"Without a leg to stand on": on the evolution and development of axial elongation and limblessness in tetrapods

Abstract

A review of the osteology of the axial and appendicular skeleton of fossil and extant tetrapods, in the context of tetrapod phylogenetic patterns, reveals common patterns of limb loss and axial elongation. A threshold number of 35–40 presacral vertebrae is linked to minor reductions in digit number and the phalangeal count. Presacral vertebral counts do not increase gradually, rather, presacral counts jump from 35–40 to 60–70. At this point, limb loss is extreme, with forelimbs being reduced to tiny appendages or lost altogether. Higher presacral counts (>90) are linked to total forelimb loss and radical rear-limb reductions culminating in total loss. A pattern of this sort is recognized in Paleozoic lepospondyls and Mesozoic to modern squamates. Developmental genetic models illuminate gene systems that are associated with morphogenesis and are linked to the evolution of limb reduction and leglessness in these tetrapods.