Temporal evolution from T = 4.2 Gyr (merger epoch) to 6.8 Gyr (present-day of the remnant M31) of randomly selected gas particle orbits at five different areas of the disk. The areas correspond to 2 kpc wide rings selected in the gas disk at T = 6.8 Gyr (snapshot 680) with radii of 10 (blue), 15 (cyan), 20 (green), 25 (pink), and 30 (magenta) kpc, respectively. Left panel: Radial orbital evolution of the gas particles from 10 kpc (top panel) to 30 kpc (bottom panel). The black dotted lines illustrate the orbital paths of characteristic gas particles in each panel. Middle panel: Ratio of the disk angular momentum (J_z/J) to the total angular momentum in the same rings as in the left panel. The black solid lines give the median, and the black dashed lines give the 1σ variations. Right panel: Same as the other panels but for the eccentricity evolution.