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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 and Argentina. 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.

Characteristics

Dromaeosaurs were small to medium-sized dinosaurs, ranging from about 2-20 feet in length. 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 dromaeosaurs 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.

Dromaeosaurid Relationship with birds and the Meaning of the Term of "Bird"

The study of dromaeosaurs was pivotal in reviving the hypothesis that birds evolved from dinosaurs.

Since the 1960’s a broad consensus of biologists (the Theropod Working Group, Ostrom, Currie, Chiappe, Xu, Benton, Novas, Pol, Gauthier, Sereno, Padian, et al.), working in anatomy and systematics, using both cladistic and non - cladistic approaches, has demonstrated that Dromaeosaurids are theropod dinosaurs, that their closest relatives are the Troodontids, and that their next closest relatives are Avialans like Archaeopteryx.

Viewed the other way, one lineage of advanced theropods evolved to the Paravian stage, which produced two lineages; one leading to Archaeopteryx and birds, the other leading to Deinonychosaurs. The Deinonychosaurs then split into two lineages - the Dromaeosaurids and the Troodontids.

This result may sound overly complex, but the relationship between dromaeosaurs and birds is clearly a family tree with many branches. Family trees, like any series of historical events, are inevitably complex. The short version is that dromaeosaurs are best thought of as cousins of birds.

One of the most interesting results of the last decade of fossil discoveries and research is that the previous definition of "bird" (the smallest clade containing Archaeopteryx and all living birds) has become a somewhat arbitrary definition. Many of the features once thought to be unique to Archaeopteryx and living birds, especially feathers, are now known in many dinosaurs. Highly bird – like features, some much more bird -like than in Archaeopteryx(pygostyles, keeled sterna, toothless beaks, etc.) are distributed among advanced theropods, either inherited in common or independently derived through convergent evolution (homoplasy).

All Coelurosaur dinosaurs (including Dilong, Sinosauropteryx, Mononykus, and Beipiaosaurus) inherited the capacity to make simple protofeathers, and many of the advanced ones (Microraptor, Sinornithosaurus,Caudipteryx, Jinfengopteryx) had tail feathers like Archaeopteryx and long, vaned, asymmetrical feathers on the arms arranged like the wings of modern birds. Indeed, genes for making feather - like dermal structures may be far older, going back to the last common ancestor between pterosaurs and dinosaurs. See Feathered dinosaurs.

Flight was once thought to have arisen in Archaeopteryx and to have been inherited by modern birds. The story may be more complex. It is not possible to demonstrate that Archaeopteryx is an actual ancestor of today's birds, only that it was a close relative of that actual ancestor. Also, Archaeopteryx’s pectoral anatomy was so primitive that it may not have been capable of flapping flight. Moreover, Dromaeosaurs like Microraptor and Rahonavis could possibly have parachuted or glided, which implies that aerodynamic adaptations may have been common in ancestral members of Paraves, and not unique to Archaeopteryx.

There are now so many fossils known for primitive or transitional "birds" that most researchers favor the use of a "protobird" clade, the Avialae, and reserving the term "bird" or Aves, for the most advanced avialans only. Aves, then, can be defined as the last common ancestor of all living birds and all of its descendants, living or extinct. This last common ancestor of all living birds probably lived in the Cretaceous, and, anatomically, it would have looked a lot more like the advanced flyer Apsaravis than like Archaeopteryx. Under this definition Aves does have a clear suite of unique anatomical features. This definition is also consistent with the common understanding of the word "bird". Dromaeosaurids cannot be considered birds under this definition.

If, instead, one arbitrarily defines Archaeopteryx as a bird (in Aves), then dromaeosaurs and troodontids may be birds, but they cannot be proven to be. This is because it is possible, but it cannot be proved, that Archaeopteryx is an actual ancestor of Dromaeosaurids, rather than a close relative of that ancestor.

The study of dromaeosaurs has been used to formulate alternative hypotheses about the relationships between birds and dinosaurs. Such alternatives include hypotheses wherein 1) birds are not related to dinosaurs (including dromaeosaurs)(Martin, 1980), 2) dromaeosaurs are secondarily flightless birds which did evolve from dinosaurs (Paul, 1988), or 3) dromaeosaurs are secondarily flightless birds which evolved from Longisquama, and not Dinosaurs (Martin, 2004). These alternatives all contradict the empirical evidence, in that they rely on as few as six pieces of evidence (usually including soft evidence like a priori reasoning or imaginary scenarios) and exclude a larger amount of contrary evidence; sometimes as many as two - hundred shared anatomical characters (which are always hard physical evidence observed in the fossils or organisms themselves). These alternative hypotheses come from and appeal to those who distrust the strong consensus among experts or feel dissatisfaction with contemporary systematic Biology.

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 was defined as a clade by Paul Sereno in 1998, as the most inclusive natural group containing Dromaeosaurus but not Troodon, Ornithomimus or Passer. Dromaeosauridae, along with Troodontidae, make up the infraorder Deinonychosauria.

Classification

The subfamilies of Dromaeosauridae frequently shift in content based on new analysis, but typically consist of the following groups. 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 subfamily of dromaeosaurs is the Microraptorinae (which is often termed Microraptoria in cladistic studies). This subfamily 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), and Norell et al. (2006).

