|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.