A regulatory variant impacting TBX1 expression contributes to basicranial morphology in Homo sapiens.

Changes in gene regulatory elements play critical roles in human phenotypic divergence. However, identifying the base-pair changes responsible for the distinctive morphology of Homo sapiens remains challenging. Here, we report a noncoding single-nucleotide polymorphism (SNP), rs41298798, as a potential causal variant contributing to the morphology of the skull base and vertebral structures found in Homo sapiens. Screening for differentially regulated genes between Homo sapiens and extinct relatives revealed 13 candidate genes associated with basicranial development, with TBX1, implicated in DiGeorge syndrome, playing a pivotal role. Epigenetic markers and in silico analyses prioritized rs41298798 within a TBX1 intron for functional validation. CRISPR editing revealed that the 41-base-pair region surrounding rs41298798 modulates gene expression at 22q11.21. The derived allele of rs41298798 acts as an allele-specific enhancer mediated by E2F1, resulting in increased TBX1 expression levels compared to the ancestral allele. Tbx1-knockout mice exhibited skull base and vertebral abnormalities similar to those seen in DiGeorge syndrome. Phenotypic differences associated with TBX1 deficiency are observed between Homo sapiens and Neanderthals (Homo neanderthalensis). In conclusion, the regulatory divergence of TBX1 contributes to the formation of skull base and vertebral structures found in Homo sapiens.

Tail-propelled aquatic locomotion in a theropod dinosaur.

In recent decades, intensive research on non-avian dinosaurs has strongly suggested that these animals were restricted to terrestrial environments1. Historical proposals that some groups, such as sauropods and hadrosaurs, lived in aquatic environments2,3 were abandoned decades ago4-6. It has recently been argued that at least some of the spinosaurids-an unusual group of large-bodied theropods of the Cretaceous era-were semi-aquatic7,8, but this idea has been challenged on anatomical, biomechanical and taphonomic grounds, and remains controversial9-11. Here we present unambiguous evidence for an aquatic propulsive structure in a dinosaur, the giant theropod Spinosaurus aegyptiacus7,12. This dinosaur has a tail with an unexpected and unique shape that consists of extremely tall neural spines and elongate chevrons, which forms a large, flexible fin-like organ capable of extensive lateral excursion. Using a robotic flapping apparatus to measure undulatory forces in physical models of different tail shapes, we show that the tail shape of Spinosaurus produces greater thrust and efficiency in water than the tail shapes of terrestrial dinosaurs and that these measures of performance are more comparable to those of extant aquatic vertebrates that use vertically expanded tails to generate forward propulsion while swimming. These results are consistent with the suite of adaptations for an aquatic lifestyle and piscivorous diet that have previously been documented for Spinosaurus7,13,14. Although developed to a lesser degree, aquatic adaptations are also found in other members of the spinosaurid clade15,16, which had a near-global distribution and a stratigraphic range of more than 50 million years14, pointing to a substantial invasion of aquatic environments by dinosaurs.

Regionalization of the vertebral column and its correlation with heart position in snakes: Implications for evolutionary pathways and morphological diversification.

Spinal regionalization has important implications for the evolution of vertebrate body plans. We determined the variation in the number and morphology of vertebrae across the vertebral column (i.e., vertebral formula) for 63 snake species representing 13 families using intracolumnar variation in vertebral shape. Vertebral counts were used to determine the position of the heart, pylorus, and left kidney for each species. Across all species we observed a conspicuous midthoracic transition in vertebral shape, indicating four developmental domains of the precloacal vertebral column (cervical, anterior thoracic, posterior thoracic, and lumbar). Using phylogenetic analyses, the boundary between the anterior and posterior thoracic vertebrae was correlated with heart position. No associations were found between shifts in morphology of the vertebral column and either the pylorus or left kidney. We observed that among taxa, the number of preapex and postapex vertebrae could change independently from one another and from changes in the total number of precloacal vertebrae. Ancestral state reconstruction of the preapex and postapex vertebrae illustrated several evolutionary pathways by which diversity in the vertebral column and heart position have been attained. In addition, no conspicuous pattern was observed among the heart, pylorus, or kidney indicating that their relative positions to each other evolve independently. We conclude that snakes exhibit four morphologically distinct regions of the vertebral column. We discuss the implications of the forebody and hindbody vertebral formula on the morphological diversification of snakes.

