Extant little owl (Athene noctua) and its relatives are iconic strigiform birds (Aves: Strigiformes) with a very poorly known evolutionary history. The anatomy and adaptation of early members of this group are poorly understood, as the fossil record of the genus Athene is very fragmentary prior to the Middle Pleistocene. The fossil taxon Athene vallgornerensis represents the earliest known species of Athene owls in Eurasia. Previously documented exclusively from the Lower Pleistocene of Mallorca and the Iberian Peninsula, this species was until now known from only a few postcranial elements, leaving its osteological characterization, adaptations and phylogenetic affinities highly uncertain. Here, I report a very rich collection of more than 350 fossil bones, including several partial skeletons, belonging to A. vallgornerensis from the Early Pleistocene (Calabrian, ~1.8-1.6 Ma) of Crimea (Eastern Europe). This material makes A. vallgornerensis the best-known pre-Late Pleistocene fossil owl species, permits the first comprehensive osteological description and reveals a mosaic morphology of A. vallgornerensis, partly similar to that of modern A. noctua, Athene brama and Athene cunicularia. Most importantly, the remarkably shortened hind limb (and tarsometatarsus in particular) aligns A. vallgornerensis with the phylogenetically basal and ecologically distinctive Athene blewitti, suggesting this fossil species could occupy a more arboreal or forest-edge niche than the open-country A. noctua and A. brama. The large sample size allows for the first quantitative assessment of individual variation in an Early Pleistocene European Athene population. The tarsometatarsus, in particular, exhibits remarkable intraspecific variability hinting at a high degree of phenotypic plasticity that may have served as raw material for adaptive evolution in the genus. These findings not only confirm the taxonomic validity of A. vallgornerensis but further provide critical insights into the early evolutionary history of Athene owls in Eurasia.
Morphological data on the eyelids, the third eyelid (nictitating membrane), and the orbital glands remain limited for many Anguimorpha, including the Komodo dragon (Varanus komodoensis). The ocular adnexa contribute to eye protection, lubrication, and immune defense, and they may also reflect ecological and phylogenetic variation within Squamata. The objective of this study was to describe the gross anatomical, histological, histochemical, and polarized-light microscopy characteristics of the upper and lower eyelids, third eyelid, Harderian gland, and lacrimal gland in the Komodo dragon. A panel of histological and histochemical stains was used to evaluate connective tissues (collagen, elastic fibers, and reticulin), muscles, cartilage, pigmentation, and mucins/glycoconjugates. The eyelids showed osteoderms and a multilayered keratin arrangement (β-keratin layer/fibrous intermediate layer/β-keratin layer/α-keratin layer). Intense palpebral scale pigmentation and folded conjunctivae with mucin-producing goblet cells may contribute to periocular protection, although this functional interpretation requires confirmation in free-ranging animals. The third eyelid contained hyaline cartilage with Type I collagen-associated birefringence in the perichondrium, Type III collagen-associated birefringence in the cartilage matrix, and conjunctiva-associated lymphoid tissue. The Harderian gland was the dominant orbital gland, but its histochemical profile showed only weak and uneven evidence of acidic mucosubstances, mainly in luminal secretory material; therefore, its secretory phenotype should be interpreted cautiously, whereas the lacrimal gland showed a clearer mucous profile dominated by acidic mucosubstances. By documenting a large non-spectacled anguimorph, these findings provide comparative data relevant to eyelid support, nictitating-membrane organization, lacrimal drainage, and orbital gland variation across Squamata.
The brachial plexus is a critical neural network governing upper limb function. However, systematic quantitative data on axon-profile counts and marker-defined axonal composition at intermediate anatomical levels, particularly trunks and divisions, remain limited. Moreover, quantitative data on fascicle numbers across different anatomical levels of the brachial plexus remain scarce. Here, we analyzed brachial plexus specimens from 10 human donors using NF200/ChAT dual immunofluorescence staining to assess axon-profile counts and fascicular organization across five sampled anatomical levels: roots, trunks, divisions, cords, and terminal branches. Within the sampled structures, NF200-positive axon-profile counts were numerically higher at proximal levels and lower at more distal sampled levels, whereas ChAT/NF200 double-positive profile counts showed less variation, with their proportional contribution increasing from 12.61% to 20.84%. Fascicle numbers were numerically highest at the trunk and cord levels. On representative qualitative inspection, no stable intrafascicular spatial pattern of ChAT/NF200 double-positive profiles was apparent. This study provides marker-defined quantitative anatomical reference data on NF200-positive axon-profile counts, ChAT/NF200-defined axonal composition, and fascicle numbers across five sampled anatomical levels of the human brachial plexus, including the under-quantified trunks and divisions. These data may serve as a foundation for future studies of brachial plexus anatomy, nerve repair planning, and neural interface design.
