Morphology and Evolution of Sesamoid Elements in Bats (Mammalia: Chiroptera)

Morphology of the stifle in agouti (Dasyprocta prymnolopha, Wagler 1831)

The agouti (Dasyprocta prymnolopha, Wagler 1831) is a wild rodent of great zootechnical potential, a fact that enables anatomical and morphological studies to support management actions with this animal. In this perspective, this study aimed to describe the anatomy and histology of the agouti stifle joint. Four adult agoutis were used, two females and two males. The animals were submitted to dissection and identification of the structures of the stifle joint. For light microscopy study, samples of the patellar ligament, cranial and caudal cruciate ligaments, medial and lateral collateral ligaments were used. Agouti has a highly congruent patellofemoral joint; elongated patella; medial and lateral fabellae at the proximal insertion of the gastrocnemius muscle; medial and lateral meniscus with lunula; in addition to the presence of the following ligament structures: patellar ligament, cranial and caudal cruciate ligaments, medial and lateral collateral ligaments, meniscofemoral ligament, caudal meniscal ligament of the medial meniscus, and medial and lateral cranial ligaments. The patellar ligament presents bundles of parallel collagen fibers with a straight path and coated fibroblasts; collateral and cruciate ligaments had loose and dense connective tissue, coated fibroblasts and collagen bundle undulations, the latter most expressive in the caudal cruciate ligament. Thus, except for the shape and angulation of the stifle, which allows specific movements, the agouti stifle has structures analogous to that of other rodents and domestic animals.

The sesamoid bone in the long abductor muscle tendon of the first digit in the dog

The sesamoid bone in the tendon of the m. abductor digiti primi longus is considered present in most dog breeds and is described to be radiologically detectable at the level of the carpus from the age of 4 months. However, an extensive investigation of this sesamoid bone has not been conducted before. The aim of this study was therefore to determine its prevalence in different dog breeds, to describe its histological development, and to determine the age at which it becomes radiologically visible. The prevalence of the sesamoid bone was assessed on radiographic images of the carpus or by dissection of the carpal region in 743 adult dogs of 115 breeds. Its development was studied by dissection and histological analysis in 45 puppies and its timing of radiological appearance was evaluated in 209 puppies. At least one sesamoid bone was present in all adult dogs, except for 14 dogs of six breeds of predominantly the small breed category. The lowest prevalence rate of 38.46% was exhibited in the French bulldog. The histological development could be divided into five stages. The first radiographic appearance corresponded to the coalescence of smaller ossification centers into one big nucleus (stage 4). The mean time of radiographic appearance was 108.4 days. This study provides extensive data on the prevalence and timing of the radiographic appearance of a sesamoid at the carpus of the dog. The data on radiographic appearance may be helpful in the age estimation of puppies.

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

An integrative definition of sesamoid bones has been recently proposed, highlighting their relationship with tendons and ligaments, their genetic origin, the influence of epigenetic stimuli on their development, and their variable tissue composition. Sesamoid bones occur mainly associated with a large number of mobile joints in vertebrates, most commonly in the postcranium. Here, we present a survey of the distribution pattern of sesamoids in 256 taxa of Caudata and Gymnophiona and 24 taxa of temnospondyls and lepospondyls, based on dissections, high-resolution X-ray computed tomography from digital databases and literature data. These groups have a pivotal role in the interpretation of the evolution of sesamoids in Lissamphibia and tetrapods in general. Our main goals were: (1) to contribute to the knowledge of the comparative anatomy of sesamoids in Lissamphibia; (2) to assess the evolutionary history of selected sesamoids. We formally studied the evolution of the observed sesamoids by optimizing them in the most accepted phylogeny of the group. We identified only three bony or cartilaginous sesamoids in Caudata: the mandibular sesamoid, which is adjacent to the jaw articulation; one located on the mandibular symphysis; and one located in the posterior end of the maxilla. We did not observe any cartilaginous or osseous sesamoid in Gymnophiona. Mapping analyses of the sesamoid dataset of urodeles onto the phylogeny revealed that the very conspicuous sesamoid in the mandibular symphysis of Necturus beyeri and Amphiuma tridactylum is an independent acquisition of these taxa. On the contrary, the sesamoid located between the maxilla and the lower jaw is a new synapomorphy that supports the node of Hydromantes platycephalus and Karsenia coreana. The absence of a mandibular sesamoid is plesiomorphic to Caudata, whereas it is convergent in seven different families. The absence of postcranial sesamoids in salamanders might reveal a paedomorphic pattern that would be visible in their limb joints.

