Which sources would preserve the soft tissues of an early mammal

Amber, permafrost and tar

Explanation:

Amber, permafrost and tar would preserve the soft tissues of an early mammal. Soft tissues of early mammals would preserve in case of amber in Russia. and frozen soil in the Arctic. Although hair and mammary glands are soft tissue but they are not readily fossilized as mammary glands and hairs are derived from dermic structures