Today, 39 years ago, on October 7, 1986 (September 4, 1986), the oldest dinosaur fossil was discovered in the United States. On October 7, 1986, scientists discovered some well-preserved dinosaur skeleton fossils in Arizona, USA. According to archaeologists, they have been estimated to have been twenty-five million years old, and are the oldest dinosaur fossil ever found in the world. Experts believe that the dinosaurs discovered are a new species of dinosaurs not yet scientifically known.