The SM neutrino hierarchies are special
  There is a noticeable difference between neutrino mixing angle hierarchy and the mass hierarchies of all three types of SM Dirac particles – charged leptons and up- and down-quarks. The hierarchies of the three Dirac experimental mass values (m1, m2, m3) at leading approximation (biminimal pattern <0, 0, 1>; hierarchy-angles being bimaximal <90, 90, 0> deg) indicate that they obey the euclidean symmetry invariant Pythagorean equation not only in the 3-dimensional space, but also in the 2-dimensional spaces for each of the two adjoining mass pairs (m1, m2) ~ <0, 1>, (m2, m3) ~ <0, 1>. In contrast, the neutrino mixing hierarchy at leading approximation is related to bimaximal solution of the Pythagorean equation in three dimensions, but not in the outlined 2-dimension ones. And more, by neutrino oscillation experimental data this distinctive feature of neutrino mixing hierarchy is probably (if the smallest neutrino mass is not zero) shared by its mass spectrum hierarchy. It points that neutrinos are not Dirac particles. Hence they should be of Majorana nature as the other possibility.