Science -- Asher et al. 307 (5712): 1091:
We describe several fossils referable to Gomphos elkema from deposits close to the Paleocene-Eocene boundary at Tsagan Khushu, Mongolia. Gomphos shares a suite of cranioskeletal characters with extant rabbits, hares, and pikas but retains a primitive dentition and jaw compared to its modern relatives. Phylogenetic analysis supports the position of Gomphos as a stem lagomorph and excludes Cretaceous taxa from the crown radiation of placental mammals. Our results support the hypothesis that rodents and lagomorphs radiated during the Cenozoic and diverged from other placental mammals close to the Cretaceous-Tertiary boundary.
Lagomorphs are rabbits, hares, and pikas. This might be referred to as a "missing link" of the rodents. Why do we care? Most mammals are rodents, and this tells us about the evolution of the most successful group of mammals. Cool!