Hindlimb and pelvic girdle reduction with associated skeletal changes in the South African limb-reduced skink Scelotes Fitzinger, 1826

Abstract

We used stained specimens to describe hindlimb and pelvic girdle reduction in a skeletal morphocline of Scelotes species, a South African scincine group, as well as how the two processes are interrelated. The findings reflect the generally varied nature of the evolutionary pathways to a limbless serpentine form among lacertilians, which presumably reflect the interplay between selective and developmental processes. The relationship between hindlimb and pelvic girdle degeneration in terms of onset, process concurrency and patterns was clarified but its applicability to lacertilians in general will have to be verified. Substantial increases in presacral vertebra counts were responsible for the increase in body length of the species. Our phylogenetic analyses suggest that Scelotes most likely descended from a limbless ancestor, with extant digit-bearing species reflecting secondary gains and subsequent losses. Forelimb and hindlimb evolution were decoupled: forelimb change was concentrated among few states (0, 3, 5) with dominant 0↔3 shifts, while hindlimbs were more labile and clustered into intermediate (3), low (1–2) and extreme (0 or 5) states, implying stepwise, multi-pathway transitions driven by developmental modularity, selective (fossorial) pressures, and constraints. Further confirmation of these findings is required by investigating the evolutionary process in the remainder of the Scelotes species, and in lacertilians in general.