What is the oldest group of placental mammals?

Answer 1

This was incorrectly presumed, by 19th century scientists, to be platypus. It is actually uniquely evolved.

If you're attempting to find out the most primitive "modern" mammals, they could be argued to be the Soricidae, the shrews and the Talpidae, the moles, but any modern creature is simply not primitive. More basic, more complex, more or less evolutionary divergent from their ancestral norms, but not more primitive.

Answer 2

No one knows. The group that includes all placental mammals and all non-placental mammals that are more similar to placental mammals than to marsupials and monotremes is called Eutheria. Today, all Eutherians are placental mammals (they have placentas) but there are extinct Eutherians that lacked placentas and so were not placental mammals. The oldest known Eutherian is Eomaia Scansoria, but it was not a placental mammal. We can trace or hypothesize the evolution of the groups of Eutherians, but it is uncertain when the placenta evolved in general or for each group.

Answer 3

The sea sponge is a basal animal - i.e., it's as far back on the evolutionary tree for animalia as anything we know. And sponges have existed for SEVEN HUNDRED AND FIFTY MILLION YEARS - at least.

If you want something a little more... "lively"... there's jellyfish (500m+) and nautiluses (500m), horseshoe crabs (450m), the coelacanth (350m), and various sharks (250m).

Answer 4

Lamprey eels are believed to be the most primitive members of vertebrate group.

Actually, the jawless fish are to be the most primitive.

Answer 5

Members of the phylum porifera family is the most primitive.