The Australasian crocodylian fossil record is dominated by the extinct endemic clade Mekosuchinae, with known members spanning the Eocene-to-Holocene. This diverse clade exhibited high morphological disparity, from early members superficially similar to Crocodylus, to more terrestrial forms later in the clade's evolutionary history. However, it remains unclear why mekosuchines went extinct whilst Crocodylus continues to thrive in the region, and how the ancestors of this terrestrial clade reached Australasia without long-distance transoceanic dispersal. One possible explanation is that there was intraclade variation in the growth pattern and life habit of mekosuchine species, and that these, in turn, differ from those of Crocodylus. Here, we present the first osteohistological study of mekosuchines, evaluating the femora of the middle-late Eocene species Kambara taraina and late Oligocene-middle Miocene Baru (B. wickeni, B. darrowi). Bone compactness profiles are more characteristic of a terrestrial habit than those of extant crocodylians, with Baru darrowi demonstrating the most terrestrialized profile. Apposition rates are similar in absolute value to those of extant crocodylians, indicating a comparable metabolism; however, high appositional rates throughout mid-cortical cyclical growth marks suggest that mekosuchines were able (or needed) to maintain a moderate rate of growth for longer than extant crocodylians. Our results indicate that mekosuchines were more terrestrial than Crocodylus and that later-diverging mekosuchines showed increased terrestrialization; however, they do not indicate that early mekosuchines were well-adapted to transoceanic dispersal. Heightened terrestrialization and an extended moderate growth period might have made mekosuchines more vulnerable to late Cenozoic aridification in Australia than semi-aquatic Crocodylus.
Composed of various minerals, the eggshell provides ideal conditions for embryonic development, and understanding its composition and structure is essential for clarifying its relationship with bone formation. This study characterized the structure and mineral profile of the Rhinoclemmys punctularia punctularia eggshell in relation to embryonic osteogenesis. Fertilized eggs were collected and incubated under controlled humidity (80%) and temperature (28.5°C ± 0.5°C). From the 15th day of incubation onward, embryos (n = 39) were analyzed at different developmental stages using a clearing and double-staining technique. Eggshell ultrastructure and mineral composition (n = 23) were evaluated by scanning electron microscopy coupled with energy-dispersive spectroscopy. Three developmental periods were established: pre-ossification (PO), with no signs of mineralization; ossification in progress (EO), marked by the first ossification centers; and complete ossification (OC), in which all bones were mineralized. Osteogenesis began on the 30th day of incubation in the dermatocranium. Ultrastructural analysis revealed three shell layers: cuticle, mineral layer, and organic layer. Calcium weight percentage on the eggshell surface decreased significantly by 5% (p = 0.037) during embryonic development. Eggshell thickness also declined significantly (p = 0.00242), from 389.86 ± 40.14 μm in PO to 323.11 ± 37.00 μm in EO and 320.65 ± 40.23 μm in OC. These findings indicate that eggshell mineral dynamics play a fundamental role in embryonic osteogenesis, contributing to a better understanding of skeletal development and providing insights for evolutionary studies.