The influence of the land-to-sea macroevolutionary transition on vertebral column disparification in Pinnipedia.

The repeated returns of vertebrates to the marine ecosystems since the Triassic serve as an evolutionary model to understand macroevolutionary change. Here we investigate the effects of the land-to-sea transition on disparity and constraint of the vertebral column in aquatic carnivorans (Carnivora; Pinnipedia) to assess how their functional diversity and evolutionary innovations influenced major radiations of crown pinnipeds. We use three-dimensional geometric morphometrics and multivariate analysis for high-dimensional data under a phylogenetic framework to quantify vertebral size and shape in living and extinct pinnipeds. Our analysis demonstrates an important shift in vertebral column evolution by 10-12 million years ago, from an unconstrained to a constrained evolutionary scenario, a point of time that coincides with the major radiation of crown pinnipeds. Moreover, we also demonstrate that the axial skeleton of phocids and otariids followed a different path of morphological evolution that was probably driven by their specialized locomotor strategies. Despite this, we found a significant effect of habitat preference (coastal versus pelagic) on vertebral morphology of crown taxa regardless of the family they belong. In summary, our analysis provides insights into how the land-to-sea transition influenced the complex evolutionary history of pinniped vertebral morphology.

Origins of mammalian vertebral function revealed through digital bending experiments.

Unravelling the functional steps that underlie major transitions in the fossil record is a significant challenge for biologists owing to the difficulties of interpreting functional capabilities of extinct organisms. New computational modelling approaches provide exciting avenues for testing function in the fossil record. Here, we conduct digital bending experiments to reconstruct vertebral function in non-mammalian synapsids, the extinct forerunners of mammals, to provide insights into the functional underpinnings of the synapsid-mammal transition. We estimate range of motion and stiffness of intervertebral joints in eight non-mammalian synapsid species alongside a comparative sample of extant tetrapods, including salamanders, reptiles and mammals. We show that several key aspects of mammalian vertebral function evolved outside crown Mammalia. Compared to early diverging non-mammalian synapsids, cynodonts stabilized the posterior trunk against lateroflexion, while evolving axial rotation in the anterior trunk. This was later accompanied by posterior sagittal bending in crown mammals, and perhaps even therians specifically. Our data also support the prior hypothesis that functional diversification of the mammalian trunk occurred via co-option of existing morphological regions in response to changing selective demands. Thus, multiple functional and evolutionary steps underlie the origin of remarkable complexity in the mammalian backbone.

Regionalization and morphological integration in the vertebral column of Eurasian small-bodied newts (Salamandridae: Lissotriton).

Serially homologous structures may have complex patterns of regionalization and morphological integration, influenced by developmental Hox gene expression and functional constraints. The vertebral column, consisting of a number of repeated, developmentally constrained, and highly integrated units-vertebrae-is such a complex serially homologous structure. Functional diversification increases regionalization and modularity of the vertebral column, particularly in mammals. For salamanders, three concepts of regionalization of the vertebral column have been proposed, recognizing one, two, or three presacral regions. Using three-dimensional geometric morphometrics on vertebra models acquired with microcomputerized tomography scanning, we explored the covariation of vertebrae in four closely related taxa of small-bodied newts in the genus Lissotriton. The data were analyzed by segmented linear regression to explore patterns of vertebral regionalization and by a two-block partial least squares method to test for morphological integration. All taxa show a morphological shift posterior to the fifth trunk vertebra, which corresponds to the two-region concept. However, morphological integration is found to be strongest in the mid-trunk. Taken jointly, these results indicate a highly integrated presacral vertebral column with a subtle two-region differentiation. The results are discussed in relation to specific functional requirements, developmental and phylogenetic constraints, and specific requirements posed by a biphasic life cycle and different locomotor modes (swimming vs. walking). Further research should be conducted on different ontogenetic stages and closely related but ecologically differentiated species.

