As an undergraduate, I used the University of Michigan’s Animal Diversity Web to learn key characters of mammal skulls (check out these great photos of a coyote skull, as an example!). It turns out, they also have a great online fossil repository too! UMORF, or the University of Michigan Online Repository of Fossils has both 3D and 2D images of fossil invertebrate and vertebrate specimens. As a newbie to paleontology, I really enjoy using their 3D viewer and marveling at the incredibly detail shown in the images!