Placenta and implantation site of Przewalski's horse
From the Benirschke web site (here)
Przewalski's horse was once listed as extinct in the wild but a successful captive breeding and release program has changed its status to endangered. There are several images of the placenta of Przewalski's horse on Benirschke's web site (see previous post). The equine placenta is epitheliochorial and has microcotyledons as pictured above.
Soft tissues usually are not preserved for posterity but a putative placenta accompanies a Middle Eocene fossil of the equid genus Propalaeotherium (cited here).