Final answer:
Alex discovered a trace fossil, which is a type of fossil that provides indirect evidence of past life activities.
Step-by-step explanation:
Alex discovered a trace fossil when he found a prehistoric bird nest on a cliff during the excavation in Greenland. A trace fossil is any indirect evidence of past life activities, such as footprints, burrows, or nests. In this case, the nest itself is a trace left behind by the bird that lived there long ago.
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