Left: time evolution of the surface density in an extended planetesimal disc (Δr_PB ≈ 45 au). The wavy structures of the Σ-curves are numerical artefacts. For the sake of comparison, the straight line shows the Σ-slope derived by MacGregor et al. (2013). Right: time evolution of the 1.3 mm profile along with the ALMA data. The 0.01 Myr surface brightness profile was scaled by a factor of 0.05 for better visibility.