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Referring to the fossil having been mobile, or "walking", long after the death of the original organism.
Etymology from Wiktionary, CC BY-SA 4.0.
zombie taxon
Today it means: A taxon that falsely appears to persist until, or reappear at, a time in the fossil record later than the taxon's actual point of extinction, due to fossils of organisms belonging to the taxon having been eroded from their original strata and redeposited in the source sediments for younger strata. Full definition →