Question Detail

The first mammals evolved from therapsid reptiles during the Triassic period. The first birds (like Archaeopteryx) evolved from theropod dinosaurs later, in the Jurassic period. So, for 'first mammals and birds', mammals came first in the Triassic. If the question asks for both appearing, the Triassic is correct for mammals, but birds appear in the Jurassic. Since both are options, and the question says 'first mammals & birds', it might be expecting Triassic for mammals, or it might be a trick. The most common answer is the Triassic for the appearance of mammals.