Period: Late Triassic
Location: South America (Brazil, Argentina)
Length: 6 1/2 feet (2 meters)
Staurikosaurus is the earliest dinosaur known. The first skeleton came from the Santa Maria Formation of southern Brazil and dates back to the Late Triassic. Its name means "cross reptile," and it was named after a group of stars called the Southern Cross, which are best seen in the southern hemisphere.
Staurikosaurus was a bipedal (two-legged) dinosaur, although its back legs and pelvis were not well made for rapid motion. It had five fingers and five toes, which is a primitive feature. The third finger and toe were the longest. The teeth were curved backwards and serrated, like a steak knife. It was a meat-eater.
Edwin Colbert described Staurikosaurus in 1970. In 1987 Don Brinkman and Hans-Dieter Sues described a second skeleton from the slightly younger Ischigualasto Formation of Argentina. Workers have found fossil pieces in Late Triassic formations in the southwest United States and in the Elliot Formation of South Africa that may belong to this animal. One partial skeleton from the Petrified Forest National Park in Arizona, nicknamed Gertie, is being studied.
Paleontologists disagree on the relationship of Staurikosaurus and the closely related Herrerasaurus. These animals either represent primitive dinosaurs that share a common ancestry with all later dinosaurs or are the earliest members of the Saurischia. Part of the uncertainty about the position of Staurikosaurus is because the skull is missing. Staurikosaurus and Herrerasaurus differ from other dinosaurs in the structure of their pelvis. Their lower legs and ankles are also different from the later dinosaurs.
Staurikosaurus fossils are not common, but they occur in formations where other land vertebrates, such as aetosaurs and rauisuchids, are common. The reason there are few Staurikosaurus fossils in these deposits could be because there were few animals; or perhaps they lived in areas where bones rarely fossilized, such as forests. It could also be a combination of both factors. Whatever the reason, there are few Staurikosaurus and Herrerasaurus fossils. Small theropod dinosaurs, such as Coelophysis, followed and perhaps replaced them by the end of the Triassic.