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This stylised bird skeleton highlights the synsacrum

The synsacrum is a skeletal structure, mainly described in birds and dinosaurs, in which the sacrum is extended by incorporation of additional fused or partially-fused caudal or lumbar vertebrae. The ilium of the pelvis is attached to the synsacrum and, in some species, may also be fused to it. Together with the pelvis this forms a rigid structure which carries the weight of the hind end of the animal. In living and some prehistoric birds, posterior to the synsacrum there are usually only a few free caudal vertebrae, on the end of which lies the pygostyle.

In terms of external morphology, the synsacrum corresponds to the rump.