A review of the osteoderms of lizards (Reptilia: Squamata)

Density-dependent colour scanning electron microscopy (DDC-SEM). Applications in the study of calcified tissues and visual impact

Scanning Electron Microscopy (SEM) is widely used as a technique for materials characterization. It has also been successfully applied to the imaging of biological samples, providing invaluable insights into the topography, morphology and composition of biological structures. A particular method combining different SEM detectors, named Density-Dependent Coloured SEM (DDC-SEM), has proven to be most useful for the identification and visualization of minerals in soft tissues. The method consists of a manipulation of original greyscale SEM images to produce coloured images that provide both topography and density information for samples with components of different densities. Here we provide a discussion on how to use DDC-SEM to aid the visualization and intuitive understanding of pathological calcification. This method has become popular not only for its scientific improvement of conventional SEM greyscale images, but also for its aesthetical merits.

Skin Appendage Proteins of Tetrapods: Building Blocks of Claws, Feathers, Hair and Other Cornified Epithelial Structures

Reptiles, birds, mammals and amphibians, together forming the clade tetrapods, have a large diversity of cornified skin appendages, such as scales, feathers, hair and claws. The skin appendages consist of dead epithelial cells that are tightly packed with specific structural proteins. Here, we review the molecular diversity and expression patterns of major types of skin appendage proteins, namely keratin intermediate filament proteins, keratin-associated proteins (KRTAPs) and proteins encoded by genes of the epidermal differentiation complex (EDC), including corneous beta-proteins, also known as beta-keratins. We summarize the current knowledge about the components of skin appendages with a focus on keratins and EDC proteins that have recently been identified in reptiles and birds. We discuss gaps of knowledge and suggest directions of future research.

Osteohistology of the unusually fast-growing theropod dinosaur Ceratosaurus

Ceratosaurus is a large-bodied non-avian theropod dinosaur known from the Upper Jurassic Morrison Formation of North America and is remarkable both for its exceptionally fast annual growth rate and its status as the only theropod currently known with postcranial osteoderms. We describe the osteohistology of three hind limb bones, two dorsal ribs, and one osteoderm representing four individuals of Ceratosaurus. In addition to describing the tissues of these bones, we compared the annual growth rates from three individuals in our sample to those of five other ceratosaurians. We fit seven growth models to two of the specimens in our sample and compared the results of the best-fit model(s) to those of two other ceratosaurians (Masiakasaurus knopfleri and Majungasaurus crenatissimus) for which sufficient growth data were available. The bone tissue of hind limbs in Ceratosaurus is highly vascularized, with dense plexiform or reticular vascular complexes and alternating strips of parallel or woven-fibered matrix. Few lines of arrested growth were recorded in hind limbs prior to specimens achieving asymptotic body size. Both sampled dorsal ribs are highly remodeled, with only small portions of primary bone visible in each section, revealing parallel-fibered bone with sparse primary osteons. Both dorsal ribs contain numerous lines of arrested growth throughout the cortex that allowed for more accurate estimates of individual age when paired with the data from hind limbs. The osteoderm is composed of a core of large Haversian canals and a perimeter of lamellar bone with dense Sharpey's fibers along the internal surface of the bone. Multiple LAGs are also present within the lamellar bone along the exterior margins. Maximum annual growth rates in Ceratosaurus were on average nine-fold faster than those of other ceratosaurians. Our sample lacks data from juveniles so confidence in inferred growth models is limited. Thus, to begin to constrain Ceratosaurus growth patterns, we averaged the results of all models that possessed an Akaike Information Criterion score corrected for small sample size (AICc) within 10 of the lowest scoring model. We found that the monomolecular model exhibited the lowest AICc value, with the von Bertalanffy and Gompertz models possessing AICc values within 10 units of it. In contrast, the logistic and Gompertz models were confidently selected for Masiakasaurus and Majungasaurus, respectively. Irrespective of growth model, maximum relative annual growth rates for Ceratosaurus were several-fold greater than those of Masiakasaurus and Majungasaurus. Both histological and growth model estimates of life history support an evolutionary trend towards more prolonged development in Ceratosauria through evolutionary time.

