Question

The amniote lineage of vertebrates is better adapted to life on land than tetrapod lineages that evolved before them. Describe features of amniote animals make them so well adapted to life on land.

Answer 1

Amniotes appeared in the later part of Carboniferous period which has vast coal swamps. The tetrapods remained near the ponds as they were fish eating. Aminotes include birds, reptiles and mammals. Various adaptations that enabled the amniotes to live on land are:

1. Amniotes have amniotic eggs that are waterproof due to presence of a shell around the egg. It is named as amniotic egg because it can form an amnion in the embryo. The shell protects the egg from water, and provides efficient gas exchange. The amnion protects the embryo from desiccation and can enclose a fluid filled cavity. Further the embryo also forms a chorion (surrounds yolk sac and embryo), yolk sac and allantois (protects against mechanical shock and involved in hydration). Mammalian eggs have albumin as protein source for the embryo. Some eggs yolk has also fats in them. Amniote egg is specialized in gas exchange, nutrition and waste removal.

2. The skin of the amniotes is water proof. This skin is made up of lipids. Skin contains keratin that makes it water proof. The skin prevents the amniotes from dehydration. Birds have feather while reptile have scales, both of which are keratinized structures. The skin also maintains the internal water filled environment within the body.

3. Lungs allow the amniotes to breathe oxygen from air in land. The alveolar sacs will help in taking in oxygen and removing CO2 rom the body.

4. Amniotes are capable of internal fertilization. Thus, sperm do not require water to swim towards the egg, as seen in external fertilization. The males will deposit the sperm directly within the female, thereby not requiring water.

5. Amniotes such as mammals develop kidneys that are efficient in preserving water on land. The kidneys can reabsorb water from the nitrogenous waste and filter it back to blood. This helps in conservation of water.