Extant turtles occupy freshwater, marine, and terrestrial habitats, and their nasal region provides important evidence for the evolution of respiration- and sensory-related cranial morphology. However, developmental data on the turtle nasal capsule remain limited, particularly in Pleurodira. Here, we describe nasal capsule development in the pleurodiran turtle Emydura subglobosa based on histological cross-sections of five distinct developmental stages and three-dimensional reconstructions. The nasal capsule underwent marked reorganization between Emyd. subglobosa developmental stages 4- and 5- (ES 4- and ES 5-), during which its basic architecture became established. Several features are notable. First, the nasal entrance shows distinctive pleurodiran specializations, including a processus supranarinus communis formed by medial fusion of the paired processus supranarini and a dorsally projecting processus rostralis medialis formed from the paired processus infranarini. In addition, Emyd. subglobosa possesses a fenestra rostralis at the nasal entrance, a feature not identified in other turtles examined to date. Second, the floor of the nasal capsule exhibits the expanded solum nasi complex characteristic of turtles, including the lamina transversalis anterior, pars paraseptalis, and pila supraglandularis, together with a canalis praepalatinus and a posteriorly facing fenestra basalis. Third, the pars ectochoanalis differs between the two major turtle clades: in pleurodires, it extends only slightly caudally, whereas in cryptodires it typically extends farther caudally and forms the ventral margin of the ductus nasopharyngeus. These findings provide a developmental framework for interpreting pleurodiran nasal morphology and a broader comparative framework for understanding turtle nasal evolution.
Previous studies in Lagidium peruanum and Lagidium viscacia reported strict right-sided dominance in ovulation and embryo implantation, although bilateral ovarian functionality may represent an intrinsic reproductive strategy to the genus. Here, we analyzed the morphology, histochemistry (periodic acid-Schiff [PAS] and Alcian blue [AB]) and lectin histochemistry of both ovaries in L. peruanum (Chinchillidae) during early, middle, and late gestation. Through gestation, the ovarian cortex contained follicles at different developmental and atretic stages, as well as interstitial glands. The medulla was composed mainly of vascularized connective tissue. Embryonic remnants, including cortical and medullary cords, were found in the cortex and the medulla during all gestational stages. A single persistent primary corpora lutea (CL) was observed in the left ovary at all gestational stages, while a secondary CL was detected only during early stages. These findings differ from previous reports describing the primary CL in the right ovary. The zona pellucida of some follicles was PAS and AB positive, and lectin histochemistry revealed dynamic glycosylation patterns in the follicles between both ovaries and throughout gestation. The ovarian morphology of L. peruanum supports the hystricomorph ovarian bauplan. However, variable ovarian laterality, persistence of embryonic structures, and differences in the number and distribution of CL among chinchillids (which includes Lagidium, Chinchilla, and Lagostomus) suggest previously unrecognized diversity in ovarian organization and reproductive strategies within the clade.
The heart is the central organ of the cardiovascular system. The myocardium is composed of a three-dimensional network of fibers arranged in clockwise and counterclockwise helices, which are essential for proper cardiac contraction. This study aimed to describe the stratigraphic and morpho-functional pattern of the porcine myocardium to provide anatomical and physiological data applicable to both human and animal studies. The projection of myocardial bridges over the interventricular subsinuosal branch was also evaluated. Twenty hearts from domestic pigs were used. After fixation and anatomical preparation, the ventricular myocardial layers and myocardial bridges were dissected and analyzed macro and microscopically. The left ventricle was found to be composed of outer, middle, inner, and cylindrical septal layers, whereas the right ventricle consisted of outer, middle, and inner layers, with its septal wall organized as a transverse septal band. The origin of the myocardial fibers was associated with the four fibrous rings of the cardiac skeleton, and their insertion occurred predominantly within the walls of the interventricular septum. Myocardial bridges were classified as dorsal, middle, or ventral and were present in 65% of the hearts analyzed. Histological and ultrastructural analyses demonstrated collagen and elastic fibers between the myocardial bridges and the associated vessels. It is concluded that the layered and structural organization of the porcine myocardium are consistent with the distinct hemodynamic demands of each ventricle. These findings reinforce the functional significance of myocardial architecture in ventricular mechanisms and support the use of the pig as a relevant model for comparative and translational cardiovascular research. The topographic analysis of myocardial bridges revealed their depth-related relationship with the vascular wall and with the outer and middle myocardial layers, suggesting a potential influence on coronary blood flow and cardiac vascularization.
