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Phylogeny of a hypothetical taxon. A crown group (containing taxa 5 and 6) appears in the fossil record 40 Ma ago (green star) but the inferred molecular age is 60 Ma (arrow). The green dashed line at 45 Ma ago shows that three stem-taxa are observed at that time. All taxa shown here may represent a single evolutionary lineage (species, under some species concepts) or clades that may include several species. What is the probability (P?) that the crown group existed 45 Ma ago?
