Dromaeosauridae Matthew and Brown, 1922

Included taxa. Adasaurus mongoliensis Barsbold, 1983; Bambiraptor feinbergi Bumham, Derstler, Currie, Bakker, Zhou, and Ostrom, 2000; Deinonychus antirrhopus Ostrom, 1969 a; Dromaeosaurus albertensis Matthew and Brown, 1922; Hulsanpes perlei Osmólska, 1982; Megaraptor namunhaiquii Novas, 1998; Microraptor zhaoianus Xu, Zhou and Wang, 2000; Saurornitholestes langstoni Sues, 1978; Sinornithosaurus millenii Xu, Wang and Wu, 1999; Utahraptor ostrommaysi Kirkland, Burge and Gaston, 1993; Velociraptor mongoliensis Osborn, 1924.

Temporal range. Barremian-Maastrichtian.

Occurrence. Nemegt Svita, Bayankhongor, Mongolia; Cloverly Formation, Montana and Wyoming, USA; Judith River Formation, Alberta, Canada, and Montana, USA; Barun Goyot Formation, Omnogov, Mongolia; Rio Neuquén Formation, Neuquén, Argentina; Judith River Formation, Alberta, Canada, and Montana, USA; Two Medicine Formation, Montana, USA; Yixian Formation, Liaoning, China; Cedar Mountain Formation, Utah, USA; Djadokhta Formation, Beds of Toogreg, Omnogov, Mongolia; Minhe Formation, Bayan Mandahu redbeds, Nei Mongol Zizhiqu, China.

Diagnosis. Quadratojugal is in the form of an inverted T-shape; paroccipital processes very long, extending laterally to the head of the quadrate; enlarged, triangular internal mandibular fenestra; fusion of interdental plates to each other and the margin of the jaws without clearly visible suture in adults; Mt. II with strongly ginglymoid distal articular facet. The following characters are not preserved in Dromaeosaurus, the type genus of the family, but probably represent synapomorphies of the family: mid-cervical vertebrae with hypertrophied epipophyses, pointing laterally; distal caudal vertebrae with extremely elongate prezygapophyses (equivalent to the length of more than two centra); distal chevrons inverted T-shaped and extremely anteroposteriorly elongated; second digit of the pes bearing a strongly enlarged ungual, with an asymmetric arrangement of the claw grooves and a sharp ventral margin.

Remarks. The first dromaeosaurid, Dromaeosaurus albertensis, was described in 1922 by Matthew and Brown, followed by the description of Velociraptor mongoliensis (Text-fig. 6d) in 1924 by Osborn. However, it was not until the discovery of Deinonychus antirrhopus that the peculiar anatomy of these animals became known and their significance for theropod phylogeny was recognized (Colbert and Russell 1969; Ostrom 1969 tz, b, 1972, 1973). Since then, our knowledge of the anatomy of these animals has rapidly increased following the discovery of more species and further studies of known taxa (Ostrom 1974 «, 1976 b, 1990; Sues 1977, 1978; Osmólska 1982; Barsbold 1983; Kirkland et al. 1993; Currie 1995; Norell and Makovicky 1997, 1999; Norell et al. 1997; Xu et al. 1999; Xu and Wang 2000).

Unfortunately, Dromaeosaurus, the type genus of the family, is based mainly on an imperfect skull and very fragmentary associated pedal remains. Originally, the recognition of Dromaeosaurus as a close relative of the much better known Deinonychus was based largely on the enlarged claw on the second pedal digit (Colbert and Russell 1969). Subsequent work showed that troÖdontids, which also have an enlarged second pedal ungual and were, therefore, thought to be very closely related to Dromaeosaurus, are sufficiently different from taxa such as Deinonychus and Velociraptor to merit their own family (Barsbold 1974, 1983; Currie 1985, 1987), so that the simple presence of this character cannot be used to define dromaeosaurids. Furthermore, Currie (1995) pointed out that the claw figured and described by Colbert and Russell (1969, fig. 15d) may actually belong to the contemporaneous dromaeosaurid Saurornitholestes. However, similarities in skull morphology between Dromaeosaurus and the better known velociraptorines seem to be sufficient to justify their treatment as a single clade (see Currie 1995).

Megaraptor was described as a coelurosaurian theropod of uncertain systematic position on the basis of extremely fragmentary material from the Upper Cretaceous of Argentina (Novas 1998). However, the presence of asymmetric claw grooves and a sharp ventral margin of the second pedal ungual indicates a dromaeosaurid relationship for this taxon (Rauhut and Werner 1995). Novas (1998) hesitated to refer the taxon to the Dromaeosauridae because of the presence of a straight shaft and an enlarged olecranon in the ulna, both regarded as primitive characters. However, a bowed ulnar shaft is present not only in maniraptorans but also in most theropods and even prosauropods, although in these groups the bend is usually less striking in lateral view due to the expansion formed by the olecranon process (see also discussion of this character in the materials and methods section), and this also seems to be the case in Megaraptor (Novas 1998, fig. 1b). An enlarged olecranon is found in some coelurosaurs, including the basal bird Mononykus (Perle et al. 1994), and its presence in Megaraptor might be an autapomorphy of this taxon within dromaeosaurids.