Snake vertebrae possess distinctive morphological features that differentiate them from those of other reptiles. While some of these structures are unique to particular snake groups, enabling direct taxonomic identifications, others are restricted to specific regions of the vertebral column. Among the latter are haemapophyses, paired structures emerging from the ventral surface of the centrum that characterize the caudal vertebrae of most, though not all, snakes. Practically confined to the caudal region, haemapophyses are absent from mid-trunk vertebrae, where other structures appear. Here, we report an extraordinary case of haemapophyses-like structures ("quasi-haemapophyses") occurring in mid-trunk vertebrae of the African elapoid snake Atractaspis corpulenta kivuensis. This bizarre taxon possesses, uniquely among all known snakes (extinct or extant), distinct "quasi-haemapophyses" throughout its mid-trunk and posterior trunk vertebrae, which diminish only in the posteriormost trunk region. Haemapophyses subsequently reappear, albeit in modified form, in the cloacal and caudal regions. In addition to a detailed analysis of vertebral morphology and intracolumnar variation, we compile and review all published figures of Atractaspis vertebrae, as well as providing documentation for additional species-rendering this the most comprehensive vertebral study conducted for the genus to date.
The aim of this study was to provide a detailed anatomical description of the aortic arch branches in Geoffroy's cat (Leopardus geoffroyi). Fifteen adult and subadult specimens were included in the study, comprising six adult males, three subadult males, two adult females, and four subadult females. Of these, five selected specimens underwent contrast-enhanced thoracic radiography, including three males (one adult and two subadults) and two females (one adult and one subadult). Latex injection followed by anatomical dissection was performed in five individuals, whereas the remaining 10 underwent direct anatomical dissection without prior injection. In all examined specimens, after the origin of the coronary arteries, the aorta first gave rise to the brachiocephalic trunk (truncus brachiocephalicus), followed by the left subclavian artery. In 13 of the 15 animals (86.7%), a bicarotid trunk originated from the brachiocephalic trunk. The length of the bicarotid trunk ranged from 0.4 to 2.5 cm (mean: 1.3 cm). The right subclavian artery originated as the continuation of the brachiocephalic trunk and gave rise to the internal thoracic, vertebral, costocervical, and superficial cervical arteries. In the remaining two individuals (13.3%), the bicarotid trunk was absent, and the left common carotid artery, right common carotid artery, and right subclavian artery arose directly from the brachiocephalic trunk in a trifurcation pattern. In these specimens, the right subclavian artery originated from the trifurcation and gave rise to the internal thoracic, vertebral, costocervical, and superficial cervical arteries. These branches were present in all individuals, though slight variations were observed in their order of origin and relative spacing. The left subclavian artery exhibited a comparable branching pattern. The combined use of radiographic and dissection techniques allowed precise characterization of the main thoracic arterial pathways. These findings expand the anatomical knowledge of L. geoffroyi and provide relevant information for diagnostic imaging, surgical approaches, and clinical management of Neotropical wild felids.
Musculoskeletal modeling offers a non-invasive approach to analyze human movement, enabling estimation of forces that are difficult to measure experimentally. In musculoskeletal modeling, the human body is represented as rigid segments connected by joints. Complex structures (i.e., the foot) are often modeled as a single rigid segment due to challenges in capturing the motion of individual bones with conventional gait analysis. Additionally, internal joint locations typically require invasive imaging to be accurately determined. Here, we propose a regression-based method to locate the in vivo positions of mid- and hindfoot joint centers using three-dimensional surface markers. Simulated weight-bearing computed tomography (SWCT) scans of 20 individuals were randomly assigned to training (n = 16) and validation (n = 4) datasets. Surface models of each foot were generated, and anatomical markers identified in Avizo Lite 9.0.1. Forty additional landmarks were placed within each foot to represent internal joint centers of 20 mid- and hindfoot joints. Best-fit models for each joint center were generated using stepwise regressions on the training set and evaluated on the validation set. Regression models showed strong predictive performance for all mid- and hindfoot joints based on external skin markers, with r2 values ranging from 0.79 to 0.98 and generally low standard errors of estimate (X: r2 = 0.91-0.99; SEE: 0.002-0.6 mm; Y: r2 = 0.93-0.99; SEE: 0.004-0.6 mm; Z: r2 = 0.70-0.96, SEE: 0.02-0.5 mm). These predictive equations thus provide a practical tool for researchers aiming to develop and validate detailed musculoskeletal models of the foot.
