Heterodontosaurus

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 (3 ft 10 in) and possibly 1.75 m (5 ft 9 in) in length, and weighing between 2 and 10 kg (4.4 and 22.0 lb). 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. 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. Heterodontosaurus is the eponymous and best-known member of the family Heterodontosauridae. This family is considered one of the most primitive or basal groups within the order of ornithischian dinosaurs. In spite of the large tusks, Heterodontosaurus is thought to have been herbivorous, or at least omnivorous. Though it was formerly thought to have been capable of quadrupedal locomotion, it is now thought to have been bipedal. Tooth replacement was sporadic and not continuous, unlike its relatives. At least four other heterodontosaurid genera are known from the same geological formations as Heterodontosaurus. Heterodontosaurus was a small dinosaur. The most complete skeleton, SAM-PK-K1332, belonged to an animal measuring about 1.18 m (3 ft 10 in) in length. Its weight was variously estimated at 1.8 kg (4.0 lb), 2.59 kg (5.7 lb), and 3.4 kg (7.5 lb) in separate studies. The closure of vertebral sutures on the skeleton indicates that the specimen was an adult, and probably fully grown. A second specimen, consisting of an incomplete skull, indicates that Heterodontosaurus could have grown substantially larger – up to a length of 1.75 m (5 ft 9 in) and with a body mass of nearly 10 kg (22 lb). The reason for the size difference between the two specimens is unclear, and might reflect variability within a single species, sexual dimorphism, or the presence of two separate species. The size of this dinosaur has been compared to that of a turkey. Heterodontosaurus was amongst the largest known members of the family Heterodontosauridae. The family contains some of the smallest known ornithischian dinosaurs – the North American Fruitadens, for example, reached a length of only 65 to 75 cm (26 to 30 in). The neck consisted of nine cervical vertebrae, which would have formed an S-shaped curve, as indicated by the shape of the vertebral bodies in the side view of the skeleton. The vertebral bodies of the anterior cervical vertebrae are shaped like a parallelogram, those of the middle are rectangular and those of the posterior show a trapezoid shape. The trunk was short, consisting of 12 dorsal and 6 fused sacral vertebrae. The tail was long compared to the body; although incompletely known, it probably consisted of 34 to 37 caudal vertebrae. The dorsal spine was stiffened by ossified tendons, beginning with the fourth dorsal vertebra. This feature is present in many other ornithischian dinosaurs and probably countered stress caused by bending forces acting on the spine during bipedal locomotion. In contrast to many other ornithischians, the tail of Heterodontosaurus lacked ossified tendons, and was therefore probably flexible. It had a long, narrow pelvis and a pubis which resembled those possessed by more advanced ornithischians. The forelimbs were robustly built and proportionally long, measuring 70% of the length of the hind limbs. The radius of the forearm measured 70% of the length of the humerus (forearm bone). The hand was large, approaching the humerus in length, and possessed five fingers equipped for grasping. The second finger was the longest, followed by the third and the first finger (the thumb). The first three fingers ended in large and strong claws. The fourth and fifth fingers were strongly reduced, and possibly vestigial. The phalangeal formula, which states the number of finger bones in each finger starting from the first, was 2-3-4-3-2. The hindlimbs were long, slender, and ended in four toes, the first of which (the hallux) did not contact the ground. Uniquely for ornithischians, several bones of the leg and foot were fused: the tibia and fibula were fused with upper tarsal bones (astragalus and calcaneus), forming a tibiotarsus, while the lower tarsal bones were fused with the metatarsal bones, forming a tarsometatarsus. This constellation can also be found in modern birds, where it has evolved independently. The tibiotarsus was about 30% longer than the femur. The ungual bones of the toes were claw-like, and not hoof-like as in more advanced ornithischians.