Question

What trait do all mammals share?

Answer 1

Class mammalia includes a diverse group of animals: there are egg laying mammals, marsupial mammals and truly placental mammals.
Hence viviparity is not a common trait.

`color(magenta)"All mammals without exception possess"`:

1. Hair and fur on body
2. Secretion of milk from modified sweat glands
3. Dentary only bone in lower jaw
4. Three ear ossicles
5. Diphyodont dentition (milk teeth are replaced once)
6. Presence of diaphragm
7. Completely four chambered heart and
8. Warm blooded metabolic condition

Egg laying mammals lack certain characters that are present in viviparous mammals: such as

1. Presence of ear pinna
2. Presence of corpus callosum
3. Separate renal and rectal outlets
4. Properly structured mammary gland with nipple
5. Development of embryo in uterus and live birth of youngs