Correct identification of Rhea americana and Rhea pennata in the fossil record has important paleoenvironmental implications; however, the strong similarity between their postcranial skeletons complicates taxonomic assignments. This study analyzes hindlimb morphology in both species to identify diagnostic characters useful for fossil and archaeological material and to reassess previously reported remains. Femora, tibiotarsi, and tarsometatarsi of adult and juvenile R. americana and R. pennata, comprising a total of 37 specimens, were comparatively analyzed using qualitative morphology and linear measurements. Several osteological differences were identified, particularly in distal tibiotarsal and tarsometatarsal morphology, including trochlear configuration and divergence patterns. Some differences are maintained in juvenile specimens, although ontogenetic variation may complicate identification in certain cases. Reevaluation of fossil and archaeological specimens supported revised taxonomic interpretations for several specimens, whereas poorly preserved or morphologically ambiguous materials were conservatively assigned to Rheidae indet. Although hindlimb bones of R. americana and R. pennata are broadly similar, distal tibiotarsal and especially tarsometatarsal morphology provide useful diagnostic characters. Trochlear configuration and divergence appear to be among the most consistent features, whereas other traits are affected by ontogenetic, intraspecific, and preservational variation. These results contribute to a more robust framework for identifying fossil rheids and highlight the importance of evaluating multiple characters when dealing with fragmentary material.
Traversodontids are non-mammaliaform cynodonts, herbivorous to omnivorous, that lived during the Triassic. In the Late Triassic, some members of this group increased in body size relative to other cynodont lineages and evolved postcranial adaptations associated with locomotion. The Late Triassic Siriusgnathus niemeyerorum is one such traversodontid. Originally described in 2018 based on cranial material, the species has since been the focus of considerable advances in our understanding of its skull anatomy; however, its postcranial skeleton remains undescribed. The present study examined the postcranial osteology of this traversodontid, focusing on the pectoral girdle and forelimb of the specimen CAPPA/UFSM 0109, paratype of the species. These remains show that Si. niemeyerorum presents a scapulocoracoid with concave anterior and posterior margins and a lateroventrally oriented glenoid fossa formed by the scapula and coracoid; a humerus with a short diaphysis and a broadly open deltopectoral crest; an ulna with a developed olecranon process; and a manus with a phalangeal formula of 2-3-3-3-3. One of the distal carpals (dc1) is greatly elongated and morphologically similar to a metacarpal, a feature observed in some gomphodonts. The combination of traits present in the scapulocoracoid (glenoid) and the robustness of the humerus suggests a lateralized ("sprawling") forelimb posture in Si. niemeyerorum, similar to that inferred for Exaeretodon argentinus and Exaeretodon riograndensis. In summary, this study expands current knowledge of traversodontid postcranial anatomy and provides new insights into the locomotor adaptations among non-mammaliaform cynodonts.
Diffusible iodine-based contrast-enhanced microCT (DiceCT) enables three-dimensional visualization of mineralized and soft tissues while preserving their spatial relationships in situ. We present a DiceCT-based digital atlas of a human hand from a consented female donor through the University of Missouri Gift of Body program, scanned at 48.8 μm resolution following Lugol's iodine staining. Bones, tendons, intrinsic muscles, neurovascular structures, the flexor retinaculum and carpal tunnel, and dorsal digital expansions were manually segmented to generate labeled multiplanar sections and three-dimensional reconstructions. The dataset resolves epidermal ridge detail on the palmar surface while capturing structures, including the carpal tunnel contents, extensor mechanism, neurovasculature, palmar fat pads, and metacarpophalangeal sesamoids. Reconstructions demonstrate relevant relationships among the median nerve, flexor tendons, and flexor retinaculum, the ulnar nerve within Guyon's canal and the radial artery within the anatomical snuffbox. Distal digital arterial anastomoses are visible near the terminal tufts, and radial artery branches supplying the dorsal and distal scaphoid poles provide context for scaphoid vascular vulnerability. Muscle volumes and physiological cross-sectional areas were calculated using all fascicles within each intrinsic muscle. Flexor pollicis brevis and adductor pollicis exhibited comparatively large relative physiological cross-sectional areas, whereas the lumbricals had the smallest values, consistent with previous architectural estimates. By documenting whole-hand anatomy and within-individual muscle architecture non-destructively, this atlas provides a high-fidelity resource for anatomical education, documentation of anatomical variation, clinical interpretation, and generation of anatomically consistent biomechanical models. These results support DiceCT as a platform bridging anatomical research, clinical translation, and pedagogical access to donor-specific human anatomy.
