  [Abridged]We present the morphological analysis based on HST-NICMOS
observations in the F160W filter of a sample of 32 early-type galaxies (ETGs)
at 1<z<2 with spectroscopic confirmation of their redshift and spectral type.
We find that ETGs at are composed of two distinct populations, an older
population (oETGs) and a younger population (yETGs) whose mean ages differ by
about 1.5-2 Gyr. Young ETGs are not denser than local ones since they follow
the size-mass relation of local ETGs and luminosity evolution brings them onto
the local Kormendy and size-luminosity relations. On the constrary, old ETGs do
not follow the size-mass relation of local ETGs and luminosity evolution does
not account for the discrepancy they show with respect to the local
size-luminosity and Kormendy relations. An increase of their effective radius
R_e by a factor 2.5-3 (a density decrease by a factor 15-30) from z~1.5-2 to
z~0 is required to bring these galaxies onto the local scaling relations. The
different behaviour shown by the two populations implies different formation
and evolution scenarios. The older population must have formed at higher-z in a
sort of dissipative gas-rich collapse ables to produce remnants which at z~2
are old and compact. The size evolution they must experience from z~2 to z~0
has to leave unchanged their mass to not exceed the local number of high-mass
ETGs. Thus, major merging cannot fit this requirement. The younger population
of ETGs can be formed later through subsequent episodes of merging which
increased progressively their size and assembled their mass down to z~2. At z<2
they evolve purely in luminosity since episodes of major merging would bring
them far from the local scaling relations.
