Dromaeosauridae

Dromaeosauridae is a family of extinct bird-like theropod dinosaurs. They were small to medium-sized, feathered carnivores that flourished during the Cretaceous Period. In informal usage they are 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-Paleogene extinction event. The presence of dromaeosaurs as early as the Middle Jurassic has been confirmed by the discovery of isolated fossil teeth, though no dromaeosaurid body fossils have been found from this epoch.

Characteristics

Body plan

Mounted skeleton of Velociraptor mongoliensis, Museum voor Natuurwetenschappen, Brussels

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, 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. 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 (the middle finger being the longest and the first finger being the shortest) 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.

Illustration of a feathered Deinonychus antirrhopus

It is now known that at least some, and probably all, dromaeosaurids were covered in feathers, including large, vaned, wing and tail feathers. This development, first hypothesized in the mid-late 1980s and confirmed by fossil discoveries in 1999, represents a significant change in the way dromaeosaurids have historically been depicted in art and film (see “Feathers” below).

Diagnosis

Dromaeosaurs 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 prezygapophyses of the caudal vertebrae elongate and spanning several vertebrae; the presence of a subglenoid fossa on the coracoid.

The dromaeosaurid foot

Model of the foot bones of a dromaeosaurid

Like other theropods, dromaeosaurids were bipedal; that is, they walked on their hind legs. However, whereas other theropods walked with three toes contacting the ground, fossilized footprint tracks show that dromaeosaurids apparently 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 "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 is 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; in Microraptor and a possible specimen of Sinornithosaurus (specimen NGMC 91, nicknamed "Dave"), elongate diamond-shaped fans of feathers are preserved on the end of the tail. In Microraptor, this may have been used as an aerodynamic stabilizer and rudder during gliding and/or powered flight (see "Flight and gliding" below).

Size

Dromaeosaurids were small to medium-sized dinosaurs, ranging from about 0.6 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.

Relationship with birds

See also: Origin of birds and Feathered dinosaurs

Comparison of the forelimbs of Deinonychus (left) and Archaeopteryx (right), one of many skeletal similarities between avians and dromaeosaurids.

Dromaeosaurids share many features with early birds (clade Avialae or Aves). 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 (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 avialans, and therefore the closest relatives of avialan birds. A consensus of paleontologists has concluded that there is not yet enough evidence to determine whether any dromaeosaurs could fly or glide, or whether they evolved from ancestors that could.

Alternative theories and flightlessness

Illustration of the bird-like Unenlagia

Dromaeosaurids are so birdlike that they have led some researchers to argue that they would be better classified as birds. First, since they had 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. 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. (See the article Avialae for a discussion about the definitions of the clades Aves and Avialae.)

Fossil cast of the extensively feathered NGMC 91, a probable specimen of Sinornithosaurus

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.

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 dromaeosaurs were proper birds, rather than possible gliders. He later issued a revised reconstruction in agreement with that of Microraptor

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, so that most of the species formerly called theropods would now not even be classified as dinosaurs.

The Thermopolis specimen of Archaeopteryx, which showed that it also had a hyperextendible second toe.

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.

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.

Systematics

Taxonomy

Dromaeosaurid fossil displayed in Hong Kong Science Museum.

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 are too poorly preserved to be placed confidently in phylogenetic analysis (see section Phylogeny below), or because they are 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 is 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 is 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).

Dromaeosaurus, a dromaeosaurine.

Microraptor, a microraptorine.

Saurornitholestes, a saurornitholestine.

Buitreraptor, an unenlagiine.

Velociraptor, a velociraptorine.

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).

• Family Dromaeosauridae
  • Dromaeosauroides
  • Luanchuanraptor
  • Mahakala
  • Pyroraptor
  • ?Unquillosaurus
  • Variraptor
  • Subfamily Dromaeosaurinae
    • Achillobator
    • Dromaeosaurus
    • Utahraptor
  • Subfamily Microraptorinae
    • Cryptovolans
    • Graciliraptor
    • Hesperonychus
    • Microraptor
    • Sinornithosaurus
    • Shanag?
    • Tianyuraptor?
  • Subfamily Saurornitholestinae
    • Atrociraptor
    • Bambiraptor
    • Saurornitholestes
  • Subfamily Unenlagiinae
    • Austroraptor
    • Buitreraptor
    • Neuquenraptor
    • Rahonavis
    • Unenlagia
  • Subfamily Velociraptorinae
    • Adasaurus
    • Deinonychus
    • Itemirus?
    • Nuthetes
    • Tsaagan
    • Velociraptor

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. Sereno offered a revised definition of the sub-group containing Microraptor to ensure that it would fall within Dromaeosauridae, and erected the subfamily Microraptorinae, attributing it to Senter et al., though this usage has only appeared on his online TaxonSearch database and has not been formally published.

The cladogram below follows a 2007 analysis by Turner and colleagues, with sub-clades labelled according to definitions by Sereno, 2005.

