Journal of Anatomy最新文献

Welcome to this special double issue of the Journal of Anatomy dedicated to advances in paleohistology, which is the anatomical study of ancient body tissues. The field of paleohistology has an unexpectedly long history that is comprehensively detailed by de Ricqlès (2021), and its many thin sections and fossils have been inherently linked since its inception. The first thin sections were produced about 210 years ago in 1815, by William Nicol in his studies of fossilized wood, including the “Great Tree” of Craigleith Quarry, Edinburgh, Scotland (de Ricqlès, 2021; Falcon-Lang & Digrius, 2014). This methodological advancement sparked a flurry of histological discovery, spurred on in tandem with refinements in microscopy by Joseph Jackson and his son, Joseph Lister, the founder of microbiology. To some degree around the turn of the 20th century, interest in paleohistology waned, albeit never completely fizzling out, with more sporadic research programs focused on foundational problems. Nevertheless, these built an important framework that allowed paleohistology, particularly vertebrate paleohistology, to be reinvigorated nearly 100 years after its birth, with the seminal works of Rodolfo Amprino (Amprino, 1947), Donald Enlow (Enlow & Brown, 1956), and Armand de Ricqlès (de Ricqlès, 1975, 1977, 1978). These works established critical links between skeletal tissues and development, growth rates, physiology, and phylogenetic evolution that entrenched vertebrate paleohistology as a core tool in the suite of paleontological investigation. Today, paleohistology is a thriving field, building on the shoulders of historical giants. Indeed, some aspects of Nicol's technique remain central to thin sectioning methods today, but as this volume highlights, two hundred years has not passed without major progress.

Our aim in this special issue is to showcase the wide range of techniques, questions, insights, and otherwise inaccessible paleobiological data that paleohistology encompasses. Among the articles included here are developments in our understanding of growth, development, and life history, with an emphasis on vertebrates, spanning across fishes, amphibians, saurians (“reptiles”), and mammals, including humans. Paleohistology continues to broaden our insight into extinct organisms by revealing the structural and developmental complexity of their hard tissues, and integrating them into the evolutionary perspective that only fossils can provide. Through a combination of methodological innovation and taxonomic diversity, the studies presented here demonstrate how fossilized microstructures can continue to uniquely illuminate the biological processes that shaped evolutionary history. This issue is arranged thematically, following a taxonomic thread beginning with fishes and progressing through younger clades from amphibians, through sa

Crocodylians have often been grouped into ecomorphological categories based on snout characters and diet, but quantitative dental morphology has rarely been used for this purpose. We collected Euclidean measurements from the teeth of 18 extant crocodylian species spanning a range of sizes and snout ecomorphotypes, normalized the data for size heterodonty using regression analyses, grouped the crowns into eight dental sections along the arcade, and ran a K-means cluster analysis to cluster individuals based on shape heterodonty. Five clusters emerged, each reflecting different degrees of gracility or robustness of crowns and their variation along the jaw arcade. These morphological clusters showed a connection to snout shape, prey preference, and feeding ecology, particularly prey size and the degree of processing necessary. Cluster assignments were, for the most part, not taxon specific; multiple families and subfamilies were found in most clusters, and members of the same species were often found in more than one cluster. For species with members in multiple clusters, the larger individuals typically were in the cluster with more robust crowns. This supports prior suggestions that dental morphotype coincides with ontogenetic niche shifts. This approach demonstrates the potential for using dental morphology to infer ecological roles in both extant and fossil crocodylians, paving the way for future comparative analyses of archosaur dentition.

Marsupials have evolved alongside other mammals on many continents, mainly in the southern hemisphere, developing their own traits and adaptations. Although the relationships between morphology, bite force, and diet have been well studied in many vertebrate groups, this has rarely been the case for marsupials until recently. Present-day American marsupials' diet and their feeding capacities, considered generalists, remain poorly understood. A better understanding of current American marsupials will lead to more accurate inference models for extinct metatherians. Here, we study and describe for the first time the masticatory apparatus of the Linnaeus' mouse opossum Marmosa murina, along with its performance. Bite forces data were collected for different marsupial species during a field mission in French Guiana in 2017. A 3D bite reconstruction model has been established through dissections and using the lever arm method, based on the static equilibrium of the muscular vectors in the jaw. The optimal gape angle and the contribution of each masticatory muscle to the closing of the mouth were determined. We identify and individualized the different fascicles of the masseter, zygomaticomandibular, temporal, and pterygoid muscles, together with their respective origin and insertion areas. The optimal gape is around 6°, supporting the use of the last molar to get the strongest bite forces. The M. masseter superficialis, the M. temporalis superficialis, and the M. temporalis profundus medialis are the muscles having the greatest impact on the maximum bite force. Our biomechanical model allows a correct approximation of the biting force. However, the muscle stress value has to be increased from 30 N.cm⁻² to 44.360 N.cm⁻² and 54.209 N.cm⁻² to match the in vivo bite forces on the last molar (m4) for Marmosa murina. These high values are rather surprising, suggesting that our model, with the use of standardized constants for all mammals, underestimates true bite forces.

