Super spirals are the most massive star-forming disk galaxies in the universe. We measured rotation curves for 23 massive spirals with the Southern African Large Telescope (SALT) and found a wide range of fast rotation speeds (240-570 km s−1), indicating enclosed dynamical masses of (0.6−4) × 1012M . Super spirals with mass in stars log M stars M > 11.5 break from the baryonic Tully-Fisher relation (BTFR) established for lower-mass galaxies. The BTFR power-law index breaks from 3.75 0.11 to 0.25 0.41 above a rotation speed of ∼340 km s−1. Super spirals also have very high specific angular momenta that break from the Fall relation. These results indicate that super spirals are undermassive for their dark matter halos, limited to a mass in stars of log M stars M < 11.8 . Most giant elliptical galaxies also obey this fundamental limit, which corresponds to a critical dark halo mass of log M halo M 12.7 . Once a halo reaches this mass, its gas can no longer cool and collapse in a dynamical time. Super spirals survive today in halos as massive as log M halo M 13.6 , continuing to form stars from the cold baryons they captured before their halos reached critical mass. The observed high-mass break in the BTFR is inconsistent with the Modified Newtonian Dynamics theory.