Tetrapod vertebrates possess skulls composed of variably articulating bones which they use to apprehend, process, and ingest food. Natural selection must therefore optimize craniomandibular sutures for load resistance, but sutural patency is required for normal craniofacial development to occur. While mammals seemingly escaped this constraint in their mandible by simplifying it into a single bony element (i.e., the dentary), sauropsids retain a composite mandible with a prominent, and occasionally flexible, intramandibular joint (IMJ) separating the rostral, dentigerous elements from the caudal elements onto which the jaw muscles insert. How sauropsids simultaneously construct a mandible robust enough for feeding that nevertheless maintains sutural patency for proper growth is a biomechanical paradox of keen interest to functional morphologists. Sauropsids may either passively reduce IMJ strain by expanding IMJ complexity or actively by using isometric contraction of specialized jaw muscles to resist excursion. American alligator (Alligator mississippiensis) mandibles possess a rather complex IMJ that must accommodate extreme magnitude and highly dynamic loads during feeding. Importantly, they also possess large m. intramandibularis (mIM) and m. pterygoideus ventralis (mPTv) muscles that may reduce IMJ strain during feeding, making them an ideal taxon to investigate the effect of joint morphology and muscle activity on IMJ and mandibular strain. We therefore constructed several 3D finite element models of Alligator mandibles with varyingly shaped IMJs to test the effect of IMJ orientation, complexity, and differential muscle activity on mandibular bending deformation and joint strain. Simple planar IMJs, regardless of orientation, reduce positive sagittal bending and medial wishboning deformation, and increase inversion of each hemimandible's dorsal margin. Changes in bending deformation during bilateral bites as the joint surface was reoriented from rostrally sloped to Vertical to caudally sloped are partially attributable to changes in joint surface area, though bending deformation is sensitive to both bite point location and joint orientation during unilateral bites. Increasing IMJ surface area reduces IMJ strain magnitudes, with a highly complex IMJ experiencing the most uniform and lowest magnitude joint ligament strains. Differential activation of mIM and mPTv do not significantly reduce IMJ strains but do affect mandibular bending deformation, suggesting that available joint surface area, and not isometric muscle contraction, is the greatest variable controlling IMJ strains in adult Alligator. Instead, mIM may significantly control bite point reaction forces due to its very long moment arm, whereas mPTv indirectly reduces medial wishboning by pulling the caudal elements against the pterygoid buttress, inducing a powerful, laterally directed reaction force on the caudal elements. However, while sauropsids appear susceptible to medial wishboning owing to the prominent medially directed pull of their jaw muscles, overall relationships between IMJ form and flexibility are unclear, as groups with complex intramandibular sutures may be akinetic (e.g., crocodylians) or kinetic (e.g., varanids). Further research will clarify IMJ morphological diversity and disparity among reptiles and divulge form-function relationships of this critical, but underappreciated, aspect of their feeding apparatus.

Gracilisuchus stipanicorum was a pseudosuchian archosaur from the Late Triassic period in Argentina. Because it was small-bodied with relatively long, slender limbs, traits that are potentially ancestral for archosaurs, such as its locomotor functions, are important to estimate. It has been illustrated as a quadruped with plantigrade autopodia, and probably with an 'erect' or 'semi-erect' stance, because it is a suchian archosaur, but there has been no deep analysis of these reconstructions. Here, we detail our reconstruction of a three-dimensional digital skeleton of Gracilisuchus from scans of the bones of four main specimens, including the holotype. In this procedure, we found hitherto unrecognised elements of the manus (metacarpals) and incorporated them in our model of the whole organism. We added estimated hindlimb musculature and body segment mass properties to form a musculoskeletal model. This model allowed us to address three key questions: Was it quadrupedal or bipedal; plantigrade or digitigrade; and more sprawling or more erect? Furthermore, we examine how its hindlimb muscle moment arms compare to those of three other small-bodied Triassic archosauriforms and an extant juvenile Nile crocodile in order to assess the diversity and potential evolutionary polarity of these traits. Our analyses of the model support the inferences that Gracilisuchus was quadrupedal (but facultative bipedalism cannot be ruled out) and plantigrade, and not strongly sprawling, but probably not strongly erect hindlimbs; although terming this posture 'semi-erect' would be an oversimplification. Gracilisuchus, as modelled here, seems to roughly be a reasonable approximation of the ancestral state of the archosaurian locomotor system. Our synthesis of numerous lines of evidence, from qualitative functional morphology to whole-body centre of mass and muscle moment arms, forms a new reconstruction of Gracilisuchus that future analyses can build on, both biomechanically and comparatively, in order to better understand archosauriform locomotor evolution.