Hidden morphological diversity among early tetrapods.

Phylogenetic analysis of early tetrapod evolution has resulted in a consensus across diverse data sets in which the tetrapod stem group is a relatively homogenous collection of medium- to large-sized animals showing a progressive loss of 'fish' characters as they become increasingly terrestrial, whereas the crown group demonstrates marked morphological diversity and disparity. The oldest fossil attributed to the tetrapod crown group is the highly specialized aïstopod Lethiscus stocki, which shows a small size, extreme axial elongation, loss of limbs, spool-shaped vertebral centra, and a skull with reduced centres of ossification, in common with an otherwise disparate group of small animals known as lepospondyls. Here we use micro-computed tomography of the only known specimen of Lethiscus to provide new information that strongly challenges this consensus. Digital dissection reveals extremely primitive cranial morphology, including a spiracular notch, a large remnant of the notochord within the braincase, an open ventral cranial fissure, an anteriorly restricted parasphenoid element, and Meckelian ossifications. The braincase is elongate and lies atop a dorsally projecting septum of the parasphenoid bone, similar to stem tetrapods such as embolomeres. This morphology is consistent in a second aïstopod, Coloraderpeton, although the details differ. Phylogenetic analysis, including critical new braincase data, places aïstopods deep on the tetrapod stem, whereas another major lepospondyl lineage is displaced into the amniotes. These results show that stem group tetrapods were much more diverse in their body plans than previously thought. Our study requires a change in commonly used calibration dates for molecular analyses, and emphasizes the importance of character sampling for early tetrapod evolutionary relationships.

Variation in vertebrae shape across small-bodied newts reveals functional and developmental constraints acting upon the trunk region.

The salamander vertebral column is largely undifferentiated with a series of more or less uniform rib-bearing presacral vertebrae traditionally designated as the trunk region. We explored regionalization of the salamander trunk in seven species and two subspecies of the salamander genus Lissotriton by the combination of microcomputed tomography scanning and geometric morphometrics. The detailed information on trunk vertebral shape was subjected to a multidimensional cluster analysis and a phenotypic trajectory analysis. With these complementary approaches, we observed a clear morphological regionalization. Clustering analysis showed that the anterior trunk vertebrae (T1 and T2) have distinct morphologies that are shared by all taxa, whereas the subsequent, more posterior vertebrae show significant disparity between species. The phenotypic trajectory analysis revealed that all taxa share a common pattern and amount of shape change along the trunk region. Altogether, our results support the hypothesis of a conserved anterior-posterior developmental patterning which can be associated with different functional demands, reflecting (sub)species' and, possibly, regional ecological divergences within species.

Postcranial skeletal pneumaticity in non-aquatic neoavians: Insights from accipitrimorphae.

Postcranial skeletal pneumaticity, air-filled bones of the trunk and limbs, is exclusive to birds among extant tetrapods and exhibits significant variation in its expression among different species. Such variation is not random but exhibits relationships with both body mass and locomotor specializations. Most species-level comparative research to date has focused on aquatic-oriented taxa (e.g., Anseriformes). The lack of data from non-aquatic birds constrains our ability to characterize global (i.e., avian-wide) patterns of this trait complex. To address this gap, the study conducted herein quantified postcranial pneumaticity in Accipitrimorphae, a mostly terrestrial clade composed of species that span a range of body sizes and exhibit diverse flight/foraging behaviors. All examined species (n = 88) invariably pneumatized the postaxial through pre-caudal vertebrae, sternum, coracoid, humerus, vertebral and sternal ribs, and pelvic girdle, a pattern herein referred to as the accipitrimorph baseline. Of the 88 sampled species, 41 expanded upon this pattern, whereas 10 species exhibited a reduction. No species deviated from the accipitrimorph baseline by more than two anatomical regions. A phylogenetically-informed regression analysis failed to identify a significant relationship between body mass and pneumaticity. However, specific pneumaticity phenotypes deviating from the baseline were correlated with aspects of wing morphology, tail length, and home range size. Results from this and previous studies provide clarity on two hypotheses: (1) aquatic taxa display distinct pneumaticity expression patterns relative to non-aquatic birds, notably with reductions in the proportion of the skeleton filled with air in diving specialists and (2) contemporary comparative studies, including the one herein, that explicitly account for phylogenetic relationships consistently fail to support the oft-cited positive relationship between pneumaticity and body mass. Instead, historical relationships and functional/ecological attributes (e.g., diving, specialized flight behaviors) appear to be the primary drivers underlying patterns of variation in this trait complex.

