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Most parsimonious optimization of sex-determining mechanisms on a phylogeny of extant tetrapods. Numbers indicate where extinct taxa of the Hell Creek Formation attach to this tree (see the first column of table 1). D, Dinosauria. Hatched branches indicate polymorphism (terminal branches) or ambiguity (internal branches). See the electronic supplementary material for studies that support this topology, the placement of extinct taxa, and for taxa whose SDM could not be reconstructed.
