  Using the most advanced model of the hadron resonance gas we reveal, at
chemical freeze-out, remarkable irregularities such as an abrupt change of the
effective number of degrees of freedom and plateaus in the collision-energy
dependence of the entropy per baryon, total pion number per baryon, and thermal
pion number per baryon at laboratory energies 6.9-11.6 AGeV. On the basis of
the generalized shock adiabat model we show that these plateaus give evidence
for the thermodynamic anomalous properties of the mixed phase at its boundary
to the quark-gluon plasma (QGP). A new signal for QGP formation is suggested
and justified.
