Stellar mass as a function of lookback time to formation (left panel) and mass histograms of extreme tertiles for ETGs. Squared symbols represent our data, where the errorbars plotted are the dispersions in the measurements. Circle points are data taken from the analysis of SDSS-DR4 by Thomas et al. (2009), showing the lookback time at which the 50% of the stellar mass has formed; the errorbars plotted are the differences between the lookback time of formation of 50% and of 80% of the stellar mass. The dashed lines represent the mass-lookback time relation computed in the extreme quartiles of overdensity. To compare data here we adopted Ω_m = 0.24, Ω_Λ = 0.76, and H₀ = 73kms^-1   Mpc^-1.