  The average profiles of cosmic ray shower development as a function of
atmospheric depth are measured for the first time with the Fluorescence
Detectors at the Pierre Auger Observatory. The profile shapes are well
reproduced by the Gaisser-Hillas parametrization at the 1% level in a 500 g/cm2
interval around the shower maximum, for cosmic rays with log(E/eV) > 17.8. The
results are quantified with two shape parameters, measured as a function of
energy. The average profiles carry information on the primary cosmic ray and
its high energy hadronic interactions. The shape parameters predicted by the
commonly used models are compatible with the measured ones within experimental
uncertainties. Those uncertainties are dominated by systematics which, at
present, prevent a detailed composition analysis.
