relationship loss: subclass portion of (AAO:0010428)[AAO]

produced by fusion of last thoracics, all lumbars, all sacrals, & first few caudals;

Complex of synostotically fused vertebrae supporting the pelvic girdle. In anurans, the synsacrum is formed by two vertebrae and the urostyle is fused to its posterior end.[AAO]

uberon

UBERON:0007805

In birds and dinosaurs, 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[Wikipedia:Synsacrum], In amphibians, a complex of synostotically fused vertebrae supporting the pelvic girdle. In anurans, the synsacrum is formed by two vertebrae and the urostyle is fused to its posterior end[AAO:Pugener_2002]