Question

Prototherians have been evolved from
(a) Reptiles
(b) Birds
(c) Amphibians
(d) Eutherians

Answer 1

Hint: Ectothermic organisms that are evolved from labyrinthodont amphibians. They are egg-laying and can survive both on land and water.

Complete answer:
Mammals belong to the kingdom Animalia which is divided into two subspecies namely Prototheria and theria. In the subclass Prototheria, organisms have a special character which is similar to birds and reptiles. Mammals in that subclass are egg-laying. Mammals in Prototheria are also called monotremes. They contain skeletons similar to reptiles and legs are present on both sides of the body. This evidence shows that monotremes or prototherians evolved from reptiles.

Additional Information: - Reproductive systems in mammals show variations in three classes. Based on this they are divided into subclasses and infraclasses
- class mammalian is divided into two subclasses namely Prototheria and theria. The subclass theria is divided into two infraclasses namely Metatheria and Eutheria.
- Like most of the non-mammalian vertebrates animals belongs to the subclass prototherians give birth to their young ones by laying eggs but they feed their young ones with milk from mammary glands like all other mammals
- They are also called monotremes because they contain a common aperture for both excretion and reproduction which is also seen in birds and reptiles
- Skeletons in monotremes are also similar to reptiles and their legs are present on either side of the body. This gives a reptile-like gait appearance.
- These characteristics in mammals give evidence that they are evolved from reptiles
- Only three species of mammals that are surviving now in subclass prototherians are the Australian platypus and two Echidna species.
- So, the correct answer is 'reptiles'

Note: Humans and all other mammals belong to subclass Eutheria. They are also called placental mammals.
Generally, these mammals carry their young ones in the uterus and protect them until they become mature enough to survive on external conditions but in kangaroo, the baby is born in the immature stage because of the rudimentary placenta.