Kromdraai, South Africa is the type locality of Paranthropus robustus. Renewed paleontological exploration of the site has yielded over 50 hominin fossils since 2014, including 21 postcranial elements. These finds nearly triple the number of postcranial remains previously known from Kromdraai. We describe here four partial humeri, five manual phalanges, two femora, two patellae, and eight pedal elements. Twenty of the fossils derive from Unit P, which dates to over 2.0 million years old and includes craniodental material from Paranthropus and Homo. One fossil, a patella, may derive from the stratigraphically older Unit O. Interestingly, while more than half of the craniodental remains from Unit P are identified as juvenile, all of the postcrania are adult. Furthermore, though none of the postcrania are directly associated with craniodental finds, we tentatively propose taxonomic assignments for some of these fossils, including two humeri and two femora that may belong to Paranthropus.
The aim of this study was to conduct a comparative evaluation of the histological and histochemical organization of the esophagus in two avian species with different feeding ecologies. A total of 6 individuals were used for this purpose (3 storks, 3 common buzzards). The esophageal samples were examined using Crossmon's triple stain, PAS with Light green combination, Alcian blue/PAS (pH 1, pH 2.5) and Aldehyde fuchsin/Alcian blue (AF/AB) staining methods following routine histological processing. In the histological evaluation, it was observed that the esophagus in both species typically consists of the mucosa, submucosa, muscularis, and adventitia/serosa layers. Significant differences were identified between the species in terms of epithelial structure and gland distribution. In storks, the mucosal folds were more distinct, and the glands were more densely distributed. In common buzzards, the epithelial structure was relatively more compact, and the distribution of the glands was limited. Histochemical analyses revealed significant inter-species variations in mucus content. PAS and AB/PAS staining showed that neutral, acidic, and mixed mucopolysaccharides coexist in the stork esophagus, whereas more limited and distinct distribution patterns were observed in common buzzards. AF/AB staining, on the other hand, revealed differences between species in the localization of sulfated and carboxylated mucopolysaccharides. The findings support the notion that the histological and histochemical characteristics of the avian esophagus are closely related to the species' feeding habits. In particular, differences in mucus composition and gland organization are thought to be important in terms of adaptive mechanisms for food transport and esophageal protection. This study contributes to the literature by providing comparative data on the structural differences of the esophagus in avian species with different trophic characteristics.
Feeding is a vital activity for all heterotrophic organisms, as life cannot be sustained without a continuous intake of energy. Survival and reproduction are directly dependent on access to food resources and consequently the need to obtain food is a strong selective pressure in evolution, driving adaptations in anatomy, physiology, and behavior. In this Special Issue of The Anatomical Record, we aim to highlight the benefits of a comparative approach beyond taxonomic borders to provide insights into evolutionary convergence or divergence and to deepen our understanding of the feeding biomechanics. It compiles original works that focus on the anatomy, functional morphology, and feeding biomechanics across diverse taxa ranging from insects to fish and mammals. They collectively address different components of the feeding system such as oropharyngeal mechanics, tooth function, bite force, masticatory musculature, craniomandibular morphology, and dental microwear patterns, applying both experimental and modeling techniques. Together, these papers demonstrate how comparative and interdisciplinary approaches reveal both convergent and lineage-specific solutions to biomechanical challenges associated with feeding. Beyond providing novel insights into feeding biology, the contributions also establish methodological frameworks for future research, particularly in the areas of dental microwear texture analysis standardization, digital modeling of elusive species, and quantitative reconstruction of muscle function from skeletal anatomy.
