Birds, reptiles, and mammals are called amniotes because

Birds, reptiles, and mammals are called amniotes because they produce eggs and embryos that are enclosed in internal membranes called the amnion.

FURTHER EXPLANATION

Mammals evolved from a group of amniotes called the synapsids while birds, reptiles, and dinosaurs evolved from the sauropsids. Reptiles, birds, mammals have four limbs and have the capacity to reproduce on land as opposed to amphibians. Amniotic eggs survive on land because it has a membrane called the amnion  that encloses the embryo as it develops and produces a fluid called the amniotic fluid. The amniotic fluid serves as a cushion and protects the embryo from bumps and sudden movements.  

The amnion also prevents the embryo from drying up because it is selectively permeable and waterproof– it only allows gases to pass but not water. This enables the embryo to breathe inside the egg without losing water.  

Amphibians are not amniotes because they can live in land and water but are limited to reproduction in water because of the lack of amnion in their eggs. Without this waterproof membrane, the embryo is in danger of drying out on land.

LEARN MORE

1.Amphibians

2.Reptiles

3.Mammals

Keywords: amniotes, amnion, amnion fluid