A fossilized rostrum fragment was recently reevaluated from the paleontological collections of the University of Modena and Reggio Emilia (Italy). The specimen, collected from the Northern Apennines of Modena province, was previously referred to an Eocene billfish due to the presence of cylindrical paired bones and small teeth. Thanks to nannoplankton analysis of the matrix, we reassign the specimen to the Upper Cretaceous (upper Campanian-lower Maastrichtian). The morphological features of the rostrum, as well as its stratigraphic provenance, led us to assign the specimen to a longirostrine plethodid tselfatiiform (Actinopterygii, Teleostei) rather than to a billfish. The fragment is assumed to have originated from the mid-posterior portion of the rostrum, with associated upper and lower jaws. The rostrum exhibits a remarkable degree of morphological convergence with extant xiphioid billfishes, together with completely unique features. The bone surface is heavily ornamented, whereas the inner structure shows a prevalence of cancellous tissue. The suture between both premaxillae and the mesethmoid is crossed by a deep longitudinal fossa not dissimilar to that of parvipelvian ichthyosaurs, a feature never reported in tselfatiiform fishes. Abundant tiny conical teeth are found between the jaw rami, separated from their sockets. Micro-teeth can be found in both smooth-stout form or thin and crossed by apicobasal ridges but always capped by a translucent acrodin tip. Comparative analysis with swordfish (Xiphias gladius) dentition provided insights on shared similarities between the two types of micro-teeth. CT scanning of the specimen revealed a large, subtriangular, and tripartite vacuity in the upper jaw. A similar internal architecture is represented by the rostral sinus of modern billfishes, which is known to host large, globose oil-producing glands to reduce drag on the skin. We showcase these anatomical similarities with CT scan analysis of postlarval swordfish and sailfish, together with the morphological comparison with adults of these groups from the available literature. The cumulative features gathered from the specimen suggest a fast, pelagic predatory ecology. The findings further confirm the homoplastic development of a billfish-like body plan in Tselfatiiformes, with independently acquired morpho-physiological adaptation that preceded the evolution of xiphioids at least since the Late Cretaceous.

Sagittal crests are observed among some primate species, including early extinct hominins, however the majority of research investigating sagittal cresting among extant primates has been confined to catarrhines. Sagittal cresting has not been well-investigated among capuchin monkeys, and understanding whether there are taxonomic differences in the frequency and pattern of sagittal cresting among gracile and robust capuchin species, or whether sagittal crest expression is only confined to the males of some species, may yield important insights in a comparative context, to better understand the underlying basis for the frequency and pattern of sagittal cresting among australopithecine species. In the research presented here, I investigate whether there are interspecific differences in the frequency and pattern of sagittal cresting, and sexual dimorphism in cresting frequency among six capuchin species, representing three gracile capuchin species (Cebus albifrons, Cebus capucinus and Cebus olivaceus) and three robust capuchin species (Cebus apella, Cebus macrocephalus and Cebus libidinosus). I collected sagittal cresting data for 279 dentally mature cranial specimens using 3D models. There are interspecific differences in the frequency of sagittal cresting among capuchin species, with four out of the six species investigated (C. capucinus, C. apella, C. macrocephalus and C. libidinosus) showing sagittal crests. There are significant sex differences in the frequency of sagittal cresting in C. capucinus and robust capuchin species (C. apella, C. libidinosus, C. macrocephalus). I further show that there are interspecific differences in the pattern of sagittal cresting among the four species that exhibit sagittal crests. Sagittal cresting in C. capucinus occurs in the posterior region of the neurocranium, in contrast to the robust capuchin species, whose sagittal crests mainly extend from anterior to bregma, to the posterior cranial region at the midline. The underlying reasons for interspecific and sex differences in the frequency and pattern of sagittal cresting among species are yet to be elucidated, and may be associated with dietary, habitat or socioecological differences among capuchin groups.

