Recent photometric studies have shown that early-type galaxies at fixed stellar mass were smaller and denser at earlier times .
In this paper we assess that finding by deriving the dynamical mass of such a compact quiescent galaxy at z=1.8 .
We have obtained a high-quality spectrum with full UV-NIR wavelength coverage of galaxy NMBS-C7447 using X-Shooter on the VLT .
We determined a velocity dispersion of 294 \pm 51 { km~ { } s } ^ { -1 } .
Given this velocity dispersion and the effective radius of 1.64 \pm 0.15 kpc ( as determined from HST-WFC3 F160W observations ) we derive a dynamical mass of 1.7 \pm 0.5 \times 10 ^ { 11 } M _ { \odot } .
Comparison of the full spectrum with stellar population synthesis models indicates that NMBS-C774 has a relatively young stellar population ( 0.40 Gyr ) with little or no star formation and a stellar mass of M _ { \star } \sim 1.5 \times 10 ^ { 11 } M _ { \odot } .
The dynamical and photometric stellar mass are in good agreement .
Thus , our study supports the conclusion that the mass densities of quiescent galaxies were indeed higher at earlier times , and this earlier result is not caused by systematic measurement errors .
By combining available spectroscopic measurements at different redshifts , we find that the velocity dispersion at fixed dynamical mass was a factor of \sim 1.8 higher at z=1.8 compared to z=0 .
Finally , we show that the apparent discrepancies between the few available velocity dispersion measurements at z > 1.5 are consistent with the intrinsic scatter of the mass-size relation .