Osteohistology of the hyperelongate hemispinous processes of Amargasaurus cazaui (Dinosauria: Sauropoda): Implications for soft tissue reconstruction and functional significance.

Dicraeosaurid sauropods are iconically characterized by the presence of elongate hemispinous processes in presacral vertebrae. These hemispinous processes can show an extreme degree of elongation, such as in the Argentinean forms Amargasaurus cazaui, Pilmatueia faundezi and Bajadasaurus pronuspinax. These hyperelongated hemispinous processes have been variably interpreted as a support structure for a padded crest/sail as a display, a bison-like hump or as the internal osseous cores of cervical horns. With the purpose to test these hypotheses, here we analyze, for the first time, the external morphology, internal microanatomy and bone microstructure of the hemispinous processes from the holotype of Amargasaurus, in addition to a second dicraeosaurid indet. (also from the La Amarga Formatin; Lower Cretaceous, Argentina). Transverse thin-sections sampled from the proximal, mid and distal portions of both cervical and dorsal hemispinous processes reveal that the cortical bone is formed by highly vascularized fibrolamellar bone interrupted with cyclical growth marks. Obliquely oriented Sharpey's fibres are mostly located in the medial and lateral portions of the cortex. Secondary remodelling is evidenced by the presence of abundant secondary osteons irregularly distributed within the cortex. Both anatomical and histological evidence does not support the presence of a keratinized sheath (i.e. horn) covering the hyperelongated hemispinous processes of Amargasaurus, and either, using a parsimonious criterium, in other dicraeosaurids with similar vertebral morphology. The spatial distribution and relative orientation of the Sharpey's fibres suggest the presence of an important system of interspinous ligaments that possibly connect successive hemispinous processes in Amargasaurus. These ligaments were distributed along the entirety of the hemispinous processes. The differential distribution of secondary osteons indicates that the cervical hemispinous processes of Amargasaurus were subjected to mechanical forces that generated higher compression strain on the anterior side of the elements. Current data support the hypothesis for the presence of a 'cervical sail' in Amargasaurus and other dicraeosaurids.

Regional differences in vertebral shape along the axial skeleton in caecilians (Amphibia: Gymnophiona).

Caecilians are elongate, limbless and annulated amphibians that, as far as is known, all have an at least partly fossorial lifestyle. It has been suggested that elongate limbless vertebrates show little morphological differentiation throughout the postcranial skeleton. However, relatively few studies have explored the axial skeleton in limbless tetrapods. In this study, we used µCT data and three-dimensional geometric morphometrics to explore regional differences in vertebral shape across a broad range of caecilian species. Our results highlight substantial differences in vertebral shape along the axial skeleton, with anterior vertebrae being short and bulky, whereas posterior vertebrae are more elongated. This study shows that despite being limbless, elongate tetrapods such as caecilians still show regional heterogeneity in the shape of individual vertebrae along the vertebral column. Further studies are needed, however, to understand the possible causes and functional consequences of the observed variation in vertebral shape in caecilians.

Phenotypic regionalization of the vertebral column in the thorny skate Amblyraja radiata: Stability and variation.

