  Recent particle-in-cell simulations suggest that a large fraction of the
energy dissipated in a relativistic shock is deposited into a Maxwellian
distribution of electrons that is connected to the high-energy power-law tail.
Here, we explore the observational implications of such a mixed
thermal-nonthermal particle distribution for the afterglow and prompt emission
of gamma-ray bursts. When the Maxwellian component dominates the energy budget,
the afterglow lightcurves show a very steep decline phase followed by a more
shallow decay when the characteristic synchrotron frequency crosses the
observed band. The steep decay appears in the X-rays at ~100 sec after the
burst and is accompanied by a characteristic hard-soft-hard spectral evolution
that has been observed in a large number of early afterglows. If internal
shocks produce a similar mixed electron distribution, a bump is expected at the
synchrotron peak of the nu*f_nu spectrum.
