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Dromaeosaurids, 'raptors' or members of the family Dromaeosauridae ('running lizards' from Greek dromeus (δρομευς) meaning 'runner' and sauros (σαυρος) meaning 'lizard') are theropod dinosaurs. They were mainly small, gracile carnivores that flourished in 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 modern carnivorous birds of prey, which are also commonly called raptors. 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 60 million years, up until the Cretaceous-Tertiary extinction event. Jurassic dromaeosaurs are known primarily from teeth (Metcalf et al., 1992).
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 second toe claw was apparently held retracted, with the third and fourth toes bearing the weight of the animal. The second toe claw is curved and bladelike, and there is currently disagreement about its function. Some have argued that it served as a hook, but its sharp ventral edges and flattened shape suggest it may have been used as a slashing weapon, like the canines of saber-toothed cats. 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 ends in a small, diamond-shaped fan of feathers which may have been used as an aerodynamic stabilizer and rudder. There is suggestive evidence that some dromaeosaurids hunted in packs: Deinonychus fossils have been uncovered in small groups near the remains of the herbivore Tenontosaurus tilletti, a larger ornithischian dinosaur. Not all paleontologists find the evidence conclusive, however. Relationship with birds Main article: Feathered dinosaurs Dromaeosaurids are theropods, and may be the sister taxon to Aves (birds), although there is mounting evidence (including the Thermopolis specimen of Archaeopteryx which preserves a retractable second toe claw) that they are true birds (see below). Evidence from dromaeosaur skin impressions (in animals such as Microraptor, Cryptovolans and Sinornithosaurus) shows modern pennaceous feathers and fully formed remiges or 'flight feathers', leading to the question of whether these animals 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. While it is extremely likely that all small dromaeosaurs had feathers, it is possible that the larger forms lost some or all of their insulatory covering (Prum & Brush, 2002). Flightless birds? While dromaeosaurids have traditionally been considered non-avian dinosaurs, some researchers (such as Martin, Paul, and Czerkas) consider dromaeosaurids and other maniraptorans to be more derived than the first bird, Archaeopteryx and therefore members of the clade/class Aves. While few published cladistic studies support this hypothesis (instead favoring a position as the closest relatives of Aves but not true members of that group), these researchers point to a number of skeletal features present in dromaeosaurs that are usually found only in flightless birds. The presence of advanced wing feathers, the possibility of flight in the most primitive dromaeosaurs (microraptorians and Rahonavis) (Czerkas, 2002) and the shoulder joints adapted for flapping in many species (such as Unenlagia) also lend support to the idea that dromaeosaurs were true birds (Paul, 2002). Mayr & Peters (2006) also consider it extremely likely that dromaeosaurs were secondarily flightless (like modern ostriches), having evolved from flying or gliding ancestors, citing the dromaeosaurid characteristics of Archaeopteryx, especially in the Thermopolis specimen. This specimen, described by Mayr, Pohl, and Peters in 2005, displays skeletal features of Archaeopteryx previously known only in dromaeosaurs and troodontids. Further, Mayr and Peters cite the presence of well-developed wings in primitive dromaeosaurs like Microraptor as evidence that the entire group containing deinonychosaurs, Archaeopteryx, and modern birds, originated from a common flying ancestor. Classification The family Dromaeosauridae was first erected by Matthew and Brown (1922) for the new genus Dromaeosaurus and was given a cladistic definition by Sereno (1998). Sereno defined it as the most inclusive clade containing Dromaeosaurus but not Troodon, Ornithomimus or Passer. Dromaeosauridae, along with Troodontidae, is a subgroup of Deinonychosauria. However, occasionally troodontids are considered more derived or more basal than dromaeosaurids, rendering Deinonychosauria redundant. The subfamilies of Dromaeosauridae frequently shift in content based on new analysis but typically consist of the following groups: 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 (1986) was among the first to name dinosaurs such as Deinonychus and Velociraptor as archaeopterygids, rather than dromaeosaurids. Genera A classification of the various genera of dromaeosaurids, based on Sereno (2005), Senter (2004) and Makovicky (2005). | |||||||||||||||||||||||||||||||||||||||
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