I don't think you'd even have to look at extinct life. There's so much life that hasn't been categorized. You can basically take a soil sample from anywhere in the world and find new microorganisms in it. Archea are a great example of this. They're small, single celled organisms. They're best known as being extremeophiles, but exist basically everywhere. But the majority of them are difficult to culture and study. What's particularly interesting is that viruses that infect archea are known to be very different from other viruses.