The specialisation of the third metacarpal and hand in arboreal frogs: Adaptation for arboreal habitat?

Abstract

Skeletal morphology is directly associated with habitat characteristics. To investigate the arboreal adaptation, we studied the osteological morphology of the forelimbs and the third metacarpals of arboreal frogs (Rhacophoridae and Hylidae) and non-arboreal frogs (Bombinatoridae, Bufonidae, Megophryidae, Ranidae, and Microhylidae) using transparent specimens or X-rays of skeletons. Our results revealed that the bony knob on the third metacarpal, which formed by a dilated and elongated lateral articular cartilage (AL) through endochondral ossification, occurred only in species of Rhacophorinae. The results of the phylogenetic comparative methods and correlation analysis strongly supported the conclusion that the bony knob is a phylogenetic independent evolution trait and had a significant correlation with the arboreal habitat. Furthermore, anatomical observation showed that a muscle adhered to the bony knob. Therefore, we speculated that the bony knob might act as an enlarged attachment point for larger or more musculatures to help with grasping. In addition, the relative length of the hand showed a significant difference between arboreal and non-arboreal species (p = .007), suggesting that longer hands might be an arboreal adaptive trait. Overall, this study leads to a deeper understanding of the arboreal adaptation. K E Y W O R D Sarboreal adaptation, bony knob, character evolution, comparative osteology, tree frog

Sesamoids in tetrapods: the origin of new skeletal morphologies

“…The secondary endochondral ossification centre in the bony knobs of Gracixalus gracilipes (see fig. in Dang et al, ) strongly support their origin as attached sesamoids (see Parsons, ).…”

“…Sesamoids‐to‐epiphyses transformation has reported to be bidirectional (arrows 1 and 2; Bernays, ; Brunner, ; Parsons, ; Pearson & Davin, ; Vaughan & France, ; Offenbecker & Case, ; Eyal et al, ). Changes from epiphyses to apophyses (arrow 3; Parsons, ; Dang et al, ) and from sesamoids to apophyses (arrow 4; Koyama et al, ) have also been reported.…”

“…a in Dang et al . (). (B) Right hand of Liolaemus chaltin in dorsal view (MCN 2223; Squamata: Liolaemidae) highlighting the distal epiphyseal modifications of metacarpals II, III and IV, which resemble the anuran bony knob shown in A. MCN: Museo de Ciencias Naturales de la Universidad Nacional de Salta, Salta, Argentina.…”

“…The presence of sesamoids in anurans is likely to be due to genetic factors, with epigenetic stimuli driving their differentiation (Le Minor, ; Felisbino & Carvalho, ; Olson, ; Kim et al, ; Ponssa et al, ; Abdala & Ponssa, ). In terms of our dynamic model, the bony knobs of the third metacarpal in species of Rhacophorinae (Dang et al, ) could be an example of a skeletal structure or sesamoid attached to a limb long bone. Identifying this bony knob during the process of detaching from a long bone epiphysis would provide supporting evidence for our hypothesis.…”

“…Certain iguanians such as Liolaemus chaltin exhibit a distal epiphyseal modification (Fig. ) that resembles the bony knob of the third metacarpal of anuran racophorids (Dang et al, ).…”