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Synopses of Research Articles Resolving the Family Tree of Placental Mammals Liza Gross | DOI: 10.1371/journal.pbio.0040111 Natural selection was particularly inventive in the appendage and accessory department during the evolution of placentals— an expansive category of mammals that bear live, fully developed offspring. A placental might sport webbed wings, a prehensile tail, fl ippers, fangs, tusks, cloven hooves, paws, claws, fl oppy ears, horns, or any number of other specialized structures. While such morphological characteristics shed light on evolutionary relationships, they can also confound classifi cations because animals might independently acquire the same traits without sharing a common ancestor. With an ever-growing repository of genome sequence data, scientists have increasingly turned to molecular techniques to help resolve evolutionary relationships. Several recent molecular analyses offer support for placing recent placentals into four major groups: Afrotheria (mostly African species, including elephants and aardvarks), Xenarthra (New World species such as armadillos and sloths), Laurasiatheria (includes carnivores, whales, and horses), and the newly reclustered Supraprimates (includes rodents and primates). Supraprimates and Laurasiatheria are further grouped together as sister taxa in a larger group called Boreotheria. Molecular approaches have also tried to resolve the hotly debated issue of where to draw the base of the placental tree, though no consensus has emerged. These studies arrived at these conclusions by analyzing different datasets of nuclear and mitochondrial genes under different models of DNA sequence evolution. But such molecular approaches have their own limitations, as genomes can also contain confounding features (called homoplasies), similar elements that look alike but do not represent common ancestry. In a new study, Jan Ole Kriegs, Jürgen Schmitz, and their colleagues used a