  We use 62,185 quasars from the Sloan Digital Sky Survey DR5 sample and
standard virial mass scaling laws based on the widths of H beta, Mg II, and C
IV lines and adjacent continuum luminosities to explore the maximum mass of
quasars as a function of redshift, which we find to be sharp and evolving. This
evolution is in the sense that high-mass black holes cease their luminous
accretion at higher redshift than lower-mass black holes. Further, turnoff for
quasars at any given mass is more highly synchronized than would be expected
given the dynamics of their host galaxies. We investigate potential signatures
of the quasar turnoff mechanism, including a dearth of high-mass quasars at low
Eddington ratio. These new results allow a closer examination of several common
assumptions used in modeling quasar accretion and turnoff.
