On the trail of the first placental mammals

Making a data-rich outline of the lineages of placental mammals how trait both living and extinct, allows researchers to interpret how traits evolved over time. Although the placental Tree of Life is currently ambiguous as to where Placentalia originated, it does reveal interesting information about how some mammals reached their present-day habitats. The group Afrothheria, originally identified using molecular data, has its oldest fossil members in the New World, both North and South America. Anatomical arguments that had brewed for decades had to be settled to launch the project. One of these was the tooth homologies of marsupials compared to placentals. Both marsupial and placental mammals have seven teeth, a mixture of premolars and molars, behind the canine tooth. The different shapes of the teeth in extinct species, however, had long invited the question of whether placental and marsupial mammals have the same seven teeth.