In a choriovitelline placenta, the yolk sac fuses with the chorion and, subsequently, wrinkles develop that hold the embryo to the uterine wall, thus forming the choriovitelline placenta.
[1] Because the yolk sac is formed earlier than the allantois in embryo development, a choriovitelline placenta can form earlier than the chorioallantoic placenta.
[1] All marsupials maintain a choriovitelline placenta.
However, this is not to say the existence of a choriovitelline placenta is a "primitive" feature: many placental mammals, including pig, horse, and ruminants, forms a choriovitelline placenta in early development before the chorioallantoic placenta forms and the choriovitelline placenta is resorbed.
Rodents and some other mammals first from a choriovitelline placenta then forms a chorioallantoic placenta, but both types are maintained throughout gestation.