A characteristic common to almost all crocodylomorphs, whether living or extinct, is the presence of a dermal skeleton. This covering is composed of bones known as osteoderms or dermal plates/scutes, which are interconnected by fibrous tissues. Osteoderms play essential roles in the biology of crocodylomorphs, and vary in size, shape, ornamentation pattern, and functions according to the species. This study analyzed the osteoderms of fossil species of four fossil crocodylomorphs taxa from the Upper Cretaceous of the Bauru Group and compared their morphological characteristics with other living and extinct lineages, including Candidodontidae and fossil Caimaninae. Sampling included osteoderms from different regions of the body and individuals of different ontogenetic stages. The osteoderms analyzed originate from distinct clades of crocodylomorphs (Peirosauridae, Baurusuchidae, Sphagesauridae, and the spaghesaurian Mariliasuchus), presenting distinct external morphologies at the macroscopic level. The histological analysis confirmed the similarities seen in distinct clades of crocodylomorphs (Peirosauridae, Baurusuchidae, Sphagesauridae, and Mariliasuchus), despite highly divergent macroscopic morphology. In all cases, the bony matrix is characterized by parallel and interwoven fibers, with secondary osteons much larger than the primary ones, indicating a process of bone resorption. Growth lines also provide information on the minimum age of the individuals at the time of death. Histological differences in the osteoderms of baurusuchid of different ontogenetic stages indicate a back-to-front progression in the reabsorption process, akin to the progressive ontogenetic fusion of neurocentral sutures seen in the vertebral spine of crocodylomorphs. A shift in the position of the parasagittal crest through ontogeny indicates that the growth process of the osteoderm is not symmetric at least in Baurusuchidae, and that individuals of different ages are expected to present meaningful macroscopic shifts in the morphology of their osteoderms. Peirosauridae ornamentation pattern also varies during ontogeny since pits present in early ontogenetic states are covered/obliterated by subsequent layers of bone that assume a more regular aspect.

Cetaceans are a lineage of marine mammals that evolved diverse modes of aquatic feeding facilitated by modifications to the ancestral mammalian feeding apparatus, including the mandibular symphysis. In mammals, the mandibular symphysis is the third joint of the lower jaw. Articulation of the joint varies across mammalian clades, ranging from fibrocartilaginous connection to complete ossification. Whales span this range, with one lineage (baleen whales) evolving an unfused, highly mobile symphysis. This study conducts a comprehensive morphological investigation of the mandibular symphysis in whales. Here, we describe diverse joint morphologies based on observations of 152 cetacean mandibles representing 74 extant and fossil taxa. We also examine the internal architecture of the joint using computed tomography (CT) cross-sectional data. Based on gross anatomical observations of the osteology of the joint, we define four broad categories of symphyses: unfused, partially fused, fully fused, and decoupled. In odontocetes, articulation ranges from unfused mandibles to full fusion of the symphysis. The decoupled, highly mobile symphysis in crown mysticetes represents a novel condition unobserved in other mammalian clades. Partial fusion of the symphysis is the most common mode of articulation among the observed extant taxa, closely followed by unfused symphyses. In extant and extinct longirostrine taxa, full fusion coincides with an elongated symphysis. However, extant sperm whales (Physeter macrocephalus) notably exhibit an elongated, unfused symphysis that likely does not play a significant role in feeding. Observations of eminences on the posterior border of the symphysis in sperm whales and other suction feeders suggest that aspects of hyolingual musculature and function may be reflected in the morphology of symphysis. We suggest that further investigation of the symphyseal joint in marine mammals and other aquatic tetrapods will advance efforts to identify phylogenetic and ecological influences on the form and function of the feeding apparatus in an aquatic environment.

