At which point on the phylogeny shown did the amnion first appear?

Question: At which point on the phylogeny shown did the amnion first appear?

The amnion, a protective membrane surrounding the embryo, first appeared in amniotes, a clade that includes reptiles, birds, and mammals. On a phylogenetic tree, this feature would emerge at the common ancestor of reptiles and mammals, distinguishing amniotes from amphibians, which lack this adaptation.