Structural and environmental constraints on reduction of paired appendages among vertebrates

Abstract

Burrowing habits or complex environments have generally been considered as potential drivers acting on reduction and loss of the appendicular skeleton among vertebrates. Herein, we suggest that this might be the case for lissamphibians and squamates, but that fin loss in fishes is usually prevented by important structural constraints, because pectoral fins are commonly used to control rolling and pitching. We provide an overview of the distribution of paired appendage reduction across vertebrates while examining the ecological affinities of finless and limbless clades. We analysed the correlation between lifestyle and fin or limb loss using the discrete comparative analysis. The resulting Bayesian factors indicate strong evidence of correlation between: (1) pectoral-fin loss and coexistence of anguilliform elongation and burrowing habits or complex habitat in teleost fishes; and (2) limb loss and a burrowing or grass-swimming lifestyle in squamate reptiles and lissamphibians. These correlations suggest that a complex environment or a fossorial habit is a driving force leading to appendage loss. The only style of locomotion that is functional even in the absence of paired appendages is the undulatory one, which is typical of all elongated reptiles and lissamphibians, but certainly less common in teleost fishes.

A farewell to arms and legs: a review of limb reduction in squamates

“…These conclusions have been validated by recent studies (e.g. Da Silva et al, 2018;Camaiti et al, 2019;Macaluso et al, 2019) linking the evolution of limb loss to the adaptation to complex three-dimensional environments such as underground burrows and crevices. Whereas these studies have been instrumental in incorporating ecological and functional constraints as main factors in the acquisition of serpentiform body shapes, they have been disproportionately focused on their implications with regard to the evolution of limbless squamates instead of intermediate limb-reduced body shapes.…”