What are some environments that placental mammals live on?
Placental mammals live in almost every environment on Earth.
They are one of the most successful groups in the animal kingdom.
Mammals in general are very successful and the majority of mammals
are placentals. This group is so successful because they are
endothermic. The only mammals that are not placentals are the
marsupials (kangaroo, opossum, etc) and the prototherians (egg
laying mammals). Environments that do contain mammals would likely
be constrained to those that are very severe: The deep ocean,
extremely high mountain peaks, etc. Otherwise you can find these
animals almost anywhere.