  We exploit recent constraints on the ages and metallicities of early-type
galaxies in the Sloan Digital Sky Survey (SDSS) to gain new insight into the
physical origin of two fundamental relations obeyed by these galaxies: the
colour-magnitude and the Mg2-sigmaV relations. Our sample consists of 26,003
galaxies selected from the SDSS DR2 on the basis of their concentrated light
profiles, for which we have previously derived median-likelihood estimates of
stellar metallicity, light-weighted age and stellar mass. Our analysis provides
the most unambiguous demonstration to date of the fact that both relations are
primarily sequences in stellar mass and that total stellar metallicity,
alpha-elements-to-iron abundance ratio and light-weighted age all increase with
mass along the two relations. For high-mass ellipticals, the dispersion in age
is small and consistent with the error. At the low-mass end, there is a tail
towards younger ages, which dominates the scatter in colour and index strength
at fixed mass. A small, but detectable, intrinsic scatter in the
mass-metallicity relation also contributes to the scatter in the two
observational scaling relations, even at high masses. Our results suggest that
the chemical composition of an early-type galaxy is more tightly related to its
dynamical mass (including stars and dark matter) than to its stellar mass. The
ratio between stellar mass and dynamical mass appears to decrease from the
least massive to the most massive galaxies in our sample.
