The duck-billed Platypus and the spiny ant-eater, both are egg-laying animals yet they are grouped under mammals.

To be classified as a mammal, the presence of hair and milk or mammary glands is mandatory. The duck-billed platypus and spiny anteaters (monotremes) possess these features, as well as the mammalian characteristics of seven cervical vertebrae and 12 pairs of cranial nerves, even though they lay eggs like reptiles. Both assertion and reason are true and the reason provides correct anatomical mammalian traits that justify their classification despite egg-laying.