A. S. Romer wrote that mammalian paleontologists seemed to think that “mammalian evolution consisted of parent molar teeth giving birth to filial molar teeth, and so on down through the ages.” It is true that most Mesozoic (250–65 million years ago (mya)) mammals are known solely from their teeth; the hardest bones in the skeleton, teeth are preserved in many sedimentary environments in which the rest of the animal is destroyed.

For this reason, the extraordinary quality of fossil preservation in the Jehol Group, a series of richly fossiliferous lake deposits in northeastern China deposited between 135 and 120 million years ago, is significant to mammalian paleontologists. The fine-grained shales of the Jehol Group, deposited in a low-energy environment with no waves or strong currents, have preserved fragile insects, delicate plants, and most prominently the long-hypothesized feathers of dinosaurs discovered in the 1990s. Almost lost in this embarrassment of riches are the partial or complete skeletons of six genera of mammals, five of them new to science.