Dromaeosauridae Totally Explained

Everything about Dromaeosaur totally explained

Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small to medium - sized, feathered carnivores that flourished in the Cretaceous Period. In informal usage they're often called "raptors" (after Velociraptor), a term popularized by the film Jurassic Park. The name Dromaeosauridae means 'running lizards', from Greek dromeus (δρομευς) meaning 'runner' and sauros (σαυρος) meaning 'lizard'.
Dromaeosaurid fossils have been found in North America, Europe, North Africa, Japan, China, Mongolia, Madagascar, Argentina, and Antarctica. They first appeared in the mid-Jurassic Period (Bathonian stage, 167 million years ago) and survived until the end of the Cretaceous (Maastrichtian stage, 65.5 ma), existing for over 100 million years, up until the Cretaceous-Tertiary extinction event. The presence of dromaeosaurs as early as the mid-Jurassic has been confirmed by the discovery of isolated fossil teeth, though no dromaeosaurid body fossils have been found from this period.

Characteristics

Body plan

The distinctive dromaeosaurid bauplan 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, The dromaeosaurid body plan includes a relatively large skull, serrated teeth, narrow snout, and forward-facing eyes which indicate some degree of binocular vision. Dromaeosaurids, like most other theropods, had a moderately long S-curved neck, and their trunk was relatively short and deep. Like other maniraptorans, they'd long arms that could be folded against the body in some species, and relatively large hands with three long fingers (the middle finger being the longest and the first finger being the shortest) ending in large claws. The dromaeosaurid hip structure featured a characteristiclly 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. The "retracted" second toe bore an unusually large, curved sickle-shaped claw, which is thought to have been used in killing prey (see "Predatory behavior" below).

The dromaeosaurid tail

Dromaeosaurids had long tails. Most of the tail vertebrae bear 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 (IGM 100/986) has an articulated tail skeleton that's curved laterally 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 and/or counterweight while running or in the air;

Size

Dromaeosaurids were small to medium-sized dinosaurs, ranging from about .6 meters in length (2 ft, in the case of Microraptor) to over 6 m (20 ft, in Utahraptor and Achillobator).
The most primitive dromaeosaurid ever described, Mahakala, is also among the smallest, at just 70 cm long. This evidence, combined with the small size of other primitive relatives indicates that the common ancestor of dromaeosaurids, troodontids, and birds – which is called the ancestral paravian (‘’Paraves’’) – may have been very small, at around 65 cm in length and 600 to 700 grams of mass. In 2002, Hwang and colleagues utilized the work of Norell et al., including new characters and better fossil evidence, to determine that birds (avialans) were better thought of as cousins to the dromaeosaurids and troodontids.
The current consensus among paleontologists agrees with the findings of Hwang et al (2002); that dromaeosaurids are most closely related to the troodontids, and together with the troodontids form the clade Deinonychosauria. Deinonychosaurians in turn are the sister taxon to birds, and therefore the closest non-avian relatives of birds. A consensus of paleontologists have concluded that there isn't yet enough evidence to determine whether any dromaeosaurs could fly or glide, or whether they evolved from ancestors that could.

Alternative theories and flightlessness

Dromaeosaurids are so birdlike in some ways that they've led some researchers to wonder if they should be better classified as birds. First, since they've feathers, dromaeosaurs (along with many other coelurosaurian theropod dinosaurs) are “birds” under traditional definitions of the word “bird”, or “Aves”, that are based on the possession of feathers. Calling a primitive theropod like Caudipteryx a bird because it has feathers, however, may stretch the word past any useful meaning. (See the article Avialae for a discussion about the definitions of the clades Aves and Avialae.)
   At least two schools of researchers have proposed that dromaeosaurs may actually be descended from flying ancestors. Hypotheses involving a flying ancestor for dromaeosaurs are sometimes called “Birds Came First” (BCF). 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 something 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 dromaeosaurs and Archaeopteryx as the closest relatives to one another.
   In 2002, Hwang et al. found that Microraptor was the most primitive dromaeosaur. Xu and colleagues in 2003 cited the basal position of Microraptor, along with feather and wing features, as evidence that the ancestral dromaeosaur could glide. In that case the larger dromaeosaurs would be secondarily terrestrial - having lost the ability to glide later in their evolutionary history.
   Other researchers, like Larry Martin believe that dromaeosaurs, along with all maniraptorans are not dinosaurs at all. Martin asserted for decades that birds were unrelated to maniraptorans, but in 2004 he changed his position, and now he agrees that the two are the closest of relatives. Martin believes that maniraptorans are secondarily flightless birds, and that birds evolved from non–dinosaurian archosaurs (or non-theropod dinosaurs in Czerkas' case), so that most of the species formerly called theropods would now not even be classified as dinosaurs.
   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 dromaeosaurs and troodontids were secondarily flightless (or had lost the ability to glide). Corfe and Butler criticized this work on methodological grounds.
   There has been a recent contradiction of these alternative scenarios. Turner and colleagues in 2007 described a new dromaeosaurid, Mahakala omnogovae, which they found to be the most basal and most primitive member of the Dromaeosauridae, more basal than Microraptor. Mahakala had short arms and no ability to glide. Turner et al. also infered that flight evolved only in the Avialae, and these two points suggest that the ancestral dromaeosaurid couldn't glide or fly. In cladistic analysis, the more primitive Mahakala suggests that the ancestral condition for dromaeosaurids is non-volant.

