We calculate excitation functions for open charm mesons in $Au+Au$ reactionsfrom AGS to RHIC energies within the HSD transport approach which is based onstring, quark, diquark ($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degreesof freedom. The open charm cross sections from $pN$ and $\pi N$ reactions arefitted to results from PYTHIA and scaled in magnitude to the availableexperimental data. From our dynamical calculations we find an approximate$m_T$-scaling for pions, kaons, $D$-mesons and $J/\Psi$ -- when discardingfinal state elastic scattering of kaons and $\phi$-mesons with pions -- incentral collisions of $Au + Au$ at 160 A$\cdot$GeV (with an apparent slope of176 MeV) without employing the assumption of a Quark-Gluon Plasma (QGP). Wedemonstrate that this result is essentially due to a relative $m_T$-scaling in$pp$ collisions at $\sqrt{s} \approx$ 17.3 GeV. At lower bombarding energies of25 A$\cdot$GeV a suppression of $D$-mesons by a factor of $\sim$ 10 relative toa global $m_T$-scaling with slope 143 MeV is expected. However, whenincorporating attractive $D$-meson self energies as suggested by QCD sum rules,an approximate $m_T$-scaling is regained even at 25 A$\cdot$GeV. The effects of$D$-meson rescattering and charmonium absorption are discussed, furthermore,with respect to rapidity and transverse mass distributions in centralcollisions of $Au + Au$ at 25, 160 A$\cdot$GeV and 21.3 A$\cdot$TeV.
