Dromaeosauridae
Dromaeosaurs had long tails. Most of the tail vertebrae bear bony, rod-like extensions, as well as bony tendons in some species. In his study of Deinonychus, Ostrom proposed that these features stiffened the tail so that it could only flex at the base, and the whole tail would then move as a single, rigid, lever. However, one well-preserved specimen of Velociraptor mongoliensis (IGM 100/986) has an articulated tail skeleton which is curved horizontally in an S-shape. This suggests that, in life, the tail could bend from side to side with some degree of flexibility. It has been proposed that this tail was used as a stabilizer and/or counterweight while running. In Microraptor, an elongate diamond-shaped fan of feathers is preserved on the end of the tail. This might have been used as a stabilizer and rudder during gliding.