Different selection regimes explain morphological evolution in fossorial lizards

Independent origins of similar phenotypes are ubiquitous to the
evolutionary process and evoke strong and recurrent environmental
associations. Snakelike lizards evolved multiple times and are often
portrayed as limb-reduced and body-elongated outcomes from shared
selection associated with fossoriality. However, a refined
evaluation including specific head traits and subtle differences in
subterranean microhabitats unveils some degree of uniqueness even among
lineages traditionally interpreted as phenotypically similar. Here we
address regimes of selection in fossorial lizards accounting for
differences in the burrowing substrate and emphasizing head shape in
addition to body and limbs. We assembled an ecomorphological
database comprising 213 species from all major lizard clades, and then
characterized contemporary morphological diversity and modeled phenotypic
evolution to test the hypothesis that fossoriality encompasses at least
two distinct selection regimes. We identified two
ecomorphological groups within the fossorial lizards: moist-soil fossorial
and dry-soil fossorial. Both groups evolved towards distinct adaptive
optima concerning head shape and limb size. Despite some degree of
uniqueness, these groups also share similar patterns in specific traits.
Dry-soil fossorial lizards present less morphological variation than
moist-soil fossorial, possibly due to the combination of distinct sets of
selective pressures with shared ancestry. Our study provides
evidence that an often-interpreted general adaptive regime (e.g.,
fossoriality) may in fact comprise enough ecological and functional
diversity to elicit several distinct ecomorphological associations despite
overall convergence among phenotypic traits.