  We define a volume limited sample of over 14,000 early-type galaxies (ETGs)
selected from data release six of the Sloan Digital Sky Survey. The density of
environment of each galaxy is robustly measured. By comparing narrow band
spectral line indices with recent models of simple stellar populations (SSPs)
we investigate trends in the star formation history as a function of galaxy
mass (velocity dispersion), density of environment and galactic radius. We find
that age, metallicity and alpha-enhancement all increase with galaxy mass and
that field ETGs are younger than their cluster counterparts by ~2 Gyr. We find
negative radial metallicity gradients for all masses and environments, and
positive radial age gradients for ETGs with velocity dispersion over 180 km/s.
Our results are qualitatively consistent with a relatively simple picture for
ETG evolution in which the low-mass halos accreted by a proto-ETG contained not
only gas but also a stellar population. This fossil population is
preferentially found at large radii in massive ETGs because the stellar
accretions were dissipationless. We estimate that the typical, massive ETG
should have been assembled at z < 3.5. The process is similar in the cluster
and the field but occurred earlier in dense environments.
