  Recently the NA60 collaboration has presented high precision measurements of
dimuon spectra double differential in invariant mass $M$ and transverse pair
momentum $p_T$ in In-In collisions at $158 {\rm AGeV}$. While the
$M$-dependence is important for an understanding of in-medium changes of light
vector mesons and is $p_T$ integrated insensitive to collective expansion, the
$p_T$-dependence arises from an interplay between emission temperature and
collective transverse flow. This fact can be exploited to derive constraints on
the evolution model and in particular on the contributions of different phases
of the evolution to dimuon radiation into a given $M$ window. We present
arguments that a thermalized evolution phase with $T > 170 {\rm MeV}$ leaves
its imprint on the spectra.
