  We have investigated the properties of a complete K-band selected sample of
35 elliptical and S0 galaxies brighter than K=20.15 in the Hubble Deep Field,
as representative of the field galaxy population. This sample has been derived
from deep K-band image by the KPNO-IRIM camera, by applying a rigorous
morphological classification scheme based on quantitative analyses of the
surface brightness profiles. The broad-band spectra of the sample galaxies
allow us to date their dominant stellar populations. The majority of bright
early-types in this field are found at redshifts z<1.3 to display colors
indicative of a fairly wide range of ages (typically 1.5 to 3 Gyrs). We find
that the major episodes of star-formation building up typical M_star galaxies
have taken place during a wide redshift interval 1<z<4 for q_0=0.5, which
becomes 1<z<3 for q_0=0.15. Our estimated galactic masses, for a Salpeter IMF,
are found in the range from a few billion to a few 10^{11} solar masses already
at z=1. So the bright end of the E/S0 population is mostly in place by that
cosmic epoch, with space densities, masses and luminosities consistent with
those of the local field E/S0 population. What distinguishes the present sample
is a remarkable absence of objects at z>1.3, which should be detectable during
the luminous star-formation phase expected to happen at these redshifts.
Obvious solutions are a) that the merging events imply perturbed morphologies
which prevent selecting them by our morphological classification filter, or b)
that a dust-polluted ISM obscures the (either continuous or episodic) events of
star-formation. We conclude that the likely solution is a combination thereof,
i.e. a set of dust-enshrouded merging-driven starbursts occurring during the
first few Gyrs of the galaxy's lifetime.
