What were Earth's earliest multicellular organisms?

The first birds evolved during the ____ era.

An animal without a backbone.

An animal with a backbone.

A vertebrate whose body temperature is determined by the temperature of its environment, and that lives its early life in water and its adult life on land.

A vertebrate whose temperature is determined by the temperature of its environment, that has lungs and scaly skin, and that lays eggs on land.

When entire species of living things die off at the same time.

A vertebrate whose temperature is regulated by its internal heat, and that has skin covered with hair or fur and glands that produce milk to feed its young.

What period did mammals first appear during?