Astyanax mexicanus is a small freshwater fish from Mexico. It is of particular interest because in addition to the ancestral surface morph, A. mexicanus has several independently evolved cave populations. These cave morphs have a strikingly different cranial morphology, most notably in the regression of the eyes. Detailed information is available on the development, ossification and morphology of the jaws and on the differences in the sensory systems that underlie the patterns of fusion and fragmentation of the infraorbital series in the skulls of cave (troglomorphic) populations of the species. However, there is limited information on the skeletogenesis of the remaining skull components in the species. The present work aims to fill this gap by providing a detailed comparative description of cranial skeletogenesis in surface and Pachón cave populations of Astyanax mexicanus. We find that, anatomically, cavefish larvae are significantly larger than surface larvae at early stages of development, developing overall larger head structures than surface individuals. Pachón cavefish also follow a heterochronic development with respect to surface populations with slower growth and a delayed ossification process compared to surface morphs.

The present study analyzed incremental markings in the enamel of eight mandibular first molars (M1) of red foxes (Vulpes vulpes) and a single M1 of a grey wolf (Canis lupus). The most prominent enamel incremental markings in both species were laminations. Based on lamination counts, a mean crown formation time of 102 days (range 99-107) was reconstructed for the red fox M1. This finding fits well with the reported M1 emergence through the gum in this species at between days 105 and 119 after birth, indicating a daily periodicity of the laminations. Based on the nature of laminations as daily growth marks, we reconstructed different crown growth parameters in the red fox molars. Overall, the enamel daily secretion rate (DSR) increased from lowest values near the enamel-dentine junction (EDJ) to highest values near the outer enamel surface. The lowest mean value (6.8 μm/day) was recorded in the inner third of the lingual enamel from the cervical crown region, the highest mean (17.4 μm/day) in the outer third of the buccal enamel from the mid-lateral crown region. While in the inner enamel third, DSR values were similar in buccal and lingual enamel, in the central and outer thirds, mean and median DSR values for buccal always exceeded those for lingual enamel. On the buccal crown side, linear enamel thickness and reconstructed ameloblast secretory lifespan were similar in the three lateral crown regions distinguished (upper, mid, lower) and only dropped in the cervical crown region. By contrast, lingually only the upper lateral crown region showed values similar to those on the buccal side, while the values in more cervical crown regions were always markedly lower than in the corresponding buccal crown regions. Enamel extension rate (EER) was highest in the upper third of the EDJ length, with mean values of 145 μm/day in buccal and 170 μm/day in lingual enamel. EER dropped to lowest values in the cervical third of the EDJ length, averaging 85 μm/day buccally and 97 μm/day lingually. The enamel formation front angle (EFFa) varied between 3.9° and 5.2°, with only slightly higher values in the lower compared with the upper crown portion. EER and the EFFa data indicate a very rapid crown elongation, which is reflected by the very steep inclination of the laminations throughout the enamel. Four to six subdaily growth increments were present between consecutive laminations, whereas long-period enamel incremental markings were not discernible. Our findings indicate that previous studies on canid enamel misidentified the incremental markings by erroneously diagnosing daily laminations as long-period markings (striae of Retzius) and subdaily prism cross-striations as daily markings. In consequence, these studies reported too low DSRs for canid enamel and too long crown formation times for the analyzed teeth.

Humans have a characteristic distribution of hair across the body. Visible, relatively long and thick terminal hair fibres are present on the scalp and eyebrows in childhood, and are stimulated to grow on other parts of the body, such as the beard and armpits, by hormones during puberty. The short and fine vellus hairs, in contrast, are not readily visible and cover most of the body, including the face. Here we report quantification of the timing and characteristics of hair follicle development in human embryogenesis, from gestational Weeks 8-19, and compare this to mouse hair follicle development. We find that human hair follicles develop first on the head, where we identify several distinct initiation sites, followed by the torso. Clustered follicular units are produced by secondary hair follicles budding off the epidermis from sites close to earlier-forming primary follicles. Arrector pili muscles are detected from gestational Week 17. Although terminal and vellus hair follicles have clear differences in the adult, both hair types initially develop from placodes and dermal condensates of similar size. Once their development is initiated, we find that human hair follicles grow and mature at the same rate, regardless of anatomical location, but have different density at different body sites. These findings demonstrate that regional differences in hair characteristics of human skin, such as the distinction between scalp and forehead, are largely caused by processes that act after the initial stages of hair follicle morphogenesis. Efforts to understand the evolution of human 'hairlessness' should, therefore, focus on genetic and cellular events that take place after hair follicle morphogenesis. We compared human skin appendages, including eccrine sweat glands, with those in mouse. We found that molecular markers, such as EDA, EDAR, SOX2 and WNT pathway components, are broadly similar in expression between both species, although species-specific differences do exist. Notably, SOX2 expression, which distinguishes different hair follicle types in the mouse, is detected in all human hair follicle types, though with a later onset of expression than that observed in mouse. Together with comparison of morphology and gene expression, these results support the use of embryonic mouse primary hair follicles as a model for human hair follicle development.

