Barrett, Paul M. Department of Palaeontology, The Natural History Museum, London, United Kingdom.
- Phylogeny and evolution
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One of two constituent clades of Dinosauria (the other being Saurischia). Ornithischian dinosaurs are characterized by the possession of numerous anatomical features, notably the configuration of the pelvis. In ornithischian hips, the pubis bone (which points downward and forward in the majority of other reptile groups) has rotated posteriorly to lie parallel to the ischium (Fig. 1). This configuration is superficially similar to that seen in living birds, but actually represents an example of evolutionary convergence (birds are, in fact, descended from saurischian dinosaurs). Other features that characterize ornithischians include the presence of ossified tendons that form a complex lattice arrangement supporting the vertebral column, a single midline bone called the predentary that lies at the front end of and connects the two lower jaws, and a palpebral bone that traverses the orbit (Fig. 2).
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