4. the first dinosaurs

We still have a lot to learn about the evolution of the first dinosaurs. The earliest known dinosaurs date to the Triassic period, possibly as old as 243 million years. The very earliest known dinosaur is thought to be Nyasasaurus from Tanzania, although its remains are too fragmentary to say for sure. This example aside, one of the oldest locations known to contain dinosaur fossils is the Ischigualasto Formation in Argentina. This formation contains an array of creatures, including reptiles known as rhynchosaurs and animals closely related to early mammals called cynodonts. Importantly it contains some of the earliest known dinosaurs, including Eoraptor and Herrerasaurus. These dinosaurs were all small, bipedal predators, however they were overshadowed by their more successful contemporaries such as pseudosuchians and aetosaurs and were far from the dominant land vertebrates dinosaurs would later become.

The Triassic period began with a mass extinction event which freed up the ecological niches that led to the evolution of the first dinosaurs. The global climate was hot and dry for most of the Triassic with much of the earth's land mass tied up in a supercontinent known as Pangaea. Deserts spanned most of Pangaea's interior until the mid-Triassic when the supercontinent began to seperate into two land masses, Laurasia to the north and Gondwana to the south, resulting in a more humid climate. Just as the Triassic began with a mass extinction event, it ended with one. This extinction event wiped out many of the early dinosaurs more successful competators and allowed dinosaurs to assume dominance in the Jurassic.