Hidden Armour: The Passive Protective Function of Caudal Osteoderms in Snakes

Dermal armour, consisting of bony dermal structures known as osteoderms (ODs), is widespread in squamate reptiles. However, in some limbless taxa such as snakes, ODs are rare, probably due to a trade-off between mechanical protection and the demands of locomotion and consumption of large prey. Recent findings of ODs restricted to the distal body regions of sand boas (Eryx, Erycidae) challenge this paradigm, suggesting they provide passive mechanical protection against aggressive prey without significantly impairing locomotion. Building on these findings, we have continued the search and identified three additional snake species that have well-developed caudal ODs, including the first-ever discovery of ODs in shield-tailed snakes (Uropeltidae). In these fossorial species, which are characterised by their unique tail morphology, ecological adaptations and colouration, the ODs at the tail tip may serve as passive protection against predators. However, an alternative role in locomotion or occasional phragmosis cannot be ruled out. In the Javelin sand boa (Eryx jaculus), the ODs are hypothesised to function as a mechanical defence against aggressive prey. These results highlight the functional and evolutionary plasticity of ODs and emphasise the urgent need for further studies on their specific role and adaptive significance in the ecology and evolution of snakes.

Histological Study of Skin Structures From Selected Body Areas in the Varanus komodoensis

The skin of the Komodo dragon (Varanus komodoensis) is covered by a form of armour formed mainly of scales, which often co-occur with osteoderms. Scales are keratinized, non-mineralized structures in the uppermost layer of the epidermis that are in contact with each other to form a system in which individual scales are isolated from each other by a softer skin fold zone. In the Varanus, the surface of the scales is flat and smooth (thoracic limb, abdomen, and tail areas), domed and smooth (head area) or domed with conical ornamentation (dorsal surface, pelvic limb-dorsal surface areas). In contrast, osteoderms are mineralized structures that are an integral part of the skin, located below the epidermal surface and positioned parallel (head, tail, thoracic limb-dorsal surface, thoracic limb-palmar surface, and tail) or obliquely (pelvic limb-dorsal surface, groin, abdomen) to the surface. Regardless of the body region, osteoderms are structures that are completely anchored in the dermis, and their surface is smooth and devoid of ornamentation. Tangential sections of the osteoderms demonstrate concentric resting lines. Histological sections of the varanid dermis show the presence of collagen bundles, parallel interlacing or crossing bundles of collagen fibers of varying thickness and degree of compactness, accompanied by muscle fibers. In the area of skin close to the osteoderm, loosely arranged bundles of collagen fibers are present, while in the zone distal to the osteoderm, a compact arrangement of these fibers is present. This study documents the morphological diversity and distribution of osteoderms and scales in selected areas of the body of V. komodoensis. Scales are characterized by a high polymorphism related to body region, while osteoderms show a high morphological similarity independent of the area of occurrence.

Microscopic structure and immunolabeling of extremely overlapped scales in some scincid, anguid, and pygopod lizards

Skink, anguid, and pygopod lizards possess an extremely flat skin, imparting a compact and solid body and shining surface that facilitates their slider and/or fossorial movements. The present morphological study, conducted using immunohistochemistry and electron microscopy, has analyzed the microscopical morphology of extremely overlapped scales in different lizards, including species with limb reduction (scincids such as Lerista bougainvilli, Scincella lateralis, Lampropholis delicata) or legless (pygopods such as Lialis burtonis and Delma molleri and the anguid Anguis fragilis). The outer surface of the epidermis shows different micro-structures of the Oberhautchen layer containing corneous beta-proteins (CBPs) with variable immunoreactivity for these proteins. The beta-layer is relatively thick in most of these species, probably in relation to the resistance against strong mechanical forces acting on scales during the movements on harsh substrates. The scincid and anguid lizards also possess and regenerate osteoderms that reinforce scales flatness and mechanical resistance during the serpentiform or fossorial movements of these reptiles. Osteoderms are absent in pygopods. Roundish cells with a granular content are detected in the deep hinge region of scales in Lerista and Lampropholis skinks. Whether these cells may secrete substances that facilitate scale anti-friction and also determine shining of the skin surface remains to be shown.

