This stylised bird skeleton highlights the synsacrum
However concerning "non-
The sacral enlargements of non-
avian dinosaurs [non-maniraptor dinosaurs] do not share much structural similarities with the lumbosacral specializations of modern birds. Furthermore these species were quadrupedal and had a long tail, i.e. there was no need for a sense organ of equilibrium. It seems that early birds with a pygostyle and a well- developped synsacrum show structural specializations which resemble that of modern birds. However, there is a need for more details and more examples of well- preserved fossils.