  We investigate the implications of a tetraquark field on chiral symmetry
restoration at nonzero temperature. In order for the chiral phase transition to
be cross-over, as shown by lattice QCD studies, a strong mixing between scalar
quarkonium and tetraquark fields is required. This leads to a light ($\sim0.4$
GeV), predominantly tetraquark state, and a heavy ($\sim1.2$ GeV),
predominantly quarkonium state in the vacuum, in accordance with recently
advocated interpretations of spectroscopy data. The mixing even increases with
temperature and leads to an interchange of the roles of the originally heavy,
predominantly quarkonium state and the originally light, predominantly
tetraquark state. Then, as expected, the scalar quarkonium is a light state
when becoming degenerate in mass with the pion as chiral symmetry is restored
at nonzero temperature.