First discovery of actinopterygian cutting-edged teeth from the middle Norian (Late Triassic) at the Tulong section, southern Tibet, China

Actinopterygians (ray-finned fishes) successfully passed through the Permian-Triassic Mass Extinction (PTME) and flourished in the Triassic with diverse feeding specializations and occupation of various trophic levels. Birgeria, one of the largest actinopterygian fish of the Triassic, was characterized by a strong, blunt rostrum and three rows of sharp cutting-edged teeth, making them the top predators in the Early Mesozoic oceanic ecosystem. These fishes rapidly radiated and diversified globally during the Early and Middle Triassic, but the fossil record is rare for the Neo-Tethys in the Late Triassic. Here, we report new actinopterygian teeth with cutting edges from Norian-age strata in the Tulong section, which was located on the northern margin of the Indian Plate at that time. The tooth features, such as the polished acrodin cap, the ratio of the acrodin cap in length, and the tiny vertical striae at the tooth base, suggest an affinity with Birgeria, which is reported in this region for the first time. Furthermore, we infer that the carnivorous Birgeria, which co-occurred with the enigmatic ichthyosaur Himalayasaurus tibetensis, played the role of predator in this part of the Neo-Tethys marine realm during the Late Triassic. These new findings increase the known diversity of actinopterygians during the Late Triassic and provide further insight into the marine fauna of this epoch.

Armed on the back: Hidden biomineralized scales in the ventral girdle of chiton Acanthopleura loochooana

Flexible protective armors are found in large animals such as fish skins, snake skins, and pangolin scales. For small-sized invertebrates, such armors are paid less attention and overlooked. Chitons, a type of marine mollusk, possess mineralized armors covering the whole dorsal body. The dorsal scales in the girdle tissue are well known, in this study, we reported hidden mineralized scales in the ventral side of chiton Acanthopleura loochooana girdles for the first time. The ventral surface is covered with scales with ca. 40 μm in length, forming continuous but overlapped scales. Additionally, scales are formed from aragonitic spicule-like and square-like scales, embedded in the cuticle layer. Nanoindentation testing results showed that the hardness and elastic modulus of ventral scales were ∼20 % higher compared to those in the dorsal scales, exhibiting good hardness and wear resistance. The combination of the ventral scales and cuticle, along with the regular arrangement of ventral scales, may allow chitons to simultaneously address complex and variable attachment interfaces while also providing wear-resistant protection. This study provides insights for designing protective structures that balance flexibility and durability. STATEMENT OF SIGNIFICANCE: Biomineralization is universal in nature and provides protection and support for animals. However, mineralization of dermal skin is not commonly seen. Herein, for the first time, we reported hidden minerals covering the whole ventral side of skin in a small marine animal, chitons. Calcium carbonate minerals are arranged regularly and manifest different morphology in different regions. Additionally, these minerals are embedded in a continuous cuticle layer covering the whole animal. The material also indicates a higher wear-resistant property. This study extends our understanding of the diverse functionality of biominerals and provides a prototype for designing wear-resistant materials.

Comparative analysis of osteoderms across the lizard body

Osteoderms (ODs) are mineralized tissue embedded within the skin and are particularly common in reptiles. They are generally thought to form a protective layer between the soft tissues of the animal and potential external threats, although other functions have been proposed. The aim of this study was to characterize OD variation across the lizard body. Adults of three lizard species were chosen for this study. After whole body CT scanning of each lizard, single ODs were extracted from 10 different anatomical regions, CT scanned, and characterized using sectioning and nanoindentation. Morphological analysis and material characterization revealed considerable diversity in OD structure across the species investigated. The scincid Tiliqua gigas was the only studied species in which ODs had a similar external morphology across the head and body. Greater osteoderm diversity was found in the gerrhosaurid Broadleysaurus major and the scincid Tribolonotus novaeguineae. