Question

Platypus and echidna is a group that forms a link between reptiles and mammals. Think and write a characteristic feature that these would have.

Answer 1

Monotremes are the subclass of mammals that share some characteristic features of amphibians and reptiles. These characteristics give us evidence that they evolved from reptiles.

Complete answer:
The class Mammalia is divided into three subclasses namely, placentas, monotremes, and marsupials. Compared to other mammalian types, monotremes have structural differences in the bone in the jaw, digestive tract, reproductive tract, and other body parts. All mammals give birth to their baby by carrying the embryo in the uterus but in monotremes, they give birth to their young ones by laying the eggs which is one of the characteristic features of amphibians and reptiles. Even though they give birth to their young ones by laying eggs they feed the baby with milk secreted by mammary glands covered by the skin because nipples are absent in them.
The only surviving species of monotremes are platypus and echidna found in New Geneva and Australia.
The absence of corpus callosum is one of the characteristic features of amphibians and reptiles which is present in all mammals. It is the communication link between two cerebral hemispheres of the brain.
Unlike mammals, monotremes have a single opening for both excretion and reproduction.

Additional Information: - Eggs of monotremes receive nutrients when they are inside the body of the mother and they hatch within 10days after laying the eggs which is the shortest incubation period when compared to other sauropsids.
- Compared to other mammals they have less developed thermoregulation.

Note: In birds vitellogenin genes (a protein necessary for eggshell formation) is seen; it suggests that the common ancestor of mammals in monotremes, marsupials, and placental was oviparous and it is retained in monotremes but lost in all other mammal groups that are extant.