Definition of transitional fossil
: a fossil that exhibits characteristics of both ancestral and derived forms During the past 30 years, new discoveries and reinvestigations of long-forgotten specimens have coalesced into a flood of transitional fossils. Paleontologists have unearthed walking whales from Pakistan, feathered dinosaurs from China … — Courier News (Bridgewater, New Jersey), 27 Feb. 2011 However, although the mammal and bird origins are both documented by numerous transitional fossils, turtles appear abruptly.— Michael S. Y. Lee, Science, 24 Sept. 1993
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