This shrew-sized Cretaceous-age animal, Ukhaatherium nessovi, which was uncovered in 1994 in the Gobi Desert by the Mongolian Academy and the American Museum of Natural History, is one of the many mammals used in the recent mammal tree-of-life study. When it was discovered, the remarkably well-preserved skeleton of this small creature showed the presence of epipubic bones, attached to both sides of the pubis. In living mammals, these bones occur only in marsupials (mammals like kangaroos that often develop their young in pouches) and monotremes (mammals like platypuses, which lay eggs). But their presence in Ukhaatherium nessovi showed that a close relative of recent placental mammals that lived many millions of years ago also had these bones. This image relates to a paper that appeared in the Feb. 8, 2013, issue of Science, published by AAAS. The paper, by Maureen A. O'Leary at Stony Brook University in Stony Brook, NY, and colleagues was titled, "The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals."