It has been suggested that environmental sex determination (ESD), more specifically temperature-dependent sex-determination, could have been the cause of dinosaur extinction. It has been also suggested that this process of sex determination is related to phylogeny. But it can be shown that ESD is not related to phylogeny, that is it is not possible to infer whether ESD was present in a given extinct group. Moreover, marked environmental fluctuations did not extinguish taxa in which ESD is ascertained. Finally, living groups with ESD traversed the K/T boundary. Therefore, ESD cannot be regarded as the demonstrable cause, of the extinctions at the Cretaceous/Tertiary boundary.