Juramaia

Juramaia is the earliest known eutherian mammal. It is a small shrew-like mammal of body length about 70100 mm. It is an extinct genus from the Upper Jurassic deposits of western Liaoning, China. 
A nearly complete skeleton, with an incomplete skull with full dentition was found. It was collected in the Daxigou site, Jianchang, from the Tiaojishan Formation. It is dated at about 160 million years ago.
The discovery of "Juramaia" provides new insight into the evolution of eutherian mammals. It shows their lineage diverged from that of the marsupials 35 million years earlier than previously thought. Furthermore, its discovery fills gaps in the fossil record and helps to calibrate modern, DNA-based methods of dating the evolution. 
The forelimb bones have climbing adaptations, so it is suggested that the basal stock of Eutheria was arboreal.