Koala (Phascolarctos cinereus)
Photo by David Iliff. License: CC-BY-SA 3.0
The koala (Phascolarctos cinereus) is a member of Vombatidae and its fetal membranes resemble those of the wombats (previous post).
Fetal membranes of the koala from Caldwell (1884)
In fact adherence of the allantois to the chorion was first described for this marsupial (here). Richard Semon soon suggested that this combination of yolk sac and simple chorioallantoic placentation was present in the common ancestor of marsupials and placentals. In other marsupials the chorioallantoic placenta was lost (bandicoot placentation had not then been described); in placentals the chorioallantoic placenta was elaborated and the yolk sac reduced or lost. In recent times this idea was revived by Freyer, Zeller and Renfree (here).
There is little of recent date though the koala is included in a review by Hughes (here) as well as a later book chapter. Based in part on the Hill Collection it includes histology of the yolk sac placenta and gross morphology of the chorioallantoic placenta.
Updated 17 July 2013