PeV Cosmic Rays: A Window on the Leptonic Era?

It is shown that a variety of characteristic features of the high-energy hadronic cosmic ray spectra, in particular the abrupt changes in the spectral index that occur around 3 PeV and 300 PeV, as well as the corresponding changes in elemental composition that are evident from kinks in the $<X_{\max }>$ distribution, can be explained in great detail from interactions with relic Big Bang antineutrinos, provided that the latter have a rest mass of $\sim 0.5$ eV/$c^2$.