Looking at modern vertebrates, their number of vertebrae varies remarkably. While some snakes have of the order of 300 vertebrae (all thoracic vertebrae), some turtles only have 18 vertebrae in total - from neck to tail. Then, in mammals the number of vertebrae and their functional division is remarkably constant.
In a recent study Johannes Müller (Berlin Natural History Museum) and co-workers surveyed 436 recent and extant amniote taxa to study the development of vertebrae in response to environmental pressure, body mass and genetic factors.
The key findings were:
- Early mammals (synapsids) had six cervical vertebrae and in total twenty thoracic plus lumbar vertebrae.
- The number of vertebrae is determined by somite formation and Hox-gene expression during embrional development. This also determines the position of the limbs along the spinal column.
- Early synapsids already showed the same number of vertebrae as recent mammals, while basal reptiles already showed a great variation in their number of vertebrae.
Müller J., Scheyer T.M., Head J.J., Barrett P., Werneburg I., Ericson P.G.P., Pol D., Sánchez-Villagra M.R: Homeotic effects, somitogenesis and the evolution of vertebral numbers in recent and fossil amniotes. In: Proceedings of the National Academy of Sciences (PNAS), doi: 10.1073/pnas.0912622107
The paper comes with substantial amounts of supporting information. Unfortunately, the direct link from the article "splash page" does not work. This, again, shows the importance of using persistent identifiers like DOI for the publication of supplementary data.