The presence of an external ear (pinna) is a characteristic feature of which of the following?

Sea cows are aquatic mammals included under

Assertion : The duck-billed Platypus and the spiny ant-eater,both are egg-laying animals yet they are grouped under mammals.
Reason : Both of them have seven cervical vertebrae and $12$ pairs of cranial nerves.

$A$: Mammals are viviparous.
$R$: Mammals possess external ear pinnae.

Select the correct set of animals belonging to the class $Mammalia$.

Which characteristic is found exclusively in mammals?