We review recent advances in our understanding of placental evolution with particular focus
on the interhaemal barrier. It seems likely that the non-invasive, epitheliochorial placentation
of living eutherians is a derived state. However, there is disagreement on whether the last
common ancestor of eutherian mammals had an endotheliochorial placenta or a
haemochorial one. Research has been stimulated by improved understanding of the
relations between the orders of mammals provided by molecular phylogenetics. In part, the …