Amniotes are vertebrates that have protective membranes around their embryos, to provide a watery environment for the development of the embryo. What other characters do amniotes have in common?

They are four-limb animals(tetraploid) with backbone (vertebrate.)Some walk close to the ground(lizard) while some e,g man, birds, walk uprightly with straighter limbs, and head far away from the ground.

They processes diaphragm for efficient muscular supply of oxygen, different from skeletal muscles for walking.

Circulation

They have efficient circulatory system of the heart and vascular system for distribution of blood containing oxygen and nutrients. The heart is made up of chambers which varies from 3 to 4. The chambers enabled the heart to pumped blood in pulmonary circulation for oxygenation, and to systemic circulation to reach all the systems of the body.

Temperature variation.

Amniotes biochemical reactions is influenced by fluctuations in the surrounding environments. They are made of enzymes which are sensitive to environmental temperatures. Based on this they can be ecothermic and endothermic. Reptile body temperature fluctuates with the immediate environments thus they lack ability to regulate their body temperature. Mammals including man and birds are endothermic. They generate their body heat and therefore temperature irrespective of fluctuations in external environment. Their body temperature are relatively constant.

Water conservation

They have efficient kidney for re-absorption of water into the blood during dehydration, a well developed Gastrointestinal tract for nutrient and food absorption, and lachrymal gland to moisten the eyes and prevent drying out.

Note some animals are tetraploids and vertebrates, but are not Amniotes e.g Amphibians, and Fishes, Therefore these animals should not be classified as Amniotes . They lack complex features listed above.