Mammalian spinal evolution was a simple transition from reptile-like side-to-side bending in early synapsids to mammal-like up-and-down bending.

A common account of mammalian evolution described a simple change in how the spine moved. Early synapsids, which often had sprawling limbs, were thought to bend their backs from side to side like living lizards. Mammals later evolved more up-and-down, or sagittal, flexion.

That story depended heavily on using modern reptiles as analogues for extinct synapsids. In 2021, researchers compared the three-dimensional shapes of fossil synapsid vertebrae with those of living reptiles and mammals.

The fossils did not fit a smooth reptile-to-mammal progression. Early synapsid vertebrae appear to have been relatively stiff, rather than strongly adapted for lizard-like lateral bending. Later mammalian evolution added different kinds of movement in different parts of the spine, including sagittal flexion, twisting and stronger regional specialization.

A 2024 follow-up used digital bending experiments and reached a similar result: the ancestral synapsid spine was comparatively stiff, and mammalian spinal function developed through several regional changes rather than one wholesale switch.

The endpoints were not the main problem with the older model. Mammals did evolve distinctive spinal movements. The mistake was assuming that their ancestors moved like living lizards and that one type of bending simply gave way to another.