The earliest known dinosaur, a relatively small creature perhaps the size of a golden retriever, lived in Tanzania around 237 million years ago during the Triassic period; other early dinosaurs were even smaller, some akin to house cats. At that time, the world was largely dominated by crocodiles and other large reptilian animals. It wasn't until about 36 million years later, during the Jurassic period, that dinosaurs became the prevailing creatures.