A fossil of the very closely related Velociraptor mongoliensis has an articulated tail skeleton that is curved laterally in a long Sâ `` shape . These features seemed to make the tail into a stiff counterbalance , but a fossil of the very closely related Velociraptor mongoliensis ( IGM 100\/986 ) has an articulated tail skeleton that is curved laterally in a long Sâ `` shape .