Dromaeosauridae	Mahakala unnamed Unenlagiinae Shanag Buitreraptor unnamed Rahonavis Unenlagia Microraptorinae Microraptor Graciliraptor Sinornithosaurus unnamed Velociraptorinae Tsaagan unnamed Saurornitholestes unnamed Deinonychus Velociraptor Dromaeosaurinae Adasaurus Dromaeosaurus unnamed Achillobator Utahraptor

Paleobiology

Predatory behavior

Dromaeosaurus foot and "sickle-claw"

Life restoration of Achillobator

There is currently disagreement about the function of the enlarged "sickle claw" on the second toe. When John Ostrom described it for Deinonychus in 1969, he interpreted the claw as a blade-like slashing weapon, much like the canines of some saber-toothed cats, used with powerful kicks to cut into prey. Adams (1987) suggested that the talon was used to disembowel large ceratopsian dinosaurs. The interpretation of the sickle claw as a killing weapon applied to all dromaeosaurids. However, Manning et al. argued that the claw instead served as a hook, reconstructing the keratinous sheath with an elliptical cross section, instead of the previously inferred inverted teardrop shape. In Manning's interpretation, the second toe claw would be used as a climbing aid when subduing bigger prey and also as stabbing weapon.

Ostrom compared Deinonychus to the ostrich and cassowary. He noted that the bird species can inflict serious injury with the large claw on the second toe. The cassowary has claws up to 125 millimetres (4.9 in) long. Ostrom cited Gilliard (1958) in saying that they can sever an arm or disembowel a man. Kofron (1999 and 2003) studied 241 documented cassowary attacks and found that one human and two dogs had been killed, but no evidence that cassowaries can disembowel or dismember other animals. Cassowaries use their claws to defend themselves, to attack threatening animals, and in agonistic displays such as the Bowed Threat Display. The seriema also has an enlarged second toe claw, and uses it to tear apart small prey items for swallowing.

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 a subsequent study published in 2007 by Roach and Brinkman suggests that the Deinonychus may have actually displayed a disorganized mobbing behavior. Modern diapsids, including birds and crocodiles (the closest relatives of dromaeosaurs), display minimal cooperative hunting; instead, they are 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 might attack smaller komodo dragons that attempt to feed; if the smaller animal dies, it is usually cannibalized. When this information is applied to the sites containing putative pack-hunting behavior in dromaeosaurs, it appears somewhat consistent with a komodo- or crocodile-like feeding strategy.Deinonychus skeletal remains found at these sites are from subadults, with missing parts that may have been eaten by other Deinonychus, which a study by Roach et al. presented as evidence against the idea that the animals cooperated in the hunt.

Illustration of a Velociraptor making a trackway

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 do not represent hunting behavior, the idea that groups of dromaeosaurs may have hunted together could not be ruled out.

Feathers

See also: Feathered dinosaurs

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 (remiges) and tail (rectrices), 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.

Fossil of Sinornithosaurus millenii, the first evidence of feathers in dromaeosaurids.

The first known dromaeosaur 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. Several even show evidence of a second pair of wings on the hind legs, including Microraptor and Cryptovolans. 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.

Flying and gliding

Rahonavis ostromi, a dromaeosaurid originally classified as an avialan bird, may have been capable of limited, powered flight.

The ability to fly or glide has been suggested for at least two dromaeosaurid species. The first, Rahonavis ostromi (originally classified as avian bird, but found to be a dromaeosaurid in later studies) may have been capable of powered flight, as indicated by its long forelimbs with evidence of quill knob attachments for long sturdy flight feathers. The forelimbs of Rahonavis were more powerfully built than Archaeopteryx, and show evidence that they bore strong ligament attachments necessary for flapping flight. Luis Chiappe concluded that, given these adaptations, Rahonavis could probably fly but would have been more clumsy in the air than modern birds.

Fossil of Microraptor gui impressions of feathered wings

Another species of dromaeosaurid, Microraptor gui, may have been capable of gliding using its well-developed wings on both the fore and hind limbs. A 2005 study by Sankar Chatterjee suggested that the wings of Microraptor functioned like a split-level "biplane", and that it likely employed a phugoid style of gliding, in which it would launch from a perch and swoop downward in a 'U' shaped curve, then lift again to land on another tree, with the tail and hind wings helping to control its position and speed. Chatterjee also found that Microraptor had the basic requirements to sustain level powered flight in addition to gliding.

Powered flight has also been suggested for the species Cryptovolans pauli (the name of which means "hidden flyer"), though Cryptovolans is probably synonymous with Microraptor.

External links

• Dromaeosauridae at DinoData.
• The Dromaeosauridae: The Raptors!, from the University of California Berkeley Museum of Paleontology.
• Dromaeosauridae, by Justin Tweet from Thescelosaurus.
• Dinosaurs - Complete and free online edition of the book "Dinosaurs" as written by W. D. Matthew (cited in this article with authorship of the family Dromaeosauridae), and former Curator of Vertebrate Paleontology at the American Museum of Natural History in New York; Originally published in 1915
• Dromaeosauridae, Dinosaur-world reference with in-depth description and pictures of many dromaeosauridae dinosaurs