On the basis of what characteristics do insectivores belong to the most primitive placental mammals?

Representatives of the order of insectivores are one of the most primitive placentals that have survived to this day. These animals have preserved a number of features that distinguish them from other mammals. First of all, it is a very small brain, if you compare it with the rest of the animals of the class. They also have a smooth brain. The structure of the skull and teeth is also reminiscent of ancient mammals that have already become extinct. Among this order there is even a cold-blooded species (the only one among mammals).