The mammals are defined are hairy quardrupeds. Their name refers to the milk secreting mammary glands in the females for sucking the young for sometimes after birth. The mammals have evolved from the mammals like reptiles like Dimetrodon in Triassic some 200 million years ago in the Jurassic period, before the first birds. There are 5000 species of living mammals known.
Prototheria vs Metatheria vs Eutheria
i. Prototheria (Egg laying mammals)
1. Ear without pinna.
2. No nipple
3.No corpus callosum
4. Testes abdominal, no scrotum
5. Digestive and urinogenital tracts open out through cloaca and cloacal aperture.
6. No marsupial pouch
8. No placenta
Example of Prototheria: Duck billed platypus (Ornithorynchus anatinus), Anteater (Tachyglossus)
ii. Metatheria (Pouched mammals or marsupials)
1. Ear with pinna
2. Nipples abdominal
3. No corpous callosum
4. Scrotum in font of penis
5. Digestive and urinogenital tracts open out by separate apertures that are controlled by a common sphincter muscle.
6. Marsupial pouch often present
8. Placenta small, intrauterine development brief, young extremely small and helpless, brought up in marsupial pouch, if present.
Example of Metatheria: Kangaroo (Macropus), Opossum (Didelphis)
iii. Eutheria (Placental mammals)
1. Ear usually with pinna.
2. Nipples abdominal or throracic.
3. Corpus callosum present.
4. Scrotum behind penis.
5. Digestive and urinogenital tracts open out by separate apertures, each with its own sphincter muscle.
6. No marsupial pounch
7. No marsupial pouch
8. Placenta large, intrauterine development prolonged, young advanced and fully formed.
Example of Eutheria: Rat(Rattus), Horse (Equus)