Morphological Variation in Anuran Limbs: Constraints and Novelties

Abstract

Anurans have three primary types of locomotion: walking, jumping, and swimming. Additionally, they may dig, climb, grasp, etc. All adult anurans have four limbs, with four fingers on the hands and five toes on the feet. We summarized and updated knowledge on the interspecific variation within anuran limbs, then discuss how developmental constraints (e.g., in size) and novelties may have influenced anuran diversification through the locomotion. We analyze morphological variation from limb bud stages up to the final limb form resulting from certain skeletal organization and growth. We find limited morphometric variations in the skeleton of different developmental modules (i.e., skull, trunk, urostyle, limbs) indicate that the anuran body shape is largely constrained. We identify specializations of the stylopodium, zeugopodium, and proximal carpals/tarsals that have evolved to facilitiate saltatorial locomotion. We show that the anuran prepollex and prehallux are not vestigial digits and that they have come to serve specialized function. Medial rotation of the manus in anurans appears to have evolved to help distribute the force of impact upon landing at the end of a jump. Additional skeletal elements in anuran limbs are intercalary elements and sesamoids. The intercalary elements appear within neobatrachians and are integrated with digital pads in lineages capable of locomotion on smooth vertical surfaces. They have allowed arboreal anurans to occupy a wide range of arboreal habitats.

Turnover or Intraspecific Trait Variation: Explaining Functional Variability in a Neotropical Anuran Metacommunity

“…Ruhi et al, 2014;Heino et al, 2015;Spasojevic et al, 2016), while limited dispersal ability imposed by limb morphology and physiological restrictions (risk of desiccation) compromise long-distance anuran dispersal (Wells, 2010;Amado et al, 2019). Differences in life history, morphology, and locomotion mode directly affect trait-environment relationships and species turnover dominance (Fabrezi et al, 2017), as observed in our study. According to our predictions, distance between ponds mostly influenced variation patterns in traits associated with habitat exploitation in arboreal species -head shape, eye position and, particularly, limb length.…”

Anuran limbs reflect microhabitat and distal, later‐developing bones are more evolutionarily labile*

“…Frog limbs develop in a proximal‐distal direction (Fabrezi et al. 2017), so the increased evolutionary rates we found may indicate that similar developmental constraints influence anuran limb evolution.…”

“…Limb development in frogs also follows a proximal‐distal direction (Fabrezi et al. 2017), but unlike in amniotes, limb formation occurs later and is decoupled from organogenesis in the phylotypic stage, relaxing constraints against changes to the number of distal elements (Galis et al. 2001; Irie and Sehara‐Fujisawa 2007).…”

Sesamoids in tetrapods: the origin of new skeletal morphologies

“…These structures have been studied using several approaches in anurans: descriptive (e.g. Gaupp, ; Parsons, ; Devanesen, ; Slater, ; Emerson, ; Nussbaum, ; de Sá & Trueb, ; Trueb et al, ; Hoyos, ; Guayasamin, ; Manzano & Barg, ; Avilan & Hoyos, ; Ponssa, , ; Chaparro, Pramuk & Gluesenkamp, ; Lehr & Trueb, ; Maglia, Púgener & Mueller, ; Manzano et al, ; Soliz & Ponssa, ), ontogenetic (Olson, ; Vickaryous & Olson, ; Ponssa et al, ; Abdala & Ponssa, ; Vera & Ponssa, ; Vera et al, ; Ponssa & Abdala, ; Soliz & Ponssa, ; Abdala et al, ; Fabrezi, Goldberg & Chuliver Pereyra, ), and functional (Emerson, , ; Clarke, ; Olson, ; Manzano & Barg, ). In some studies, characters derived from sesamoids are used in phylogenetic analyses (Laurent, , , ; Nussbaum, ; Cannatella & Trueb, ; Scott, ; Wiens et al, ; Fabrezi, ; Báez, Moura & Gómez, ), or optimized in phylogenies (Olson, ; Ponssa et al, ; Soliz & Ponssa, ).…”