Heterodontosauridae

…of several subgroups, including 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.…

In heterodontosaurs the cheek teeth were crowded together into long rows and set inward slightly from the outer cheek surface. The inset, which persisted through all later ornithopods, has been interpreted to suggest the presence of cheeks that may have held plant food in the mouth…