The evolution of fetal membranes and placentation in carnivores and ungulates (Ferungulata)

Molecular phylogenetics has made a substantial contribution to our understanding of the relationships between mammalian orders and has generated trees that can be used to examine the evolution of anatomical and physiological traits. We here summarize findings on fetal membranes and placentation in Ferungulata, a clade comprising carnivores, pangolins, and even- and odd-toed ungulates. Their early ontogeny shows several conserved traits such as superficial attachment of the blastocyst, amnion formation by folding, a large allantoic sac and a temporary yolk sac placenta. In contrast, several characters of the chorioallantoic placenta are derived, including the diffuse and cotyledonary placental types in ungulates and zonary placenta in carnivores, specializations of the interhaemal barrier, the presence of areolae or haemophagous regions and lack of stromal decidual cells. Ungulates produce large amounts of placental proteins including placental lactogens and pregnancy-associated glycoproteins. Evolutionary innovations of the placental system may contribute to the high diversity of lifestyles within Ferungulata and be linked to the evolution of highly precocial offspring in ungulates.