Masticatory muscle architectural properties (i.e., fascicle length [FL] and physiological cross-sectional area [PCSA]) relate to an animal's diet. However, the traditional gross dissection approach to evaluating them destroys the three-dimensional relationships between and within muscles. Recent advances in three-dimensional analytical approaches, particularly diffusible iodine-based contrast-enhanced computed tomography (DiceCT), have been used to evaluate, in primates, both traditional and in situ variables, including tortuosity and fascicular orientation-which can inform the interpretation of a muscle's position on the length-tension curve and its vector of pull, respectively. To evaluate this method's applicability in carnivorans, we used DiceCT to analyze the masticatory musculature of seven dietarily diverse members of the superfamily Musteloidea. Digitally reconstructed muscle masses corresponded to gross dissection values from conspecifics in all but one specimen, a discrepancy we attribute to intraspecific body size variation. Digitally derived FL and PCSA values also broadly agreed with previously reported dissection-based values. No architectural variables (FL, PCSA, or muscle mass) showed consistent correlations with diet in this small sample. As seen in primates, fascicular orientation revealed an anterior-vertical-posterior division among musteloid masseteric components. Tortuosity values also displayed functionally coherent trends: abductors exhibited the lowest tortuosity at occlusion, while the temporalis components showed the highest, aligning with hypothesized gape-dependent activation sequences, for example, the "triplet" motor pattern. These findings demonstrate that DiceCT reliably reproduces traditional architectural variables while revealing additional aspects of muscle structure that relate to important functional activation patterns supporting the broader utility of DiceCT for comparative musculoskeletal research across Mammalia.
Despite being a highly advantageous trait, tympanic hearing was secondarily lost multiple times within tetrapods. Typically, loss of a sensory structure implies a shift to new environments where selection for another trait inhibits the development of said sensory structure, or a relaxation of selection on the sensory organ. However, the loss of tympanic hearing within North American sand lizards (Phrynosomatidae: Callisaurini) has eluded explanation. To explore the evolutionary reduction and loss of tympanic hearing within this clade, we created 3D models of the stapes to analyze morphological differences. Then, we determined if those morphological differences impact stapedial movement using harmonic response analysis (HRA). We also included the stapes of the callisaurin sand lizards along with those of a diversity of other squamatans in a series of linear discriminant analyses to determine if stapes of tympanic and atympanic species differ morphologically, and if the presence or absence of limbs, lifestyle, or diet influence stapedial morphology. We found that both earless taxa (Cophosaurus, Holbrookia) possess stapes that are relatively short with a large footplate, similar to those of other atympanic squamatans, while the eared taxon Uma has a long and slender stapes, similar to those of other tympanic squamatans. However, the eared taxon Callisaurus possesses a stapes that is intermediate between those of Uma and the atympanic species. Similarly, it was less prone to displacement within the HRA than other eared species. We found that neither lifestyle nor diet could explain differences in ear morphology within this clade. This suggests that ear loss in this clade is either due to a relaxation of selection on the ear, or more likely, differences in substrate use when the lizards bury themselves resulted in selection on the ear that influences both stapedial shape and tympanum loss, perhaps for the transmission of low-frequency vibrations through substrate. The combination of a tympanum with a broad stapes in Callisaurus, in the context of the other sand lizard genera, suggests that modification to the stapes and quadrate may precede the formation or loss of a tympanum, which has implications for interpreting the evolutionary origins of hearing from fossils.
Assimilation of coccygeal vertebrae with the sacrum can be an obstetrical impediment in humans. This study evaluates homeotic transformation of the thoracic-lumbar (T-L) and lumbar-sacral (L-S) boundaries and position of the diaphragmatic vertebra (DV) as proximate determinants of this assimilation. Sample is 311 females and 791 males, ages 20-49 years, who died in the United States from the 19th to 21st centuries. Results show 33% of females and 42% of males have fusion of coccygeal vertebrae with the sacrum. In both sexes, cranial transformation of the T-L and L-S boundaries is associated with assimilation of coccygeal vertebrae with the sacrum, whereas caudal transformation of these boundaries is associated with inhibition of this assimilation. Males, but not females, also show assimilation of coccygeal vertebrae with the sacrum with cranial transformation of DV, and inhibition of this assimilation with caudal transformation of DV. Females have higher prevalence of caudal transformation of the T-L and L-S boundaries than males (15% and 7%, respectively), and males have higher prevalence of cranial transformation of these boundaries than females (22% and 11%, respectively). Assimilation of coccygeal vertebrae with the sacrum contracts posterior space of the pelvic outlet. Dual contraction of posterior and anterior spaces of the outlet can be obstetrically perilous. High prevalence of assimilation of coccygeal vertebrae with the sacrum contributes to selection for sexual dimorphism of the pubic arch and angulation of the sacrum, with females larger than males.
