Date of Award


Document Type


Degree Name

Master of Science (MS)


Biological Sciences

Committee Chair/Advisor

Susan Chapman

Committee Member

Julia George

Committee Member

Emily Rosowski


Avian dinosaurs evolved skeletal adaptations resulting in numerous fused spinal structures, like the synsacrum, present in extant birds. The synsacrum is a highly fused region necessary for stiffening the spine, and it fuses the spine to the pelvic ilium. Conventional wisdom suggests that fusion occurred in ovo during embryonic development via the cartilage matrix that patterns the future vertebrae. We show that post hatch birds have intervertebral discs separating all vertebrae of the spine, leading us to re-examine this notion in chickens up to 10 weeks of age. Intervertebral discs undergo constant remodeling, then degenerate and undergo tissue clearance, followed by subsequent ossification of adjacent vertebrae resulting in fusion by five months of age. Surprisingly, we discovered that the vertebral formula for the chicken synsacrum comprises 15 vertebrae: the final thoracic, all four lumbar, four sacral and six sacralized caudal vertebrae. The six sacralized caudal vertebrae are in part, or wholly derived from tailbud mesenchyme. In contrast to prevailing wisdom, the distal three sacralized caudal intervertebral discs all contain a nucleus pulposus, similar to our discovery of the same in the caudal tail discs. These novel findings extend our understanding of avian spinal fusion, and support comparative studies of taxa to discover universal and species-specific mechanisms of vertebral fusion.



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