Discovered in 1997, this rodent belongs to the family Diatomyidae that was thought to have died out in the late Miocene (background here). Molecular data place it basal to hystricognath rodents such as the guinea pig and capybara.
Cross section through the rock rat uterus, placenta and fetal membranes
Reproduced from Carter et al. © (2013) with permission from Elsevier
The internal structure of the placenta was rather simple with a separate labyrinth and spongy zone. In contrast, the hystricognath placenta is highly folded with lobules of labyrinth separated by interlobular areas of spongiotrophoblast.
Thin section through the exchange area of a rock rat placenta
Reproduced from Carter et al. © (2013) with permission from Elsevier
The interhemal barrier was hemodichorial, i.e. with two trophoblast layers. The maternal blood spaces (mbs) were lined by syncytiotrophoblast (syn tr) and below this was a layer of cytotrophoblast (ctr). This pattern is different from all other rodents that have been studied so far.
Some distinctive features of hystricognath placentation, such as the subplacenta, were not found in the Laotian rock rat. On the other hand this species does resemble hystricognaths in giving birth to a single precocial pup.
Disclosure: I am lead author of this paper and must acknowledge my colleagues Jean-Pierre Hugot (Paris), Allen C. Enders (Davis), Carolyn J. P. Jones (Manchester) and Par Kham Keovichit (Vientine).
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