The forelimb and hindlimb of tetrapods are serially homologous structures that share a conserved developmental framework. Despite their evolutionary divergence in form and function, the two sets of limbs retain parallel genetic and morphogenetic patterning, enabling comparisons across and within species. While serial homology has been well established for proximal limb elements (i.e., girdle and long bones), the mesopodium (the carpals and tarsals) has received comparatively little attention. This gap is due in part to the morphological complexity of these regions and the absence of a clear, consistent framework for one-to-one comparison. In this paper, we present a framework for morphological comparisons between the limbs by proposing explicit serial homologies between individual carpal and tarsal elements. Building on Owen's classical criteria of homology (i.e., composition, relative position, and development), we integrate evidence from comparative anatomy, embryology, and the fossil record to justify specific pairings. In doing so, we extend previous efforts that relied primarily on gross morphology by incorporating developmental timing, positional shifts, and patterns of fusion and reduction of elements observed across taxa. This proposed framework enables new comparative analyses of the mesopodium, with implications for future studies of covariation, modularity, and functional-adaptive evolution, expanding the anatomical toolkit for addressing long-standing questions in vertebrate limb evolution. By refining the criteria for mesopodial homology, this work contributes to a more complete understanding of integration between sets of limbs and their process of evolutionary divergence in tetrapods.

Natural variations in cochlear anatomy have substantial implications for both clinical care and research in the fields of otology, neurotology, and audiology. While precise anatomic characterization is essential for a multitude of applications, comprehensive reference dimensions of both osseous and membranous cochlear structure obtained from a large and morphologically heterogeneous sample set do not currently exist. In this study, one hundred healthy human cadaveric temporal bone samples, without historical or visible pathology, underwent high-resolution three-dimensional synchrotron radiation phase-contrast imaging (SR-PCI) to develop a morphometric compendium of the human cochlea. Measurements of both bone and soft tissue in the cochlea were obtained, including basal turn diameter and width, cochlear height, and cochlear length along multiple anatomic paths (lateral wall, basilar membrane, and modiolar wall). The hook region, scalar geometry (diameter, area, tilt, width, and volume), and round window dimensions were also comprehensively characterized. Normative tonotopic frequency distributions of the basilar membrane and spiral ganglion were derived using cochlear length measurements and Greenwood's frequency-position function. These anatomic benchmarks establish invaluable reference data which may be used for anatomically informed, precision medicine approaches, including patient-specific surgical planning, intracochlear pharmaceutical delivery optimization, the development of automated image analysis algorithms, and the investigation of cochlear structure-function relationships in pathological conditions.

Facial morphology results from coordinated developmental and evolutionary processes that produce structured patterns of covariation among traits. Therefore, individual faces reflect integrated anatomical configurations rather than collections of isolated traits. However, discrete facial variation is often examined in a disconnected manner without explicitly accounting for these biological interdependencies. Given that such interdependencies shape which trait combinations are more or less likely, this study evaluates how modularity and integration structure the joint occurrence of facial features. Frontal facial photographs of adult males (n = 462) were analyzed. Twenty-nine anatomical landmarks were digitized and analyzed using geometric morphometrics. Procrustes coordinates were used to test three modularity hypotheses, to quantify integration between facial modules, and to explore patterns of continuous covariation through principal component analysis. Then, morphometric variables were categorized into discrete variables to evaluate. The results confirm that the face is organized into relatively distinct anatomical modules, with varying degrees of covariation. The highest integration observed was between the facial outline and the eyes modules. Observed frequencies of discrete trait combinations further support the structured co-occurrence of facial features by showing deviations from independence expectation. These findings highlight the importance of interpreting discrete facial phenotypes as integrated systems, providing a framework that complements trait-based approaches and insights for understanding facial discrete variation.

Pterosaurs were the first vertebrates to achieve powered flight, a result of various unique anatomical adaptations, and significant morphological diversity throughout their evolutionary history. Although the pterosaurs body structure was specialized for powered flight, the clade exhibited great disparity from its origins, with later evolution giving rise to multiple phylogenetic lineages and a wide range of body sizes, from small forms such as Anurognathus ammoni to truly giant species like Quetzalcoatlus northropi. This variation led to distinct flight strategies within the group. Here, we assess allometry in the skeletal elements of Rhamphorhynchus muensteri in comparison with Eupterodactyloidea and Euctenochasmatia, examining their implications for paleoecology and flight capacity. The analysis included 127 specimens and assessed 13 skeletal variables using statistical methods, principal components analysis (PCA), and standardized major analysis (SMA). The results reveal differences in size-shape trajectories and functional adaptations related to activity flight. R. muensteri exhibits a trend of negative allometry (PCRW/body, humerus, radius/ulna, metacarpal IV, and femur), indicating distinct allometric scaling patterns and flight style. These patterns suggest behavior and ecological differences, as well differing flight capabilities between R. muensteri likely relied on active flapping flight interspersed with gliding, whereas other taxa may have exhibited greater migratory adaptations within Eupterodactyloidea, and a greater dependence on flappingflight in Euctenochasmatia. The negative allometry in R. muensteri suggests a flight style characterized by early flight capability (precocial flight), whereas Eupterodactyloidea showed positive allometry and Euctenochasmatia isometry. Additionally, the study highlights how allometric patterns influence pterosaur phylogeny, underscoring the importance of incorporating them into future analysis.