Lattice anomalies in the FeAs$_{rm 4}$ tetrahedra of the NdFeAsO$_{rm 0.85}$ superconductor that disappear at T$_{rm c}$

High resolution synchrotron X-ray powder diffraction (SXRPD) was used to study the temperature dependence of the oxygen deficient NdFeAsO$_{0.85}$ superconducting compound. By employing a dense temperature sampling we have managed to reveal unnoticed structural modifications that start around $sim$180K, and disappear at the transition temperature. The data show minor changes of the structural characteristics in the Nd-O charge reservoir layer while in the superconducting Fe-As layer the FeAs$_{4}$ tetrahedron shows gradual modifications below $sim$180K, which suddenly disappear at T$_{rm c}$ strongly indicating a connection with superconductivity.