Definition and Unique Characteristics of Ornithopoda

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

Mesure Arche Interne Pied
FIGURE 11.1 Important characters for Clade Ornithopoda: offset tooth row; occlusal surface higher than jaw joint; crescent-shaped paraoccipital process; premaxilla with an elongate process.

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