1) Transitional fossil 
primitive genus of extinct mammals found as fossils in Late Cretaceous rocks (66.4 to 97.5 million years old) of Mongolia. Deltatheridium, a tiny ...
(pronounced DEL-tah-ther-ID-ee-um) Deltatheridium was an early mammal from the late Cretaceous period , about 80 million years ago. This opossum-like quadruped was about 6 inches (15 cm) long and had a long tail. Its had sharp canine teeth and its cheek teeth were wide with triangular crowns. Deltatheridium was an insectivore that may have also eat...