Intercalary Elements, Treefrogs, and the Early Differentiation of a Complex System in the Neobatrachia

Abstract

Intercalary elements are additional skeletal structures of digits of many anuran amphibians. Twelve terminal clades in the neobatrachian lineage of frogs have intercalary elements revealing it is a homoplastic character with five to seven gains and two to four losses along a consensus phylogeny of the Neobatrachia. We analyzed anatomical variation of intercalary elements, related structures (distal phalanges, tendons, and muscles), and articulations of digits of 45 anuran species, representing eight suprageneric terminal taxa. The intercalary elements are integrated in a complex system that is probably related to different types of movements, which are produced by a similar set of muscles and tendons with limited variation among the studied taxa. Species in the clades Hyloides and Ranoides show distinctive patterns of morphostructural features in their intercalary elements that are usually wedge-shaped and composed of hyaline cartilage in Ranoides, and biconcave and composed of embryonic cartilage in Hyloides. Features derived from the typical hyloid condition may only be interpreted in some Hylidae (Pseudis and Lysapsus) and Centrolenidae. In Ranoides, the described features of the intercalary elements are found in all taxa examined with the exception of Leptopelis, which have an intercalary element similar to the other Ranoides but formed by connective tissue. Several features are shared by all taxa having intercalary elements: (1) the intercalary elements differ from the phalanges by lacking terminal epiphyses, (2) they are present in hands and feet, and (3) they appear in all digits. This finding suggests that the genetic basis for presence of intercalary elements may be homologous in all these taxa and may have evolved only once early in neobatrachian history. Anat Rec,

Heterochrony during skeletal development of Pseudis platensis (anura, hylidae) and the early offset of skeleton development and growth

“…6. As was described by Manzano et al (2007) the intercalary elements of species of Pseudis are different from all Hyloides in: a) shape and function, in being cylindrical with plane articular surfaces that allow limited movements; and b) histological composition, as they are formed by hyaline cartilage. Both shape and histological structure suggest that the intercalary of Pseudis are peramorphic features.…”

“…The resulting embryonic cartilage remains during the whole ontogeny of Hyloides. It may be mineralized in some species but never acquires the proteoglycan matrix typical of the hyaline cartilage of P. platensis and Ranoides (Manzano et al, 2007). In respect to Hyloides, the intercalary elements of P. platensis show an accelerated developmental rate that defines early its elongated shape and the proteoglycan matrix of hyaline cartilage.…”

“…The species of Pseudis possess elongated intercalary elements, belong to Hylidae, the most diverse group of neotropical treefrogs, and are included in the clade Dendropsophini (Faivovich et al, 2005). The intercalary elements are formed by hyaline cartilage, differing from most hylodine anurans (Manzano et al, 2007). Pseudis displays complete webbed feet with a distinct paddle-like morphology, a consequence of isochronic development of Toe IV with respect to other digits.…”

The effect of substrate diameter and incline on locomotion in an arboreal frog

“…Moreover, arboreal frogs are extremely diverse and have radiated independently in many different lineages (Frost et al, 2006). Among arboreal frogs many ecological specialists exist, with some species of tree frogs being fine branch specialists and showing a walking-type gait rather than jumping from branch to branch (Manzano et al, 2007;Manzano et al, 2008). Species of the genus Phyllomedusa are of special interest given their known ability to move across branches of narrow diameter and the fine motor control they appear to have over their fingers, allowing them to perform a precision grip (Burton, 1998;Herrel et al, 2008).…”

The Systematic Status of Rhacophorid Frogs

“…The species are united morphologically in possessing intercalary elements between the two distal phalanges and expanded digit discs. These characters are shared with hylids, mantellids, and some ranid lineages (Frost et al, 2006;Liem, 1970;Manzano et al, 2007). Toe pads help in climbing due to their ectodermal morphology and secreted proteins which aid in their adherence to surfaces (Duellman and Trueb, 1986).…”