Instructions
Play through the game until you battle Bullwort in Richmond's room. He'll summon a Frigisaurus, which will utterly annihilate you.
Speak with the Digadig Chieftain and he'll tell you about the ancient rivalry between Frigisaurus and Ignosaurus. He'll also tell you that Ignosaurus's fossil can be found in Mt. Lavaflow.
Go to the Fossil Center and speak with your teammates. Rex will debrief you on the dangers of Mt. Lavaflow, namely molten lava, and how you need to drain the lava. Your dog Rex will guide you to the lava-draining valves. Go to Mt. Lavaflow.
Walk down the right-hand path. To get past the locked door, touch the keys' rusty edges on the DS touch pad. Proceed until you start encountering rocks. Smash them until you find the valve. Turn it to drain the lava.
Return to the starting path of Mt. Lavaflow and head left. Pay attention to your radar. Ignosaurus's fossil will blink red. Collect it. Bullwort will suddenly appear and corrupt the fossil. Take it back to the Fossil Center anyway. Show it to Diggins. He tells you that it's no longer of use and that you need another fossil.
Go back to Mt. Lavaflow and collect a second Ignosaurus fossil. Bring this back to the Fossil Center. After a long cut-scene, Ignosaurus will be revived and you'll have a legendary vivosaur capable of defeating Frigisaurus.