We simulate the dynamics of fractal star clusters , in order to investigate the evolution of substructure in recently formed clusters .
The velocity dispersion is found to be the key parameter determining the survival of substructure .
In clusters with a low initial velocity dispersion , the ensuing collapse of the cluster tends to erase substructure , although some substructure may persist beyond the collapse phase .
In clusters with virial ratios of 0.5 or higher , initial density substructure survives for several crossing times , in virtually all cases .
Even an initially homogeneous cluster can develop substructure , if it is born with coherent velocity dispersion .

These results suggest that the simple initial conditions used for many sophisticated N -body simulations could be missing a very important and dramatic phase of star cluster evolution .