  The Fundamental Plane has finite thickness and is tilted from the virial
relation, indicating that dynamical mass-to-light ratios (Mdyn/L) vary among
early type galaxies. We use a sample of 16,000 quiescent galaxies from the
Sloan Digital Sky Survey to map out variations in Mdyn/L through the 3D
Fundamental Plane space defined by velocity dispersion (sigma), effective
radius (R_e), and effective surface brightness. We consider contributions to
Mdyn/L variation due to stellar population effects, IMF variations, and
variations in the dark matter fraction within one R_e. Along the FP, we find
that the stellar population contribution scales as M*/L ~ f(sigma), while the
dark matter and/or IMF contribution scales as Mdyn/M* ~ g(Mdyn). The two
contributions to the tilt of the FP rotate the plane around different axes in
the 3D space, with dark matter/IMF variations likely dominating. Through the
thickness of the FP, we find that Mdyn/L variations must be dominated either by
IMF variations or by real differences in dark matter fraction with R_e. Thus
the finite thickness of the FP is due to variations in the stellar mass surface
density within R_e, not the fading of passive stellar populations. These
structural variations are correlated with galaxy star formation histories such
that galaxies with higher Mdyn/M* at a given sigma have higher [Mg/Fe], lower
metallicities, and older mean stellar ages. It is difficult to explain the
observed correlations by allowing the IMF to vary, suggesting difference in
dark matter fraction dominate. These can be produced by variations in the
"conversion efficiency" of baryons into stars or by the redistribution of stars
and dark matter through dissipational merging. A model in which some galaxies
experience low conversion efficiencies due to premature truncation of star
formation provides a natural explanation for the observed trends.
