Dilophosaurus ( dy-LOHF-o-SOR-əs) is a genus of theropod dinosaur that lived in what is now North America during the Early Jurassic Period, about 193 million years ago. In 1940 three skeletons were found in northern Arizona. The two best preserved of these were collected in 1942, with the most complete later made the holotype specimen of a new species of the genus Megalosaurus, named M. wetherilli by Samuel P. Welles in 1954. Welles found a larger skeleton belonging to the same species in 1964. Realizing it bore crests on its skull, he moved the species to the new genus Dilophosaurus in 1970, as D. wetherilli. The genus name means "two-crested lizard", and the species name honors John Wetherill, a Navajo councilor. More specimens have since been found, including an infant, and footprints have also been attributed to the animal. Another species from China, D. sinensis, was named in 1993, but was later found to belong to the genus Sinosaurus.
At about 7 meters (23 ft) in length, with a weight of about 400 kilograms (880 lb), Dilophosaurus was one of the first big predatory dinosaurs, though it was smaller than some later theropods. It was slender and lightly built, and though the skull was proportionally large, it was delicate. The snout was narrow, and the upper jaw had a gap or kink below the nostril. It had a pair of longitudinal, plate-shaped crests on its skull, similar to a cassowary with two crests. The mandible was slender and delicate at the front, but deep at the back. The teeth were long, curved, thin, and compressed sideways. The teeth of the lower jaw were much smaller than those of the upper jaw. Most of the teeth had serrations at their front and back edges, and were covered in enamel. The neck was long, and its vertebrae were hollow, and very light. The arms were powerful, with a long and slender upper arm bone. The hands had four fingers: the first was short but strong and bore a large claw, the two following fingers were longer and slenderer with smaller claws, and the fourth was vestigial. The thigh bone was massive, the feet were stout, and the toes bore large claws.
Dilophosaurus is considered a member of the family Dilophosauridae along with Dracovenator, a group that has been placed between the Coelophysidae and later theropods. Dilophosaurus would have been active and bipedal, and may have been able to hunt large animals, though it could also have fed on smaller animals and fish. The function of the crests is unknown; they were too weak for battle, but may have been used in visual display, such as species recognition and sexual selection. It may have grown rapidly, attaining a growth rate of 30 to 35 kilograms (66 to 77 lb) per year early in life. The holotype specimen had multiple paleopathologies, including healed injuries and signs of a developmental anomaly. Dilophosaurus is known from the Kayenta Formation, and lived alongside dinosaurs such as Megapnosaurus and Sarahsaurus. Dilophosaurus was featured in the novel Jurassic Park and its movie adaptation, wherein it was given the fictional abilities to spit venom and expand a cowl on its neck. It was designated as the state dinosaur of Connecticut in 2017.