The spine of the first mammals was unique and developed along its own path, in contrast to early theories.
A group of researchers led by Harvard University used advanced techniques to find out that non-mammalian synapsids moved the spine differently than any other animal.
The authors of the work found that although the degree of sagittal bending did increase during the evolution of mammals, the dorsal bones of the earliest synapsids were optimized for rigid attachment, and the evolutionary transition to mammals did not include the stage characterized by lateral bending of reptiles.
Instead, they found that modern lizards and other reptiles have unique spinal morphologies and functions that do not represent ancestral locomotion. And also the fact that the very ancestors of mammals did not move like lizards, as scientists previously believed.
There is a long-standing idea that the evolution of mammals took place directly from the lateral to the sagittal bend, but it is too simple. Lizards and mammals separated from each other millions of years ago, and each of them embarked on their own evolutionary journey.
Stephanie Pearce, Associate Professor in the Department of Organism and Evolutionary Biology of Thomas D. Cabot
The authors first measured the shape of the vertebrae of some reptiles, mammals, salamanders, and some non-mammalian synapsid fossils. Samples came from museum collections around the world, including modern skeletons.
The researchers then examined how the vertebrae might function: They used data from their previous work, which compared the shape of the vertebrae to the degree of movement of the vertebrae in living lizards and mammals.
The findings allowed the researchers to map the variation in vertebral function across a large sample of animals, including fossils, and reconstructed the exact combination of functional traits that describe each group of animals.