  We study the dependence of extensive air shower development on the first
hadronic interactions at ultra-high energies occurring in the startup phase of
the air shower cascade. The interpretation of standard air shower observables
depends on the characteristics of these interactions. Thus, it is currently
difficult to draw firm conclusions for example on the primary cosmic ray mass
composition from the analysis of air shower data. On the other hand, a known
primary mass composition would allow us to study hadronic interactions at
center of mass energies well above the range that is accessible to accelerators
measurements.
