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We measure radial gradients of the Mg2 index in 15 E-E/S0 and 14 S0 galaxies.Our homogeneous data set covers a large range of internal stellar velocitydispersions (2.0<logsigma<2.5) and Mg2 gradients (dMg2/dlogr/re* up to-0.14mag/dex). We find for the first time, a noticeable lower boundary in therelation between Mg2 gradient and sigma along the full range of sigma, whichmay be populated by galaxies predominantly formed by monolithic collapse. Athigh sigma, galaxies showing flatter gradients could represent objects whichsuffered either important merging episodes or later gas accretion. Theseprocesses contribute to the flattening of the metallicity gradients and theirincreasing importance could define the distribution of the objects above theboundary expected by the ``classical'' monolithic process. This result is inmarked contrast with previous works which found a correlation betweendMg2/dlogr/re* and sigma confined to the low mass galaxies, suggesting thatonly galaxies below some limiting sigma were formed by collapse whereas themassive ones by mergers. We show observational evidence that a hybrid scenariocould arise also among massive galaxies. Finally, we estimated d[Z/H] from Mg2and Hbeta measurements and single stellar population models. The conclusionsremain the same, indicating that the results cannot be ascribed to age effectson Mg2.Comment: 11 pages, 2 figures, to appear in ApJLetter

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