The Tree of Life web project is a growing online collaborative project to:
to contain a page with pictures, text, and other information for every species and for each group of organisms, living or extinct.
It’s a great starting point for looking at the tree of life because each page has links to a wealth of online resources. One of the links on the Mammalia page, for example, is to Faunmap, an online database that produces maps of where different modern and extinct mammalian species can be found in the United States.
All the pages on the Tree of Life website are linked by the branches of the tree of life. The Class Mammalia links up to its parent Therapsida and down to the its Orders such as Monotremata (one of my favorites) and Eutheria, the placenta mammals.
The site is authored by professional scientists and science educators so has that credibility. Most of the images also allow free, non-commercial use. Thanks to Anna C. for the link.