Regionalization of the vertebral column occurred early during vertebrate evolution and has been extensively investigated in mammals. However, less data are available on vertebral regions of crown gnathostomes. This is particularly true for batoids (skates, sawfishes, guitarfishes, and rays) whose vertebral column has long been considered to be composed of the same two regions as in teleost fishes despite the presence of a synarcual. However, the numerous vertebral units in chondrichthyans may display a more complex regionalization pattern than previously assumed and the intraspecific variation of such pattern deserves a thorough investigation. In this study, we use micro-computed tomography (µCT) scans of vertebral columns of a growth series of thorny skates Amblyraja radiata to provide the first fine-scale morphological description of vertebral units in a batoids species. We further investigate axial regionalization using a replicable clustering analysis on presence/absence of vertebral elements to decipher the regionalization of the vertebral column of A. radiata. We identify four vertebral regions in this species. The two anteriormost regions, named synarcual and thoracic, may undergo strong developmental or functional constraints because they display stable patterns of shapes and numbers of vertebral units across all growth stages. The third region, named hemal transitional, is characterized by high inter-individual morphological variation and displays a transition between the monospondylous (one centrum per somite) to diplospondylous (two centra per somite) conditions. The posteriormost region, named caudal, is subdivided into three sub-regions with shapes changing gradually along the anteroposterior axis. These regionalized patterns are discussed in light of ecological habits of skates.

Three-dimensional topology optimization model to simulate the external shapes of bone.

Elucidation of the mechanism by which the shape of bones is formed is essential for understanding vertebrate development. Bones support the body of vertebrates by withstanding external loads, such as those imposed by gravity and muscle tension. Many studies have reported that bone formation varies in response to external loads. An increased external load induces bone synthesis, whereas a decreased external load induces bone resorption. This relationship led to the hypothesis that bone shape adapts to external load. In fact, by simulating this relationship through topology optimization, the internal trabecular structure of bones can be successfully reproduced, thereby facilitating the study of bone diseases. In contrast, there have been few attempts to simulate the external structure of bones, which determines vertebrate morphology. However, the external shape of bones may be reproduced through topology optimization because cells of the same type form both the internal and external structures of bones. Here, we constructed a three-dimensional topology optimization model to attempt the reproduction of the external shape of teleost vertebrae. In teleosts, the internal structure of the vertebral bodies is invariable, exhibiting an hourglass shape, whereas the lateral structure supporting the internal structure differs among species. Based on the anatomical observations, we applied different external loads to the hourglass-shaped part. The simulations produced a variety of three-dimensional structures, some of which exhibited several structural features similar to those of actual teleost vertebrae. In addition, by adjusting the geometric parameters, such as the width of the hourglass shape, we reproduced the variation in the teleost vertebrae shapes. These results suggest that a simulation using topology optimization can successfully reproduce the external shapes of teleost vertebrae. By applying our topology optimization model to various bones of vertebrates, we can understand how the external shape of bones adapts to external loads.

Why a Constant Number of Vertebrae? Digital Control of Segmental Identity during Vertebrate Development: The Somite Cycle Controls a Digital, Chromatin-Based Counter That Defines Segmental Identity and Body Plans in Vertebrate Animals.

It is not understood how the numbers and identities of vertebrae are controlled during mammalian development. The remarkable robustness and conservation of segmental numbers may suggest the digital nature of the underlying process. The study proposes a mechanism that allows cells to obtain and store the segmental information in digital form, and to produce a pattern of chromatin accessibility that in turn regulates Hox gene expression specific to the metameric segment. The model requires that a regulatory element be present such that the number of occurrences of the motif between two consecutive Hox genes equals the number of segments under the control of the anterior gene. This is true for the recently discovered hydroxyl radical cleavage 3bp-periodic (HRC3) motif, associated with histone modifications and developmental genes. The finding not only allows the correct prediction of the numbers of segments using only sequence information, but also resolves the 40-year-old enigma of the function of temporal and spatial collinearity of Hox genes. The logic of the mechanism is illustrated in the attached animated video. How different aspects of the proposed mechanism can be tested experimentally is also discussed.

Characteristics of the sagittal spinal balance in the asymptomatic elderly Chinese population.

