Hypsilophodontidae

Fabrosauridae, Heterodontosauridae, Hypsilophodontidae, Iguanodontidae, and Hadrosauridae (the duck-billed dinosaurs). The fabrosaurs were the earliest and most primitive of the ornithopods; these small, lightly built dinosaurs reached lengths of 60–120 cm (2–4 feet). The heterodontosaurs began to develop the horny beaks and specialized teeth typical of ornithischians. The…

The hypsilophodonts had cheek teeth arranged in tightly packed rows set well inward from the outer cheek surfaces. The teeth occluded directly, and the opposing rows formed a long shearing edge similar to that of the heterodontosaurs. There was, however, no “tusk” either above or below.…