromaeosaurids is non-volant. However, in 2012, an expanded and revised study incorporating the most recent dromaeosaurid finds recovered the Archaeopteryx-like Xiaotingia as the most primitive member of the clade Dromaeosauridae, which appears to suggest the earliest members of the clade may have been capable of flight.