Delimiting the boundaries of sesamoid identities under the network theory framework

Abstract

Sesamoid identity has long been the focus of debate, and how they are linked to other elements of the skeleton has often been considered relevant to their definition. A driving hypothesis of our work was that sesamoids’ nature relies deeply on their connections, and thus we propose an explicit network framework to investigate this subject in Leptodactylus latinasus (Anura: Leptodactylidae). Through the dissection of L. latinasus’ skeleton, we modeled its anatomical network where skeletal elements were considered nodes while joints, muscles, tendons, and aponeurosis were considered links. The skeletal elements were categorized into canonical skeletal pieces, embedded sesamoids, and glide sesamoids. We inquired about the general network characterization and we have explored further into sesamoid connectivity behavior. We found that the network is structured in a modular hierarchical organization, with five modules on the first level and two modules on the second one. The modules reflect a functional, rather than a topological proximity clustering of the skeleton. The 25 sesamoid pieces are members of four of the first-level modules. Node parameters (centrality indicators) showed that: (i) sesamoids are, in general terms, peripheral elements of the skeleton, loosely connected to the canonical bone structures; (ii) embedded sesamoids are not significantly distinguishable from canonical skeletal elements; and (iii) glide sesamoids exhibit the lowest centrality values and strongly differ from both canonical skeletal elements and embedded sesamoids. The loose connectivity pattern of sesamoids, especially glides, could be related to their evolvability, which in turn seems to be reflected in their morphological variation and facultative expression. Based on the connectivity differences among skeletal categories found in our study, an open question remains: can embedded and glide sesamoids be defined under the same criteria? This study presents a new approach to the study of sesamoid identity and to the knowledge of their morphological evolution.

An overview of the osseous palmar sesamoid in Anura, with the particular case of someRhinellaspecies

“… Fontanarrosa, Fratani & Vera (2020) assigned the palmar sesamoid to the pectoral—forelimb module, formed by the coracoid elements and the forelimb, showing the high intermediacy of this sesamoid, comparable to that of the canonical bones. This high value of this network parameter suggests a functional significance, probably related to the requirements of the strongly terrestrial and burrowing mode of life.…”

“…Sesamoids have been considered structures that do not belong to the tetrapod body plan ( Le Minor, 1987 ; Giori, Beaupré & Carter, 1993 ; Carter et al, 1998 ; Carter, Mikic & Padian, 1998 ; Sarin et al, 1999 ). They are elements that appear separated from the limb’s skeleton, usually associated with or embedded in a collagenous structure, such as ligaments or tendons ( Vickaryous & Olson, 2007 ; Regnault et al, 2016 ; Abdala et al, 2019 ).…”

Sesamoids in Caudata and Gymnophiona (Lissamphibia): absences and evidence

“…Further research is needed to find out whether fibrocartilaginous sesamoids are absent in these groups as well. Sesamoids are identified “in relation to” other anatomical elements, such as ligaments, tendons, and joints ( Fontanarrosa, Fratani & Vera, 2020 ). In disarticulated fossil remains sesamoids are challenging to identify, which might explain their report in association with postcranial joints in only two fossils of the anuran Xenopus ( Henrici & Baéz, 2001 ; Wang, Jones & Evans, 2007 ).…”

Peer Review #1 of "Sesamoids in Caudata and Gymnophiona (Lissamphibia): absences and evidence (v0.1)"

“…Further research is needed to find out whether fibrocartilaginous sesamoids are absent in these groups as well. Sesamoids are identified "in relation to" other anatomical elements, such as ligaments, tendons, and joints (Fontanarrosa et al, 2020). In disarticulated fossil remains, sesamoids are challenging to identify, which might explain their report in association with postcranial joints in only two fossils of the anuran…”