Dromaeosauridae
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| Dromaeosaurids
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Achillobator |
Dromaeosaurids, "raptors" or members of the family Dromaeosauridae ("running lizards") are theropod dinosaurs. They were small, fast and agile dominant carnivores throughout the Cretaceous period. In popular usage they are often called "raptors" after the Velociraptor, which was made famous by the film Jurassic Park.
Dromaeosaurids have been found in North America, Europe, North Africa, Japan, China, Mongolia and Argentina. They were a very successful group in the Cretaceous, existing for over 60 million years, up until the Cretaceous-Tertiary extinction event.
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Characteristics
The characteristics of the Dromaeosauridae are distinctive. Unlike their relatives, such as the Troodontidae, dromaeosaurids were muscular animals. Long fleshy tails held out behind could help them swing quickly to change direction. They were lightly built, agile, bipedal hunters, with large eyes. They also had the biggest brains for their body weight among all dinosaurs excepting the troodontids, with an encephalization quotient of about 5.0¹, which indicates that they were among the most intelligent dinosaurs. They were far more intelligent than the plant-eating sauropods, with their intelligence perhaps approaching that of modern-day birds of comparable sizes (Hopson, 1980).
One of the more unusual features of dromaeosaurids is that the tail is stiffened by elongated bones. Though sometimes referred to as ossified tendons, they are actually incredibly lengthened versions of bones most amniotes use to articulate vertebrae together, termed prezygopophyses. The chevrons or hemal arches (bones that attach to the bottom of the tail vertebrae) were also highly elongated. This resulted in a stiff, but not rigid tail, perhaps used to steer the animal when in hot pursuit of prey.
There is evidence that some dromaeosaurids hunted in packs: Deinonychus fossils have been uncovered in small groups that seem to have been killed while attacking Tenontosaurus tilletti, a larger ornithischian dinosaur. Not all paleontologists find the evidence conclusive, however.
The enlarged raptorial claw on the second toe possessed by all dromaeosaurids was a raking weapon that could open a deep disembowelling gash, as it was about twice the length of the other claws. Then the agile predator could withdraw and wait for a victim to weaken from bleeding.
Relationship with birds
Main article: Feathered dinosaurs
Dromaeosaurids are theropods, and may be the sister taxon to Avialae (birds and proto-birds). Dromaeosauridae is a subgroup of Deinonychosauria, and includes such animals as Deinonychus, Dromaeosaurus, Velociraptor, Saurornitholestes, and Utahraptor. Dromaeosauridae is sometimes seen as synonymous to Deinonychosauria, when Troodontidae is placed in less derived maniraptorans.
Recently described Chinese "dromaeosaurids", such as Microraptor, Cryptovolans and Sinornithosaurus had modern pennaceous feathers and fully formed remiges or "flight feathers", leading to the question of whether these animals were capable of powered flight. This also leads to the question whether all dromaeosaurids bore feathers, which at this point is uncertain.
Discussion about the relationship between birds and dinosaurs has mostly narrowed to whether bird ancestors lie within Dromaeosauridae or not. In order to exclude them, one recent cladistic analysis (Senter, 2004) has gone so far as to remove these three genera from Dromaeosauridae in the strict sense, and the authors created a new closely related taxon Microraptoria for them. Thus under this re-classification, it can still be claimed that there have been no reports of fossil feathers in Dromaeosauridae.
References
- Colbert, E. and D. A. Russell (1969). "The small Cretaceous dinosaur Dromaeosaurus." American Museum Novitates 2380: 1-49.
- Hopson, J. A. 1980, "Relative brain size in dinosaurs: implications for dinosaurian endothermy", American Association for the Advancement of Science Symposium no. 28, pp. 287–310.
- Ji, Q., M. A. Norell, et al. (2001). "Distribution of integumentary structures in a feathered dinosaur." Nature 410: 1084-1088.
- Kirkland, J. I., R. Gaston, et al. (1993). "A large dromaeosaur (Theropoda) from the Lower Cretaceous of Eastern Utah." Hunteria 2: 1-16.
- Norell, M. A. and P. J. Makovicky (2004). Dromaeosauridae. The Dinosauria. D. B. Weishampel, P. Dodson and H. Osmolska. Berkeley, University of California Press: 196-209.
- Ostrom, J. H. (1969). "Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana." Peabody Museum of Natural History, Bulletin 30(1-165).
- Perle, A., M. A. Norell, et al. (1999). "A new maniraptoran theropod Achillobator giganticus (Dromaeosauridae) from the Upper Cretaceous of Burkhant, Mongolia." Department of Geology, National University of Mongolia, Ulaanbaatar: 1-105.
- Senter, P., R. Barsold, B.B. Britt and D.A. Burnham. 2004. "Systematics and evolution of Dromaeosauridae (Dinosauria, Theropoda)" in Bulletin of the Gunma Museum of Natural History, 8: 1-20.
- Xu, X., X.-L. Wang, et al. (1999). "A dromaeosaurid dinosaur with filamentous integument from the Yixian Formation of China." Nature 401: 262-266.
- Xu, X., Z. Zhou, et al. (2003). "Four winged dinosaurs from China." Nature 421: 335-340.
External links
- The Dromaeosauridae: The Raptors! (http://www.ucmp.berkeley.edu/diapsids/saurischia/dromaeosauridae.html), from the University of Calfornia Berkeley Museum of Paleontology.
- Dromaeosauridae (http://personal2.stthomas.edu/jstweet/dromaeosauridae.htm), by Justin Tweet from Thescelosaurus!de:Dromaeosauridae