HISTORICAL REMNANTS. “Retained tails in humans, or external hindlimbs in cetaceans and snakes—lost from ancestors but present again—are other telltale clues to organismal history, both phylogenetic and ontogenetic, as are the re-emergent “hen’s teeth and horse’s toes” that Gould made famous in his book of the same name.”
https://evolution-outreach.springeropen.com/articles/10.1186/s12052-014-0012-5