Why did mammals evolve before birds?

The first thing to address is that there probably isn't any reason for anything. Evolution isn't goal driven, it does what it does and all the processes that drive evolution have no end goal. read more

Mammals evolved from an amniote group known as the synapsids (or therapsids, they refer to the same group, but later in evolutionary time) and the birds evolved from sauropsids. The supposed 125 million year gap between when mammals and birds evolved is also misleading. read more

Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is likely that they simply did not exist, and that the molecular clock runs fast during major evolutionary radiations. read more

ANSWER ABOVE IS INCORRECT Mesozoic is the correct answer small mammals evolved during the Mesozoic era, with reptiles, as did birds. The full potential and releasing of mammals came in the Cenozoic, when the mass extinction of 20% of the Mesozoic's species happened. read more