The earliest conclusive evidence of the evolution of dinosaurs dates their origins to 230 million years ago. They are believed to have evolved from other reptilian species and developed certain defining characteristics, such as specialized hip and leg bones.
Earlier fossils have been discovered which may predate previously uncovered archaeological evidence. A previously unknown genus of archosaur could push the time period back another 10 million years, while footprints discovered in Europe may place the date back to 250 million years. So little evidence remains from so long ago that it makes it difficult to pinpoint the exact date, year or millennium for a given animal type to have evolved.
What is known, as of 2014, is that the animals referred to as dinosaurs first appeared in the late Triassic period and remained in existence as a group for more than 190 million years. This has been determined based on the depth at which fossils have been found and through the use of radio-carbon dating which allows archaeologists to determine how old an item is based on the constant decay rate of carbon isotopes contained in all matter on Earth. This method is also used in geology when determining the age of rock formations or sedimentary layers.