page_title: Heterodontosauridae
section_title: Skull and teeth
hierarchical_section_title: Heterodontosauridae / Description / Skull and teeth
caption_attribution_description: Evolution of key masticatory specializations among heterodontosaurids. A Heterodontosauridae B Simple-crowned heterodontosaurids C Heterodontosaurinae D Advanced heterodontosaurines E Abrictosaurus and Heterodontosaurus F Heterodontosaurus. Abbreviations: 1 arched inset diastema 2 heterodont dentition with caniniform teeth 3 cingular ectoloph 4 postcaniniform diastema 5 external mandibular fossa 6 lowered jaw articulation 7 mobile predentary articulating against saddle-shaped dentary articular surface 8 increased differential in crown size 9 arched alveolar margins in cheek dentition 10 asymmetrical enamel 11 hollow crown base.
context_page_description: Heterodontosauridae is a family of early ornithischian dinosaurs that were likely among the most basal members of the group. Although their fossils are relatively rare and their group small in numbers, they lived across all continents except Australia for approximately 100 million years, from the Late Triassic to the Early Cretaceous.
  Heterodontosaurids were fox-sized dinosaurs less than 2 metres in length, including a long tail. They are known mainly for their characteristic teeth, including enlarged canine-like tusks and cheek teeth adapted for chewing, analogous to those of Cretaceous hadrosaurids. Their diet was herbivorous or possibly omnivorous.
context_section_description: Both Abrictosaurus and Heterodontosaurus had very large eyes. Underneath the eyes, the jugal bone projected sideways, a feature also present in ceratopsians. As in the jaws of most ornithischians, the anterior edge of the premaxilla (a bone at the tip of the upper jaw) was toothless and probably supported a keratinous beak (rhamphotheca), although heterodontosaurids did have teeth in the posterior section of the premaxilla. A large gap, called a diastema, separated these premaxillary teeth from those of the maxilla (the main upper jaw bone) in many ornithischians, but this diastema was characteristically arched in heterodontosaurids. The mandible (lower jaw) was tipped by the predentary, a bone unique to ornithischians. This bone also supported a beak similar to the one found on the premaxilla. All the teeth in the lower jaw were found on the dentary bone.
   
  Heterodontosaurids are named for their strongly heterodont dentition. There were three premaxillary teeth. In the Early Jurassic Abrictosaurus, Heterodontosaurus, and Lycorhinus, the first two premaxillary teeth were small and conical, while the much larger third tooth resembled the canines of carnivoran mammals and is often called the caniniform or 'tusk'. A lower caniniform, larger than the upper, took the first position in the dentary and was accommodated by the arched diastema of the upper jaw when the mouth was closed. These caniniforms were serrated on both the anterior and posterior edges in Heterodontosaurus and Lycorhinus, while those of Abrictosaurus bore serrations only on the anterior edge. In the Early Cretaceous Echinodon, there may have been two upper caniniforms, which were on the maxilla rather than the premaxilla, and Fruitadens from the Late Jurassic may have had two lower caniniforms on each dentary.
  Like the characteristic tusks, the cheek teeth of derived heterodontosaurids were also unique among early ornithischians. Small ridges, or denticles, lined the edges of ornithischian cheek teeth in order to crop vegetation. These denticles extend only a third of the way down the tooth crown from the tip in all heterodontosaurids; in other ornithischians, the denticles extend further down towards the root. Basal forms like Abrictosaurus had cheek teeth in both maxilla and dentary that were generally similar to other ornithischians: widely spaced, each having a low crown and a strongly-developed ridge (cingulum) separating the crown from the root. In more derived forms like Lycorhinus and Heterodontosaurus, the teeth were chisel-shaped, with much higher crowns and no cingula, so that there was no difference in width between the crowns and the roots.
  These derived cheek teeth were overlapping, so that their crowns formed a continuous surface on which food could be chewed. The tooth rows were slightly inset from the side of the mouth, leaving a space outside the teeth that may have been bounded by a muscular cheek, which would have been necessary for chewing. The hadrosaurs and ceratopsians of the Cretaceous Period, as well as many herbivorous mammals, would convergently evolve somewhat analogous dental batteries. As opposed to hadrosaurs, which had hundreds of teeth constantly being replaced, tooth replacement in heterodontosaurids occurred far more slowly and several specimens have been found without a single replacement tooth in waiting. Characteristically, heterodontosaurids lacked the small openings (foramina) on the inside of the jaw bones which are thought to have aided in tooth development in most other ornithischians. Heterodontosaurids also boasted a unique spheroidal joint between the dentaries and  the predentary, allowing the lower jaws to rotate outwards as the mouth was closed, grinding the cheek teeth against each other. Because of the slow replacement rate, this grinding produced extreme tooth wear that commonly obliterated most of the denticles in older teeth, although the increased height of the crowns gave each tooth a long life.
caption: Evolution of key masticatory specialisations in heterodontosaurids, according to Sereno, 2012
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