Dinosaurs did not start out being large. Probably the oldest dinosaur ever found going back 243 million years was only as big as the average dog. This is only supposition of course. Some scientists say this animal, the Asilisaurus, was only a relative of dinosaurs.
Research into the matter is difficult. Only fossil fragments have been found. The specimen in question called the Asilisaurus lived in Tanzania. Another similar animal was found in the Tanzanian Manda Beds in the 1930s. Named a Nyasasaurus, it had a crest of bone along 30 per cent of its upper arm where large chest muscles would have been attached. This is a major feature of dinosaurs.
Many kinds of dinosaur seem to have suddenly appeared 10 million years after the Tanzanian fossils. This indicates a common origin from one creature. This ancestor could have been the early dinosaur Nyasasaurus or its relative the Asilisaurus.