Different drivers of diversification for body elongation and limb reduction in convergently snake-like lizards

Convergence is the evolution of similar phenotypes often due to similar selective pressures or constraints limiting evolutionary options. Snake-like morphologies, characterized by elongated bodies and reduced limbs, have evolved repeatedly among vertebrates, including numerous times in squamate reptiles (lizards and snakes). It has been suggested that elongation facilitates locomotion through substrates while limb reduction typically occurs in clade-specific patterns, but this has not been tested. We compared the fit of a series of habitat- and clade-specific models for the evolution of digits, phalanges. and trunk vertebrae in lizards. We found that species inhabiting fossorial and cluttered habitats differed in numbers of vertebrae, digits, and phalanges from species in other habitats. A model with habitat-specific rates fit best for vertebral evolution, with sand swimmers, litter dwellers, and burrowers having higher rates of vertebral evolution than non-fossorial taxa. However, we found digits and phalanges evolved in a clade-specific manner, with higher rates of limb evolution in certain clades. This suggests that limb reduction in snake-like lizards is dictated by clade-specific constraints. In contrast, fossoriality appears to relax functional constraints on vertebral number, facilitating body form diversification. These results suggest that the relaxation of constraints may be an additional mechanism for convergent evolution.