|Decidua was present in the most recent common ancestor of placental|
mammals but was lost in some lineages. Data from A. M. Mess and A. M. Carter
|Gray Four-eyed Opossum (Philander opossum)|
Wikimedia Commons CC-BY-3.0 (André de Souza Pereira)
The authors of the latter paper are part of a consortium that just published an extensive analysis of gene expression by the endometrium across mammals (here). The study included a frog, chicken, lizard, monotreme (Duck-billed Platypus), marsupial (Gray Short-tailed Opossum), and seven different placental mammals. It identified a huge number of genes that were recruited during the evolution of pregnancy in mammals, including many that are associated with the decidualization process.
The main thrust of the new paper is the central role played in evolution by transposable elements. These were co-opted into regulatory elements that coordinate the endometrial progesterone response.