An overview of the osseous palmar sesamoid in Anura, with the particular case of some Rhinella species

MANZANO A.S.; ABDALA V.

Peer J

SAGE Publications, O?Reilly Media, Inc.

Año: 2023

2167-8359

Background. Sesamoids are generally regarded as structures that are not part of thetetrapod body plan. The presence of a palmar sesamoid is assumed to serve as adistribution point for the forces of the flexor digitorum communis muscle to the flexortendons of the digits, which are embedded in the flexor plate. It has been consideredthat the palmar sesamoid is present in most anuran groups, and it has been suggestedthat it acts by inhibiting the closing of the palm, preventing grasping. Typical arborealanuran groups lack a palmar sesamoid and flexor plate, a pattern shared with othertetrapod groups, which can retain a reduced sesamoid and flexor plate. We focus onthe anatomical structure of the Rhinella group, which includes species that present anosseous palmar sesamoid and climb bushes or trees to avoid depredation or escapedangerous situations and can exhibit scansorial and arboreal behaviors. We also adddata on the bony sesamoids of 170 anuran species to study the anatomy and evolution ofthe osseous palmar sesamoid within this amphibian group. Our objective is to bring anoverview of the osseous palmar sesamoid in anurans, unveiling the relationship betweenthis element of the manus, its phylogeny, and the anuran habitat use.Methods. Skeletal whole-mount specimens of Rhinella were cleared and double-dyed todescribe the sesamoid anatomy and related tissues. We review and describe the palmarsesamoid of 170 anuran species from CT images downloaded from Morphosource.org,representing almost all Anuran families. We performed a standard ancestral statereconstruction by optimizing two selected characters (osseous palmar sesamoidpresence, distal carpal palmar surface) along with the habitat use of the sampled taxa,using parsimony with Mesquite 3.7.Results. Our primary finding is that sesamoid optimization in the anuran phylogenyrevealed that its presence is associated with certain clades and is not as widespread aspreviously anticipated. Additionally, we will also be delving into other important outcomes of our study that are relevant to those working in the field of anuran sesamoids. The osseous palmar sesamoid is present in the clade Bufonidae Dendrobatidae-Leptodactylidae-Brachicephalidae which we named as PS clade, and also in the archeobatrachian pelobatoid Leptobranchium, all strongly terrestrial and burrowing species, though with exceptions. The osseous palmar sesamoid is always present in Bufonidae, but varies in form and size, depending on the mode that they use their manus, such as in the Rhinella margaritifera which has a cylindrical one and also grasping abilities that involve closing the manus. The scattered presence of the bony palmar sesamoid among anuran clades raises the question of whether this sesamoid can be present with a different tissular composition in other groups.