Placentation in gibbons

Agile Gibbon (Hylobates agilis) Bristol Zoo Gallery

A recent paper on a fossil ape (here) highlights the divergence of the lesser apes (gibbons and siamangs) from the great apes (orangutans, gorilla, chimpanzee, bonobo, human). The fossil (Pliobates cataloniae) has a combination of primitive and derived features that make it difficult to place on the evolutionary tree (discussed here).

 

Gibbons themselves have placentation with some monkey-like features and others shared with the great apes.

 

Uterus of an agile gibbon (H. agilis) opened to show the decidua
capsularis enclosing the embryo. From Selenka 1899

The most important shared characteristic is that the fetus develops beneath a decidua capsularis (see previous post) implying that implantation is interstitial as in great apes. In Old World monkeys implantation is superficial and no decidua capsularis is found. 

 

Placental bed of a Javan gibbon (Hylobates moloch)
Reproduced from Carter et al. (c) Museum for Naturkunde Berlin

However, when we examined the placenta of a Javan gibbon we found a continuous trophoblastic shell and a sharp boundary between the shell and the underlying endometrium - just as in Òld World monkeys. In great apes, the boundary is less distinct because trophoblast cells invade the endometrium by this route.