page_title: Heterodontosaurus
section_title: Growth and proposed sexual dimorphism
hierarchical_section_title: Heterodontosaurus / Palaeobiology / Growth and proposed sexual dimorphism
caption_attribution_description: Flesh reconstruction of the head and neck of Heterodontosaurus tucki from the Lower Jurassic Upper Elliot and Clarens formations of South Africa. Flesh head and neck and skull reconstructions with similar gape. The flesh reconstruction was modeled over a skull cast with skin texture based on bone surface details in the skull of Heterodontosaurus tucki (SAM-PK-K1332) and bristles based on those preserved in Tianyulong confuciusi (STMN 26-3).
context_page_description: Heterodontosaurus is a genus of heterodontosaurid dinosaur that lived during the Early Jurassic, 200–190 million years ago. Its only known member species, Heterodontosaurus tucki, was named in 1962 based on a skull discovered in South Africa. The genus name means "different toothed lizard", in reference to its unusual, heterodont dentition; the specific name honors G. C. Tuck, who supported the discoverers. Further specimens have since been found, including an almost complete skeleton in 1966.
  Though it was a small dinosaur, Heterodontosaurus was one of the largest members of its family, reaching between 1.18 m and possibly 1.75 m in length, and weighing between 2 and 10 kg. The skull was elongated, narrow, and triangular when viewed from the side. The front of the jaws were covered in a horny beak. It had three types of teeth; in the upper jaw, small, incisor-like teeth were followed by long, canine-like tusks. A gap divided the tusks from the chisel-like cheek-teeth. The body was short with a long tail. The five-fingered forelimbs were long and relatively robust, whereas the hind-limbs were long, slender, and had four toes.
context_section_description: The ontogeny, or the development of the individual from juvenile to adult, is poorly known for Heterodontosaurus, as juvenile specimens are scarce. As shown by the juvenile skull SAM-PK-K10487, the eye sockets became proportionally smaller as the animal grew, and the snout became longer and contained additional teeth. Similar changes have been reported for several other dinosaurs. The morphology of the teeth, however, did not change with age, indicating that the diet of juveniles was the same as that of adults. The length of the juvenile skull was suggested to be 45 mm (2 in). Assuming similar body proportions as adult individuals, the body length of this juvenile would have been 450 mm (18 in). Indeed, the individual probably would have been smaller, since juvenile animals in general show proportionally larger heads.
  In 1974, Thulborn suggested that the large tusks of heterodontosaurids represented a secondary sex characteristic. According to this theory, only adult male individuals would have possessed fully developed tusks; the holotype specimen of the related Abrictosaurus, which lacked tusks altogether, would have represented a female. This hypothesis was questioned by palaeontologist Richard Butler and colleagues in 2006, who argued that the juvenile skull SAM-PK-K10487 possessed tusks despite its early developmental state. At this state, secondary sex characteristics are not expected. Furthermore, tusks are present in almost all known Heterodontosaurus skulls; the presence of sexual dimorphism however would suggest a 50:50 ratio between individuals bearing tusks and those lacking tusks. The only exception is the holotype specimen of Abrictosaurus; the lack of tusks in this individual is interpreted as a specialisation of this particular genus.
caption: Model, showing hypothetical display structure on the snout, and skull cast
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