This is an interesting find in that they actually compared placentals and marsupials. There are other examples of transposable elements and placental development which illustrate a wonderful case of convergent evolution.
Two endogenous retroviral envelope genes are required for placental formation in humans and non-human primates (Syncytin-1 and 2) which are a HERV-W and HERV-FRD envelope gene respectively. Mice (and other mammals) don't have these HERVs..however, distantly related ERV envelope genes perform exactly the same function in mice (syncytin A and B).
It is not so surprising as envelope genes are highly fusiogenic and placental formation requires synctiotrophoblast fusion. But it is a nice example of beneficial mutations caused by retroelements that can cause huge evolutionary events.
References
Dupressoir A, Marceau G, Vernochet C, Benit L, Kanellopoulos C, Sapin V, Heidmann T. Related Articles, Links
Syncytin-A and syncytin-B, two fusogenic placenta-specific murine envelope genes of retroviral origin conserved in Muridae.
Proc Natl Acad Sci U S A. 2005 Jan 18;102(3):725-30. Epub 2005 Jan 11.
Blaise S, de Parseval N, Benit L, Heidmann T. Related Articles, Links
Genomewide screening for fusogenic human endogenous retrovirus envelopes identifies syncytin 2, a gene conserved on primate evolution.
Proc Natl Acad Sci U S A. 2003 Oct 28;100(22):13013-8. Epub 2003 Oct 13.