The dewetting of ultrathin polystyrene films on a liquid substrate is studied in the vicinity of the glass transition. This technique leads to the measurement of the extensional creep compliance of the film. The results reported in this article show that the rubbery plateau value of the compliance is unchanged for films as thin as 20 nm. Thus, we conclude that the entanglement density is unaffected by the surface at a scale of the coil size. These results are discussed in the context of previous results that report the reduction of the viscosity in the same films. Furthermore, the creep compliance of ultrathin films in the segmental relaxation regime exhibits a small reduction of the characteristic times. For high molecular weight thin films, the time scale reduction in the transition zone is much weaker than the reduction of the terminal relaxation time. This observation shows that the different parts of the time spectrum are not equally sensitive to the confinement. This is consistent with expected effects of the confinement at different length scales.