| Method |
The soft maximum constraint is set by older fossiliferous beds with fossil mammals, but not placentals or marsupials, or members of the stem groups of either clade. For example, an older therian, neither marsupial nor placental, is Vincelestes from the La Amarga Formation of Argentina, dated as Hauterivian (136.4 Ma ± 2.0 Myr – 130.0 Ma ± 1.5 Myr). Thus researchers' soft maximum constraint is 138.4 Ma. Beds of similar age in North America and Europe have also produced such basal therians that are neither marsupials nor placentals according to present evidence. |