Clade-specific drivers of rapid vertebral evolution in squamate reptiles

In vertebrates, larger body length can evolve through two mechanisms: the addition of vertebrae (pleomerism) versus the enlargement of vertebrae (proportional change). These two processes have significant but distinct functional consequences for locomotion and stem from different developmental mechanisms. Squamate reptiles (lizards and snakes) show tremendous variation in precloacal vertebral number, ranging from 14 to nearly 400 vertebrae. By compiling vertebral counts for 2,357 squamate species from radiographs of museum specimens and supplementing with data from primary literature records, we tested how body size and habitat may influence variation in vertebral number across clades. We found that much of the variation in vertebral number reflects phylogenetic relatedness, likely due to other conserved causal variables or "macroevolutionary drift". Size and habitat also explained a small proportion of variance in elongate clades. Vertebral number in amphisbaenians and blind snakes differed significantly from patterns observed in other elongate clades, showing little influence of body size or phylogenetic history. Their extreme fossoriality may explain the decoupling between vertebrae and body size in these groups. Taken together, our results indicate a surprising complexity to vertebral evolution and we find little evidence that any single factor affects vertebral variation consistently across all squamates.