Ornithopoda (= "bird foot"), first named as a group by O. C. Marsh in the late nineteenth century (Chapter 3), was described in early literature as bipedal ornithis-chians. However, considering that bipedalism is now recognized as a probable ancestral trait for all dinosaurs, saurischians included, the definition of ornithopods has been refined considerably. Ornithopoda is now distinguished by the following characters, among others (Fig. 11.1):
■ Offset tooth row, where the maxillary teeth are higher (more dorsal) than those in the premaxillary, although teeth in the latter might be missing altogether.
■ Occlusal surface is higher (more dorsal) than the jaw joint.
■ Crescent-shaped paraoccipital process is located in the posterior of the skull.
■ Premaxilla has an elongate process that touches either (or both) the pre-frontal or lachrymal.
As mentioned earlier, ornithopods are ornithischians, meaning that they shared a common ancestor with thyreophorans (Chapter 12) and marginocephalians (Chapter 13). This common ancestry places the ornithopods, thyreophorans, and marginocephalians in the node-based clade Genasauria. Genasauria splits into stem-based and sister clades Thyreophora (ankylosaurs and stegosaurs) and Cerapoda (Marginocephalia and Ornithopoda). Ornithopoda itself is a stem-based clade, having two main branches to Heterodontosauridae and Euornithopoda (Fig. 11.2). The majority of ornithopod species compose the latter, and most of
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