Dromaeosauridae
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Categories: Dinosaurs, Birds

Dromaeosaurids Fossil range: Jurassic - Cretaceous
Model of Microraptor at the American Museum of Natural History
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Superorder: Dinosauria Order: Saurischia Suborder: Theropoda Infraorder: Deinonychosauria Family: Dromaeosauridae Matthew & Brown, 1922
Genera
See text.

Dromaeosauridae is a family of bird-like theropod dinosaurs. They were mainly small, gracile carnivores that flourished in 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'.

Dromaeosaurids have been found in North America, Europe, North Africa, Japan, China, Mongolia, Madagascar, Argentina, and Antarctica.^[1] 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. Jurassic dromaeosaurs are known primarily from teeth.^[2]

Contents 1 Characteristics 2 Relationship with birds 3 Systematics 3.1 Taxonomy 3.2 Phylogeny 4 Paleobiology 4.1 Predatory behavior 4.2 Feathers 5 References 6 External links

[edit] Characteristics

Dromaeosaurids were small to medium-sized dinosaurs, ranging from about .5 meters in length (2 feet, in the case of Microraptor) to over 6 m (20 ft, in Utahraptor and Achillobator). Like other theropods, they walked on their hind legs. However, the large, curved second toe claw was apparently held retracted, with the third and fourth toes bearing the weight of the animal. The long tail of dromaeosaurids had a flexible base, but most of its length was stiffened by bony tendons. It has been proposed that this tail was used as a stabilizer; in Microraptor gui, the tail ended in a small diamond-shaped fan of feathers which may have been used as an aerodynamic stabilizer and rudder.

[edit] Relationship with birds

Dromaeosaurids were members of the clade Maniraptora, and may be the sister taxon to Aves (birds), although there is mounting evidence that they are true birds themselves. Evidence from dromaeosaurid skin impressions (in animals such as Microraptor, Cryptovolans and Sinornithosaurus) and quill knobs (the anchor points for large wing feathers, seen in Rahonavis and Velociraptor^[3]) show that dromaeosaurids had modern pennaceous feathers and fully formed remiges or 'flight feathers', leading to the question of whether the smaller, larger-winged species were capable of active flight. Modern feathers are a primitive trait of the Maniraptora and primitive dromaeosaurids and dromaeosaur relatives (like Jinfengopteryx, Pedopenna and Archaeopteryx) show evidence of feathers.

Whether dromaeosaurids were birds or non-avian dinosaurs depends on their position relative to Archaeopteryx, on which most scientific definitions of "bird" are based. In phylogenetic nomenclature, all members of the clade Aves are dinosaurs, but since dromaeosaurids are found to be slightly less advanced than Archaeopteryx by most studies, they are excluded from Aves.^[4] However, some researchers (such as Alan Feduccia, Larry Martin, Gregory S. Paul,^[5] and Stephen Czerkas^[6]) have considered dromaeosaurids to be more advanced than Archaeopteryx, and therefore members of the clade (or class) Aves. Paul, for example, pointed out numerous features of the dromaeosaurid skeleton which he interpreted as evidence that the entire group had evolved from flying ancestors.^[7] Mackovicky and colleagues also found that primitive dromaeosaurids, such as Microraptor and Rahonavis, were more bird-like than advanced forms like Velociraptor, indicating that the larger dromaeosaurids were secondarily flightless, like the modern ostrich.^[8] Additionally, the discovery in 2005 of the Thermopolis specimen of Archaeopteryx, which preserved a dromaeosaurid-like hyperextendible second toe, may mean that Archaeopteryx itself is more primitive than the dromaeosaurids.^[9]

[edit] Systematics

[edit] 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.^[10] 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 confidantly 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.^[11] 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).

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).^{[12][4][8][13][3]}

• Family Dromaeosauridae
  • Atrociraptor
  • Dromaeosauroides
  • Luanchuanraptor
  • Mahakala
  • Pyroraptor
  •  ?Unquillosaurus^[14]
  • Variraptor
  • Subfamily Dromaeosaurinae^[10]
    • Achillobator
    • Adasaurus
    • Dromaeosaurus
    • Utahraptor
  • Subfamily Microraptorinae^[4]
    • Bambiraptor
    • Cryptovolans
    • Graciliraptor
    • Microraptor
    • Sinornithosaurus
  • Subfamily Unenlagiinae^[15]
    • Buitreraptor
    • Neuquenraptor
    • Rahonavis
    • Shanag
    • Unenlagia
  • Subfamily Velociraptorinae^[16]
    • Deinonychus
    • Saurornitholestes
    • Tsaagan
    • Velociraptor

[edit] 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.^[4] 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.^[12]

The cladogram below follows a 2007 analysis by Turner and colleagues, with sub-clades labelled according to definitions by Sereno, 2005.^[3]

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

[edit] Paleobiology

[edit] Predatory behavior

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 disembowel prey. This interpretation was commonly 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.^[17] 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
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.^[18] 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 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 will attack smaller komodos that attempt to feed; if the smaller animal dies, it is 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.^[19] No evidence for any kind of social behavior has been reported for dromaeosaurs other than Deinonychus.

[edit] Feathers

The first known dromaeosaur with definitive evidence of feathers was Sinornithosaurus, reported from China by Xu et al. in 1999.^[20] Many other dromaeosaurid fossils have been found with feathers covering their bodies, some with fully-developed feathered wings. Some even show evidence of a second pair of wings on the hind legs, including Microraptor and Cryptovolans.^[21] 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 attatchment 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.^[7][3] 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 used as evidence that all members of the family retained feathers.^[22][3]

[edit] References

[edit] 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

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