Dromaeosauridae

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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) and their similarities to carnivorous birds who are also commonly called raptors.

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.

Characteristics

First discovered by the palaeontologist Dr A. Waldman, the characteristics of the Dromaeosauridae are distinctive; among other things they were notoriously fast and small. Unlike their relatives, such as the Troodontidae, dromaeosaurids were muscular animals. Long bony 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 tail chevrons or hemal arches (bottom part of a vertebra) were also highly elongated. This resulted in a stiff, but not rigid tail, perhaps used to steer the animal when in 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 large, raptorial claw on the second toe appears to have been used anchor the raptor to its prey rather than slash at prey as originally thought. Computer models show that the size and shape of the claw makes it poorly suited to slashing but ideal for peircing and holding, similar to a fish hook.

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 microraptorians, 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. It is almost certain that all dromaeosaurids bore feathers of some type.

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

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• Template:Journal reference (doi:10.1038/45769)
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External links

• " Dromaeosauridae names and info". DinoData.
• The Dromaeosauridae: The Raptors!, from the University of California Berkeley Museum of Paleontology.
• Dromaeosauridae, by Justin Tweet from Thescelosaurus.