PURPOSE: To investigate the normal values of the sagittal spinal parameters and analyze the distribution of the global spinal profiles in a Chinese population with age over 75. METHODS: Two hundred and twelve sets of the whole spine lateral radiographs were obtained from a database of an asymptomatic elderly population. Global and regional spinal parameters were measured. Sagittal profiles were determined according to the Roussouly classification compared with previous studies involving different populations. RESULTS: A total of 102 elderly subjects (≥ 75 years) were enrolled with an average age of 79.24 ± 3.53 years. The mean values of the spinopelvic parameters were 42.89 ± 11.64° for TK, 13.84 ± 10.78° for TLK, 44.48 ± 12.88° for LL, 44.76 ± 9.84° for PI, 17.19 ± 8.08° for PT, 28.35 ± 7.94° for SS, 3.47 ± 3.56 cm for SVA, 14.75 ± 7.85° for TPA, -0.27 ± 11.95° for PI-LL, respectively. Subjects ≥ 75 years were found to have significantly smaller LL and SS, but greater TLK, PT, SVA, TPA, and PI-LL than those 60-74 years (p < 0.05). Significant age- and sex-dependent differences were found in the Roussouly classification's distribution between the two subgroups. CONCLUSIONS: The normal values of the sagittal parameters were presented in the elderly Chinese asymptomatic population (≥ 75 years). Ethnic, age, and sex displayed significant effects on the behaviors of the sagittal spinal balance and profiles. These results could be served as physiological references for the planning of surgical strategies in elderly Chinese patients over 75 years.

Vertebral pneumaticity of the North American therizinosaur Nothronychus.

Nothronychus was a large, derived therizinosaur from the Upper Cretaceous of Utah and New Mexico. The genus is known from elements that have been referred to single individuals. Therizinosaurs were unusual maniraptoran theropods close to the origin of birds. The axial skeleton is extensively pneumatized, but CT scans reveal an apneumatic synsacrum. Inferred air sacs invade the basicranium, the presacral vertebrae, and the proximal caudal vertebrae, but bypassed the sacrum resulting in a caudosacral hiatus similar to some sauropods and reflecting the development of multiple diverticula from the abdominal air sac. The vertebral pneumatic chambers are described here and compared with those observed in the theropod Allosaurus and the recent avian Dinornis. The vertebrae of Nothronychus are intermediate between those two theropods. It is inferred to have possessed avian-like abdominal air sacs. This theropod would have had unidirectional lungs, as in birds, but this character cannot be related to endothermy.

Morphometry of the kangaroo spine and its comparison with human spinal data.

The upright posture of the kangaroo suggests that the spine of the kangaroo could be a possible substitute model for biomechanical studies of the human spine. A prerequisite for this should be the agreement of anatomy in humans and kangaroos. The purpose of this study was to investigate the anatomical parameters of the kangaroo spine from C4 to S4 and compare them with existing anatomical data of the human spine. Eight complete spines of the red giant kangaroo were obtained and 21 anatomical parameters were measured from the vertebral bodies, spinal canal, endplate, pedicles, intervertebral discs, transverse, and spinous processes. Most similarities between kangaroo and human spines were found for the vertebral bodies in the cervical and the lumbar spine. The largest differences were evident for the spinous processes. Although both species are somehow upright, these differences may be explained by the way how they move. Jumping probably requires more muscle strength than walking on two legs.

Bendy to the bone: Links between vertebral morphology and waterfall climbing in amphidromous gobioid fishes.

