  Using HST/ACS slitless grism spectra from the PEARS program, we study the
stellar populations of morphologically selected early-type galaxies in the
GOODS North and South fields. The sample - extracted from a visual
classification of the (v2.0) HST/ACS images and restricted to redshifts z>0.4 -
comprises 228 galaxies (F775W<24 ABmag) out to z~1.3 over 320 arcmin2, with a
median redshift zM=0.75. This work significantly increases our previous sample
from the GRAPES survey in the HUDF (18 galaxies over ~11 arcmin2; Pasquali et
al. 2006b). The grism data allow us to separate the sample into `red' and
`blue' spectra, with the latter comprising 15% of the total. Three different
grids of models parameterising the star formation history are used to fit the
low-resolution spectra. Over the redshift range of the sample - corresponding
to a cosmic age between 5 and 10 Gyr - we find a strong correlation between
stellar mass and average age, whereas the **spread** of ages (defined by the
RMS of the distribution) is roughly ~1 Gyr and independent of stellar mass. The
best-fit parameters suggest it is formation epoch and not formation timescale,
that best correlates with mass in early-type galaxies. This result - along with
the recently observed lack of evolution of the number density of massive
galaxies - motivates the need for a channel of (massive) galaxy formation
bypassing any phase in the blue cloud, as suggested by the simulations of Dekel
et al. (2009).
