Intravertebral vs. intervertebral integration and modularity in the vertebral column of mammalian carnivorans

The vertebral column is a multicomponent structure whose organization results from developmental and functional demands. According to their distinct somitic origins, individual vertebrae exhibit intravertebral modularity between the centrum and neural spine. However, vertebrae are also organized into larger units called intervertebral modules that result from integration between adjacent vertebrae due to locomotory demands or from common developmental origins due to resegmentation. A previous hypothesis suggested that the boundaries of intervertebral modules coincide with changes in the patterns of intravertebral integration. Here, we explicitly test whether the patterns of modularity and integration between the centrum and neural spine (i.e., intravertebral) in the boundary vertebrae among previously defined intervertebral modules change with respect to those in the vertebrae within intervertebral modules. We quantified intravertebral modularity patterns and quantified the strength of intravertebral integration for each vertebra of the presacral region in 41 species of carnivoran mammals using 3D geometric morphometrics. Our results demonstrate a significant intravertebral modular signal between the centrum and neural spine in all post-cervical vertebrae, including the boundary vertebrae among intervertebral modules. However, the strength of intravertebral integration decreases at the boundary vertebrae. We also found a significant correlation between the degree of intravertebral integration and intervertebral integration. Following our results, we hypothesize that natural selection does not override the integration between the centrum and neural spine at the boundary vertebrae, a pattern that should be influenced by their distinct somitic origins and separate ossification centers during early development. However, natural selection has probably influenced (albeit indirectly) the integration between the centrum and neural spine in the vertebrae that compose the intervertebral modules.