Morphological peculiarities in the integument of enigmatic anomalurid gliders (Anomaluridae, Rodentia)

Scaly-tailed squirrels, the most poorly known group of gliding mammals, hold the record for variety of remarkable integument peculiarities. One of the most striking of these features is the scales on the tail, which apparently allow them to reduce energy costs when positioning themselves on a tree trunk. No less interesting is a peculiar spur that supports the flying membrane: the unciform element ('spur'). Despite the peculiarity of such elements, their nature has not yet been studied. Using anatomical, histological methods and scanning electron microscopy we studied the structure of the skin and its derivatives in five of the six species from both genera of extant gliding scaly-tailed squirrels (Anomaluridae, Rodentia): Idiurus macrotis, Idiurus zenkeri, Anomalurus beecrofti, Anomalurus pusillus and Anomalurus derbianus. In addition to the common mammalian skin structures, such as hair, vibrissae, sebaceous glands, meibomian glands of eyelids and eccrine sweat glands of the palmar and plantar pads, these animals have unique species-specific skin derivatives (the tail scaly organ and its specific glands, vibrissae of the withers, patagium and its hair brush) that play a significant role in their adaptation to gliding and to their environment in general. The structure of the elbow spur is also described and hypotheses on its evolutionary origin from the tendon of the triceps muscle are presented.

Functional morphology and identity of the thenar pad in the subterranean genus Ctenomys (Rodentia, Caviomorpha)

As in many other fossorial tetrapods, the most obvious adaptations to scratch-digging in the subterranean tuco-tuco (Rodentia, Ctenomyidae, Ctenomys) are found in the hands, which among other adaptations, present the mesaxonic condition; i.e. the central digits are more developed, and also their claws, which are curved and elongated. The thumb is atrophied and aligned with the rest of the digits, showing a small and flat claw. This configuration of digits and claws seems to be in accordance with what it is expected for rodents: rudimentary movements when handling food items. However, on the palmar side of the hand, tuco-tucos have several pads, the thenar (located under the thumb) being the most developed. In this study, we investigated the functional morphology of the thenar pad through different approaches: musculoskeletal anatomy, histology and functionality. The analysis of radiographs and clarified and double-stained hand samples of Ctenomys talarum and C. australis showed that the thenar pad is supported by a paddle-shaped bone that articulates with a protrusion in the scapholunate bone. This bone, flat and long, continues in a flat cartilaginous structure, with a shape similar to a claw. Dissections showed that the thenar pad has several associated muscles: the m. palmaris longus, the m. abductor pollicis longus, and a massive muscular complex located between the thumb and the thenar pad. By topology it might be inferred that this complex is formed by the m. abductor pollicis brevis, the m. flexor pollicis brevis and the m. adductor pollicis brevis. Longitudinal histological sections of the thenar pad stained with hematoxylin-eosin showed a thick layer of keratin at the distal end, external face. The observation of live specimens of C. talarum foraging on two food items of different size and filmed at 300 fps showed that the thenar pad acts as an opposable thumb, with digit-like movements. Tuco-tucos are able to perform more precise movements than expected, and to grasp and manipulate the food with one hand. In previous studies, it was suggested that the thenar pad was supported by a 'palmar ossicle', or 'prepollex' (= radial sesamoid bone). Our results suggest that this sesamoid underwent a radical change on its morphology, making the thenar pad a part of the food handling system in Ctenomys, so the thenar pad might be considered a 'false thumb', rather than a palmar pad. It is suggested to advance on the description and functional analysis of the thenar pad, redefining the structure, since the terms used so far to define it would not be accurate.

Sesamoids in tetrapods: the origin of new skeletal morphologies

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.

The Role of Developmental Integration and Historical Contingency in the Origin and Evolution of Cypriniform Trophic Novelties

While functional morphologists have long studied the evolution of anatomical structures, the origin of morphological novelties has received less attention. When such novelties first originate they must become incorporated into an integrated system to be rendered fully functional. Thus, developmental integration is key at the origin of morphological novelties. However, given enough evolutionary time such integration may be broken, allowing for a division of labor that is facilitated by subsequent decoupling of structures. Cypriniformes represent a diverse group of freshwater fishes characterized by several trophic novelties that include: kinethmoid-mediated premaxillary protrusion, a muscular palatal and post-lingual organ, hypertrophied lower pharyngeal jaws that masticate against the base of the neurocranium, novel pharyngeal musculature controlling movement of the hypertrophied lower pharyngeal jaws, and in a few species an incredibly complex epibranchial organ used to aggregate filtered phytoplankton. Here, we use the wealth of such trophic novelties in different cypriniform fishes to present case studies in which developmental integration allowed for the origin of morphological innovations. As proposed in case studies 1 and 2 trophic innovations may be associated with both morphological and lineage diversification. Alternatively, case studies 3 and 4 represent a situation where ecological niche was expanded but with no concomitant increase in species diversity.