  Recently the NA60 collaboration has reported the transverse mass spectra of
dimuons coming from In-In collisions at 158 GeV/A. The measured yields display
a strong invariant mass dependence not typical of radial flow, suggesting that
different sources contribute in different mass regions. We interpret the dimuon
transverse mass spectra from an early thermalized partonic phase and hadronic
phase constrained by the strictures of broken chiral symmetry. Each phase
develops a specific transverse momentum dependence by hydrodynamical expansion.
We show that a measurement of the momentum anisotropy at NA60 could provide
information on the dominant emission source (hadronic or partonic) in the
intermediate mass region 1.5 <= M <= 3.0 GeV.