• Family Dromaeosauridae
  • Atrociraptor
  • Dromaeosauroides
  • Pyroraptor
  • Saurornitholestes
  • Shanag
  • Variraptor
  • Subfamily Unenlagiinae
    • Buitreraptor
    • Neuquenraptor
    • Rahonavis
    • Unenlagia
  • Subfamily Microraptorinae
    • Bambiraptor
    • Cryptovolans
    • Graciliraptor
    • Microraptor
    • Sinornithosaurus
  • Subfamily Velociraptorinae
    • Deinonychus
    • Tsaagan
    • Velociraptor
  • Subfamily Dromaeosaurinae
    • Achillobator
    • Adasaurus
    • Dromaeosaurus
    • Utahraptor

Note: Some authors and cladistic studies, especially those that include the small flying species of dromaeosaurid, find the first bird Archaeopteryx to be an early member of Dromaeosauridae. If this is the case, since its family was named before the family Dromaeosauridae, ICZN rules state that its family name has priority. Therefore, when Archaeopteryx is included, the name "Dromaeosauridae" becomes invalid in favor of the name Archaeopterygidae. Paul was among the first to name dinosaurs such as Deinonychus and Velociraptor as archaeopterygids, rather than dromaeosaurids.

Phylogeny

Cladogram after Turner et al. 2007, with subfamilies added according to definitions by Sereno, 2005.

Paleobiology

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. 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. 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. No evidence for any kind of social behavior has been reported for dromaeosaurs other than Deinonychus.

Feathers

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. Some even show evidence of a second pair of wings on the hind legs, including Microraptor and Cryptovolans. In light of this, it is most likely that even the large, ground-dwelling dromaeosaurids bore feathers, since even flightless birds today retain most of their plumage. While it is extremely likely that all dromaeosaurs had feathers, it is also possible that the larger forms lost some or all of their insulatory covering.

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, a theory that has since been largely rejected. 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, though Jurassic Park III addressed this last oversight.

References

1. Metcalf, S.J., Vaughan, R.F., Benton, M.J., Cole, J., Simms, M.J. and Dartnall, D.L. (1992). "A new Bathonian (Middle Jurassic) microvertebrate site, within the Chipping Norton Limestone Formation at Hornsleaslow Quarry, Gloucestershire". Proceedings of the Geologists’ Association. 103: 321–342.{{cite journal}}: CS1 maint: multiple names: authors list (link)
2. ^ Matthew, W. D., and Brown, B. (1922) "The family Deinodontidae, with notice of a new genus from the Cretaceous of Alberta." Bulletin of the American Museum of Natural History, 46: 367-385.
3. ^ Turner, A.S. (2007). "A small derived theropod from Öösh, Early Cretaceous, Baykhangor Mongolia" (PDF). American Museum Novitates. 3557: 1–27. Retrieved 2007-03-29. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
4. Sereno, P. C. (2005). "The logical basis of phylogenetic taxonomy". Systematic Biology. 51: 1–25.
5. ^ * Senter, P., R. Barsold, B.B. Britt and D.A. Burnham (2004). "Systematics and evolution of Dromaeosauridae (Dinosauria, Theropoda)". Bulletin of the Gunma Museum of Natural History. 8: 1–20.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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7. Norell, M.A., Clark, J.M., Turner, A.H., Makovicky, P.J., Barsbold, R., and Rowe, T. (2006). "A new dromaeosaurid theropod from Ukhaa Tolgod (Omnogov, Mongolia)." American Museum Novitates, 3545: 1-51.
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12. Manning, P.L., Payne, D., Pennicott, J., Barrett, P.M., and Ennos, R.A. (2005). "Dinosaur killer claws or climbing crampons?". Biology Letters. 2: 110–112. doi:10.1098/rsbl.2005.0395.{{cite journal}}: CS1 maint: multiple names: authors list (link)
13. Maxwell, W. D. (1995). "Taphonomy and paleobiological implications of Tenontosaurus-Deinonychus associations". Journal of Vertebrate Paleontology. 15 (4): 707–712. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
14. Roach, B. T. (2007). "A reevaluation of cooperative pack hunting and gregariousness in Deinonychus antirrhopus and other nonavian theropod dinosaurs". Bulletin of the Peabody Museum of Natural History. 48 (1): 103–138. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
15. Xu, X., Wang, X.-L., and Wu, X.-C. (1999). "A dromaeosaurid dinosaur with a filamentous integument from the Yixian Formation of China". Nature. 401: 262–266. doi:10.1038/45769.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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18. Prum, R., and Brush, A.H. (2002). "The evolutionary origin and diversification of feathers". The Quarterly Review of Biology, 77: 261-295.
19. Bakker, Robert T. (1995). Raptor Red. New York: Bantam Books. pp. pg. 4. ISBN 0-553-57561-9. {{cite book}}: |pages= has extra text (help)
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21. Pérez-Moreno, B.P. (1994). "A theropod dinosaur from the Lower Cretaceous of southern France". Dinosaurs and Other Fossil Reptiles of Europe, Second Georges Cuvier Symposium, Montbéliard; Revue de Paléobiologie, Volume spécial. 7: 173–188. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
22. Currie, P. J. (1995). "New information on the anatomy and relationships of Dromaeosaurus albertensis (Dinosauria: Theropoda)". Journal of Vertebrate Paleontology. 15 (3): 576–591. (abstract)
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External links

• See entry on "Dromaeosauridae" at DinoData (registration required, free).
• 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

• Feathered dinosaurs
• Dromaeosaurs