Synsacrum

File:Synsacrum.PNG

File:UnidentifizierterTierschaedel.jpg

The synsacrum is a skeletal structure of birds and other dinosaurs,{{Cite journal |last=Wedel |first=Mathew J. |date=June 2003 |title=Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs |url=https://www.cambridge.org/core/product/identifier/S0094837300018091/type/journal_article |journal=Paleobiology |language=en |volume=29 |issue=2 |pages=243–255 |doi=10.1666/0094-8373(2003)029<0243:VPASAT>2.0.CO;2 |bibcode=2003Pbio...29..243W |issn=0094-8373}} pterosaurs,{{Cite journal |last=Hyder |first=Elaine |date=2012 |title=Evolution of the pterosaur pelvis |url=https://doi.org/10.4202/app.2011.1109 |journal=Acta Palaeontologica Polonica |doi=10.4202/app.2011.1109 |issn=0567-7920|doi-access=free }} as well as xenarthran mammals,{{Cite journal |last1=Galliari |first1=Fernando Carlos |last2=Carlini |first2=Alfredo Armando |date=December 2019 |title=Xenarthran Synsacrum Morphology and Evolution |url=http://link.springer.com/10.1007/s10914-018-9442-0 |journal=Journal of Mammalian Evolution |language=en |volume=26 |issue=4 |pages=493–504 |doi=10.1007/s10914-018-9442-0 |issn=1064-7554|url-access=subscription }} in which the sacrum is extended by incorporation of additional fused or partially fused caudal or lumbar vertebrae. Some posterior thoracic vertebrae, the lumbar, sacral and a few anterior caudal vertebrae are fused to form a complex bone called synsacrum. In birds, inate bones are fused with the synsacrum to a greater or lesser extent, according to species, forming an avian pelvis. This forms a more extensive rigid structure than the pelvis of a mammal, fulfilling requirements for flight, locomotion and respiration. Posterior to the bird synsacrum there are a few free caudal vertebrae, the last of which is the pygostyle to which the long, stiff tail feathers are attached. The central section of the synsacrum is swollen to accommodate the glycogen body, an organ whose function is as yet unclear but which may be associated with balance. {{cite journal | last1=Stanchak | first1=K E | last2=French | first2=C | last3=Perkel | first3=D J | last4=Brunton | first4=B W | title=The Balance Hypothesis for the Avian Lumbosacral Organ and an Exploration of Its Morphological Variation | journal=Integrative Organismal Biology | publisher=Oxford University Press (OUP) | volume=2 | issue=1 | date=2020-01-01 | pages=obaa024 | issn=2517-4843 | doi=10.1093/iob/obaa024 | pmid=33791565 | pmc=7751001 }}

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