Ligaments are important connective tissues within the musculoskeletal system that connect bone to bone and provide support and stability. The spine contains a number of ligaments that predominantly function in mechanical stabilisation and allow for certain ranges of spinal motion. Establishment of mechanical stability provided by spinal ligaments has not been described, and it is not known to what extent failure or inadequate spinal ligaments contribute to spinal conditions, such as scoliosis. While there are many similarities between ligaments and tendons, there is no experimental evidence investigating the development of these stability-bearing tissues. This study uses the embryonic chick model Gallus gallus and investigates the development of spinal ligaments in the thoracic spine, examining structure and molecular expression across development. Findings show organisational changes in spinal ligaments in association with vertebral shape changes from cranial to caudal, with the anatomical identification of six vertebral ligaments in the thoracic spine. As development proceeds, the size of the anterior longitudinal ligament, on the ventral surface of the vertebral body, and the supraspinous ligament, on the dorsal side of the spine, becomes greater, with the orientation of collagen fibres in the supraspinous ligament becoming more aligned. In addition, this study demonstrates that cell density decreases and nuclei become smaller and more circular across development. This study provides evidence that the embryonic chick is an appropriate model to study spinal ligament development and has added knowledge on the structural hallmarks of embryonic vertebral ligament tissues. These findings allow for subsequent investigation of the mechanical and molecular characteristics of spinal ligament development, for example useful for determining if in utero movement is important for the establishment of spinal ligament stability. Use of this model and integration of findings with additional models will provide knowledge of the contribution of spinal ligaments in spinal failure conditions.

Among squamates, hemipenes are known to evolve rapidly and exhibit diverse shapes, sizes, and ornamentation. Croaking geckos (Aristelliger) are unique among geckos in exhibiting mineralized structures (hemibacula) in their hemipenes. We here describe the gross morphology of the hemibacula of each currently recognized species of Aristelliger, document hemibacular histology, and report on hemibaculum development. We confirm the presence of hemibacula in all currently recognized species and demonstrate that three distinct morphologies correspond to three putative clades in the genus. Histology revealed that hemibacula are superficially similar to chondroid bone and composed of mineralized dense connective tissue covered in a thin layer of epidermis with alcian-positive cells embedded within a mineralized matrix. Additionally, we demonstrate that hemibacula do not develop until past the onset of sexual maturity and that hemibaculum length scales isometrically with body size. We hypothesize that hemibacula of Aristelliger develop via peramorphosis, a phenomenon also expressed in the cranial morphology of this genus. Additionally, we speculate on the functional significance of these enigmatic structures.

The ankle joint of lizards has a complex structure, and its features help to define the Lacertilia. The configuration of this joint in its ancestral state entrains conjoint flexion-extension and long-axis rotation of the pes relative to the long axis of the crus. In Gekko gecko these actions can be decoupled because of derived features of the ankle joint. The increased degrees of freedom of the motions of the pes are associated with the operation of the adhesive toe pads carried on the digits. Among iguanian lizards, the genus Anolis has independently acquired a digital adhesive system that employs toe pads. Geckos and anoles are thus regarded as being convergent in the possession of a digital adhesive apparatus. This raises the question of whether anoles exhibit a similar ankle structure to that of geckos to allow them to deploy their toe pads in a mechanically similar fashion. Comparative analysis reveals that this is not the case, and that Anolis retains an ankle structure very similar to that of its iguanian relatives and non-gekkotan lizards in general. Some differences set its ankle and foot structure apart from those of its closest relatives, but these exaggerate the differences between geckos and anoles rather than lessen them: its ankle joint architecture is more sharply contoured than that of its close iguanian relatives; the ventral peg on the fourth distal tarsal is more extensive; its metatarsals are more gracile in form, relatively longer, and their distal joints are all unicondylar; its fifth metatarsal has a longer shaft and a less prominently sculpted ventral surface; and the meniscus that intervenes between the anterodistal extremity of the astragalocalcaneum and the more medial of the metatarsals is more extensive. These attributes combine to limit degrees of freedom at the ankle joint but provide the digits with greater mobility relative to the metatarsals. Such derived features may prove to be associated with enhanced capabilities for grasping narrow perches, sprinting and jumping, activities common to anoles but much less evident for geckos. The ways in which geckos and anoles negotiate their locomotor environments may be associated with the differences evident in their ankle and tarsometatarsus structure-anoles seemingly using the combination of their toe pads and claws to navigate along and between relatively narrow branches and geckos using broader, more expansive sectors of the substratum. Anoles and geckos have incorporated adhesive toe pads into their locomotor apparatus from structurally different starting points, with the former integrating the adhesive system into a pedal configuration that departs little from the ancestral lacertilian pattern. Beyond the possession of toe pads the pedal structure of anoles exhibits little in the way of convergence with that of geckos.