No abstract

Bats are the only mammals capable of powered flight. One of the oldest bats known from a complete skeleton is Onychonycteris finneyi from the Early Eocene (Green River Formation, Wyoming, 52.5 Ma). Estimated to weigh approximately 40 g, Onychonycteris exhibits the most primitive combination of characters thus far known for bats. Here, we reconstructed the aerofoil of the two known specimens, calculated basic aerodynamic variables and compared them with those of extant bats and gliding mammals. Onychonycteris appears in the edges of the morphospace for bats, underscoring the primitive conformation of its flight apparatus. Low aerodynamic efficiency is inferred for this extinct species as compared to any extant bat. When we estimated aerofoil variables in a model of Onychonycteris excluding the handwing, it closely approached the morphospace of extant gliding mammals. Addition of a handwing to the model lacking this structure results in a 2.3-fold increase in aspect ratio and a 28% decrease in wing loading, thus greatly enhancing aerodynamics. In the context of these models, the rapid evolution of the chiropteran handwing via genetically mediated developmental changes appears to have been a key transformation in the hypothesized transition from gliding to flapping in early bats.

No abstract

Along with supernumerary bones, sesamoids, defined as any organized intratendinous/intraligamentous structure, including those composed of fibrocartilage, adjacent to an articulation or joint, have been frequently considered as enigmatic structures associated with the joints of the skeletal system of vertebrates. This review allows us to propose a dynamic model to account for part of skeletal phenotypic diversity: during evolution, sesamoids can become displaced, attaching to and detaching from the long bone epiphyses and diaphysis. Epiphyses, apophyses and detached sesamoids are able to transform into each other, contributing to the phenotypic variability of the tetrapod skeleton. This dynamic model is a new paradigm to delineate the contribution of sesamoids to skeletal diversity. Herein, we first present a historical approach to the study of sesamoids, discussing the genetic versus epigenetic theories of their genesis and growth. Second, we construct a dynamic model. Third, we present a summary of literature on sesamoids of the main groups of tetrapods, including veterinary and human clinical contributions, which are the best-studied aspects of sesamoids in recent decades. Finally, we discuss the identity of certain structures that have been labelled as sesamoids despite insufficient formal testing of homology. We also propose a new definition to help the identification of sesamoids in general. This review is particularly timely, given the recent increasing interest and research activity into the developmental biology and mechanics of sesamoids. With this updated and integrative discussion, we hope to pave the way to improve the understanding of sesamoid biology and evolution.

Bats are atypical small mammals. Size is crucial for bats because it affects most aerodynamic variables and several key echolocation parameters. In turn, scaling relationships of both flight and echolocation have been suggested to constrain bat body size evolution. Previous studies have found a large phylogenetic effect and the inclusion of early Eocene fossil bats contributed to recover idiosyncratic body size change patterns in bats. Here, we test these previous hypotheses of bat body size evolution using a large, comprehensive supermatrix phylogeny (+800 taxa) to optimize body size and examine changes reconstructed along branches. Our analysis provides evidence of rapid stem phyletic nanism, an ancestral value stabilized at 12 g for crown-clade Chiroptera followed by backbone stasis, low-magnitude changes inside established families, and massive body size increase at accelerated rate in pteropodid subclades. Total variation amount explained by pteropodid subclades was 86.3%, with most changes reconstructed as phyletic increases but also apomorphic decreases. We evaluate these macroevolutionary patterns in light of the constraints hypothesis, and in terms of both neutral and adaptive evolutionary models. The reconstructed macroevolution of bat body size led us to propose that echolocation and flight work as successive, nested constraints limiting bat evolution along the body size scale.

No abstract

Sesamoids are skeletal elements found within a tendon or ligament as it passes around a joint or bony prominence. Here we review the distribution of sesamoids in bats, the only mammals capable of powered flight. Our survey included bat species representing most extant families as well as two key Eocene fossil bats in which sesamoids are exquisitely preserved, Onychonycteris finneyi and Icaronycteris index. We identified 46 separate sesamoid elements (or sets of elements) from dissections of selected bat taxa, with no more than 23 of these present in any given species. Among the sesamoids identified in our survey, 12 have not previously been described in bats. We also identified seven sesamoids previously described in the literature that are not present in our sample of species. No sesamoids were found to be exclusive to the fossil taxa in our study; all the sesamoids observed in Onychonycteris and Icaronycteris have apparent homologs among extant species. We mapped the presence/absence of the 46 sesamoids onto a bat phylogeny. Based on these optimizations, we discuss homology issues and evolutionary history of some of the most taxonomically widespread sesamoids. Functional inferences regarding some sesamoids can be made based on what is known about bat musculoskeletal morphology, although further biomechanical studies are required to test the hypotheses proposed here. Sesamoids will continue to be a source of interesting insights about the evolution of bats and their unique locomotor abilities.

Sesamoids are skeletal elements found within a tendon or ligament as it passes around a joint or bony prominence. Here we review the distribution of sesamoids in bats, the only mammals capable of powered flight. Our survey included bat species representing most extant families as well as two key Eocene fossil bats in which sesamoids are exquisitely preserved, Onychonycteris finneyi and Icaronycteris index. We identified 46 separate sesamoid elements (or sets of elements) from dissections of selected bat taxa, with no more than 23 of these present in any given species. Among the sesamoids identified in our survey, 12 have not previously been described in bats. We also identified seven sesamoids previously described in the literature that are not present in our sample of species. No sesamoids were found to be exclusive to the fossil taxa in our study; all the sesamoids observed in Onychonycteris and Icaronycteris have apparent homologs among extant species. We mapped the presence/absence of the 46 sesamoids onto a bat phylogeny. Based on these optimizations, we discuss homology issues and evolutionary history of some of the most taxonomically widespread sesamoids. Functional inferences regarding some sesamoids can be made based on what is known about bat musculoskeletal morphology, although further biomechanical studies are required to test the hypotheses proposed here. Sesamoids will continue to be a source of interesting insights about the evolution of bats and their unique locomotor abilities.