Locomotor force production imposes strong demands on organismal form. Thus, the evolution of novel locomotor modes is often associated with morphological adaptations that help to meet those demands. In the goby lineage of fishes, most species are marine and use their fused pelvic fins to facilitate station holding in wave-swept environments. However, several groups of gobies have evolved an amphidromous lifecycle, in which larvae develop in the ocean but juveniles migrate to freshwater for their adult phase. In many of these species, the pelvic fins have been co-opted to aid in climbing waterfalls during upstream migrations to adult habitats. During horizontal swimming, forces are produced by axial musculature pulling on the vertebral column. However, during vertical climbing, gravity also exerts forces along the length of the vertebral column. In this study, we searched for novel aspects of vertebral column form that might be associated with the distinctive locomotor strategies of climbing gobies. We predicted that stiffness would vary along the length of the vertebral column due to competing demands for stability of the suction disk anteriorly and flexibility for axial thrust production posteriorly. We also predicted that derived, climbing goby species would require stiffer backbones to aid in vertical thrust production compared to non-climbing species. To test these predictions, we used microcomputed tomography scans to compare vertebral anatomy (centrum length, centrum width, and intervertebral space) along the vertebral column for five gobioid species that differ in climbing ability. Our results support our second prediction, that gobies are more flexible in the posterior portion of the body. However, the main variation in vertebral column form associated with climbing ability was the presence of larger intervertebral spaces in Sicyopterus stimpsoni, a species that uses a distinctive inching behavior to climb. These results build on past kinematic studies of goby climbing performance and lend insights into how the underlying vertebral structure of these fishes may enable their novel locomotion.

Development and evolution of the notarium in Pterosauria.

The notarium is the structure formed by fusion of the dorsal vertebrae which occurred independently in pterosaurs and birds. This ankylosis usually involves two to six elements and in many cases, also includes the last cervical vertebra. Fusion can occur in different degrees, uniting the vertebral centra, the neural spines, the transverse processes, the ventral processes, or a combination of these sites. A detailed assessment of the fusion process of pterosaur dorsal vertebrae is still lacking. Here we identify the fusion sequence of pterosaur notarial elements, demonstrating the order of ossification in vertebral bodies and neural spines based on fossils and extant birds. In both Pterosauria and Aves, the notarium generally develops in a antero-posterior direction, but the actual order of each fusion locus may present slight variations. Based on our data, we were able to identify seven developmental stages in the notarium formation, with broad implications for the prediction of ontogenetic stages for the Pterosauria. In addition, we report the occurrence of a notarium in Ardeadactylus longicollum (Kimmeridgian, Southern Germany), the oldest occurrence of this structure in pterosaurs.

Multiscale structural characterization of the vertebral endplate in animal models.

Research in the field of spinal biomechanics, including analyses of the impact of implants on the stability of the spine, is conducted extensively in animal models. One of the basic problems in spinal implantation is the transfer and distribution of loads carried by the spine on the surfaces of the vertebral bodies. An important factor in proper cooperation of spinal implants with the vertebrae is the endplate (EP), which is why the EP in the animal model used for testing should be as similar as possible to the human EP. Therefore, this study involved multiscale structural and morphometric analyses of the animal models most commonly used in spinal biomechanics research, i.e. pig, ovine, and bovine tail. The tests were performed on 28 lumbar porcine, ovine, and bovine vertebrae. Both cranial and caudal EPs were analysed in three selected areas: anterior, middle, and posterior EPs. The conducted tests included a morphometric analysis of the trabecular bone (TB) layer of the EP as well as microscopic analysis at the mesoscale (total thickness) and microscale (thickness of the individual EP layers). The porcine EP had a characteristic increased circumferential thickness (~3 mm) with a significant narrowing in the central region (50%-60%). The convex cranial ovine EP had a constant thickness throughout the cross-section and the concave caudal EP showed ~35% narrowing in the central region. The thickest EPs were observed in the bovine tail model with negligibly small narrowing in the central region (~5%). The thickness of the cartilaginous layer in the porcine and bovine models reached up to 1 mm in the peripheral regions and decreased in the central part. The growth plate layer had a similar thickness in all the models. On the other hand, the narrowing of the total thickness of the EPs in the central region was mainly due to a decrease in the VEP thickness. In the ovine and bovine models, the central region of the EP was characterized by large isotropy and trabeculae of mixed or rod-like shape. By contrast, in the pig, this region had plate-like trabeculae of anisotropic nature. The porcine model was identified as best reflecting the shape and structure of the human EP and as the best surrogate model for the human EP model. This choice is particularly important in the context of biomechanical research.