Bagheera
Bagheera.com: Extinct in the Wild Heath Hen
This article is about the now extinct heath hen. There is a lot to be learned from organisms that have died out to help prevent other extinctions. The questions for thought are excellent discussion starters.
Other
Conservation Ecology: Assessing Extinction Risk
This site discusses how genetic variability is correlated with the risk of organism extinction. It also discusses how bottlenecks can reduce genetic variability in populations leading to extinctions.
Sophia Learning
Sophia: Evolution: Lesson 3
This lesson will introduce the concept of evolution and describe how and why organisms evolve.
University of California
Ucmp: Introduction to Trilobita
One of the most famous fossil organisms, trilobites were once a complex group. Learn to tell them apart in this illustrated article that tells how they lived, their body structure, there place in evolution, and the like.
Tree of Life Project
Reptiles: Synapsida
This site contains general information about the extinct link between reptiles and mammals. The Synapsida organisms are the link.