We plot the ages of the known fossils to isolate the transition from water to land
Once we have researched all the fossils relevant to the water-to-land transition, we begin to get an idea of what timeframe this transition happened in. We know the lobe-finned fish are from 390-380 million year old rocks. The first tetrapods appear around 363 million years ago. Common sense tells us that the transitional form must have arisen 380-363 million years ago.
STEP 2: Determine the age of the rocks the transitional fossil should be in
In order to find our transitional fossil, we'll need to find rocks that are between 380 and 363 million years old. The world is an awfully big place. How do we know where to start looking? Luckily, most of the land on earth has been mapped by geologists exploring for valuable natural resources like oil and coal. So, we need to do some more research, this time, in the map room of our local library.