Systematics

Taxonomy

The authorship of the family Dromaeosauridae is credited to W.D. Matthew and Barnum Brown, who erected it as a subfamily (Dromaeosaurinae) of the now-defunct family Deinodontidae in 1922, containing only the new genus Dromaeosaurus. Dromaeosauridae, along with Troodontidae, make up the infraorder Deinonychosauria.
The subfamilies of Dromaeosauridae frequently shift in content based on new analysis, but typically consist of the following groups. A number of dromaeosaurids have not been assigned to any particular subfamily, often because they're too poorly preserved to be placed confidantly in phylogenetic analysis (see section Phylogeny below), or because they're basal relative to the primary subdivisions of Dromaeosauridae (Mahakala, for example, is the most primitive known dromaeosaurid and falls outside any named sub-group). The most basal subfamily of dromaeosaurids is often found to be the Unenlagiinae. This enigmatic group is the most poorly-supported subfamily of dromaeosaurs and it's possible that some or all of its members belong outside of Dromaeosauridae. The larger, ground-dwelling members like Buitreraptor and Unenlagia show strong flight adaptations, although they were probably too large to 'take off'. One member of this group, Rahonavis, is very small, with well-developed wings that show evidence of quill knobs (the attachment points for flight feathers) and it's very likely that it could fly. The next most primitive clade of dromaeosaurs is the Microraptoria. This group includes many of the smallest dromaeosaurs, which show adaptations for living in trees. All known dromaeosaur skin impressions hail from this group and all show an extensive covering of feathers and well-developed wings. Like the unenlagiines, some species may have been capable of active flight. The subfamily Velociraptorinae has traditionally included Velociraptor, Deinonychus, and Saurornitholestes, and while the discovery of Tsaagan lent support to the this grouping, the inclusion of Saurornitholestes is still uncertain. The Dromaeosaurinae is usually found to consist of medium to giant-sized species, with generally box-shaped skulls (the other subfamilies generally have narrower snouts).
The following classification of the various genera of dromaeosaurids is based on studies by Sereno (2005), Senter (2004), Makovicky et al. (2005), Norell et al. (2006), and Turner et al. (2007).

Variraptor

Subfamily Dromaeosaurinae

Subfamily Velociraptorinae

Phylogeny

Dromaeosauridae was first defined as a clade by Paul Sereno in 1998, as the most inclusive natural group containing Dromaeosaurus but not Troodon, Ornithomimus or Passer. The various "subfamilies" have also been re-defined as clades, usually defined as all species closer to the groups namesake than to Dromaeosaurus or any namesakes of other sub-clades (for example, Makovicky defined the clade Unenlagiinae as all dromaeosaurids closer to Unenlagia than to Velociraptor). The Microraptoria is the only dromaeosaurid sub-clade not converted from a subfamily. Senter and colleagues expressly coined the name without the subfamily suffix -inae to avoid perceived issues with erecting a traditional family-group taxon, should the group be found to lie outside dromaeosauridae proper. In Manning's interpretation, the second toe claw would be used as a climbing aid when subduing bigger prey and also as stabbing weapon.

Pack hunting and trackway

Deinonychus fossils have been uncovered in small groups near the remains of the herbivore Tenontosaurus, a larger ornithischian dinosaur. This had been interpreted as evidence that these dromaeosaurs hunted in coordinated packs like some modern mammals. However, not all paleontologists found the evidence conclusive, and subsequent studies suggest that the Deinonychus were more likely to have been engaged in disorganized mobbing behavior. Modern birds and crocodiles (the closest relatives of dromaeosaurs) display little cooperative hunting; instead, they're usually either solitary hunters, or are drawn to previously-killed carcasses, where conflict often 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 dies, it's cannibalized. When this information is applied to the sites containing putative pack-hunting behavior in dromaeosaurs, it appears consistent with 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, evidence against the idea that the animals cooperated in the hunt.
In 2007, scientists described the first known extensive dromaeosaur trackway, in Shandong, China. In addition to confirming the hypothesis that the sickle-claw was held retracted off the ground, the trackway (made by a large, Achillobator-sized species) showed evidence of six individuals of about equal size moving together along a shoreline. The individuals were spaced about one meter apart, and retained the same direction of travel, walking at a fairly slow pace. The authors of the paper describing these footprints interpreted the trackways as evidence that some species of dromaeosaurs lived in groups. While the trackways clearly don't represent hunting behavior, the idea that groups of dromaeosaurs may have hunted together couldn't be ruled out. Many other dromaeosaurid fossils have been found with feathers covering their bodies, some with fully-developed feathered wings. Several even show evidence of a second pair of wings on the hind legs, including Microraptor and Cryptovolans.

In popular culture

The dimensions of the supposed Velociraptor in the film Jurassic Park are much larger than the largest members of the genus. Robert Bakker recalled that Steven Spielberg had been disappointed with the dimensions of Velociraptor and so upsized it, adding that soon afterwards he named Utahraptor which was more the size depicted. Gregory S. Paul, in his book Predatory Dinosaurs of the World, concluded that Deinonychus was a species of Velociraptor and rechristened the species Velociraptor antirrhopus, Michael Crichton continued to synonymize the two genera in his novels, on which the first two films were based. The depiction of the dromaeosaurid in the original Jurassic Park film, while accurate for its time, is now known to have been inaccurate in many respects, including the lack of feathers. While Jurassic Park III attempted to address this last oversight by adding quill-like structures around the head of some of its dromaeosaurs, they didn't resemble the structure or distribution of actual dromaeosaurid feathers known from fossil remains.

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