The equation of state ( EOS ) in quartessence models interpolates between two stages : p \simeq 0 at high energy densities and p \approx - \rho at small ones .
In the quartessence models analyzed up to now , the EOS is convex , implying increasing adiabatic sound speed ( c _ { s } ^ { 2 } ) as the energy density decreases in an expanding Universe .
A non-negligible c _ { s } ^ { 2 } at recent times is the source of the matter power spectrum problem that plagued all convex ( non-silent ) quartessence models .
Viability for these cosmologies is only possible in the limit of almost perfect mimicry to \Lambda CDM .
In this work we investigate if similarity to \Lambda CDM is also required in the class of quartessence models whose EOS changes concavity as the Universe evolves .
We focus our analysis in the simple case in which the EOS has a step-like shape , such that at very early times p \simeq 0 , and at late times p \simeq const < 0 .
For this class of models a non-negligible c _ { s } ^ { 2 } is a transient phenomenon , and could be relevant only at a more early epoch .
We show that agreement with a large set of cosmological data requires that the transition between these two asymptotic states would have occurred at high redshift ( z _ { t } \gtrsim 38 ) .
This leads us to conjecture that the cosmic expansion history of any successful non-silent quartessence is ( practically ) identical to the \Lambda CDM one .