  In the last years a general consensus has emerged on the use of ultra-high
energy cosmic rays (UHECR) data as a powerful probe of the validity of special
relativity. This applies in particular to the propagation of cosmic rays from
their sources to Earth, responsible for energy suppressions due to pion
photoproduction by UHE protons (the Greisen-Zatsepin Kuzmin limit) and photo
disintegration of UHE nuclei (the Gerasimova-Rozental limit). A suppression in
the flux of UHECRs at energies above 40 EeV -- as expected from both these
interactions -- has been established experimentally beyond any doubt by current
experiments. However, such an observation is still not conclusive on the origin
of the suppression. In particular, data from the Pierre Auger Observatory can
be interpreted in a scenario in which the suppression is due to the maximum
acceleration energy at the sources rather than to interactions in the
background radiation. In this scenario, UHECR data can no longer yield bounds
on Lorentz invariance violations which increase the thresholds for interactions
of nuclei on background photons, in particular through modification of the
dispersion relations. Here we argue in turn that the study of UHECRs still
represents an opportunity to test Lorentz invariance, by discussing the
possibility of deriving limits on violation parameters from UHECR phenomena
other than propagation. In particular we study the modifications of the shower
development in the atmosphere due to the possible inhibition of the decay of
unstable particles, especially neutral pions.
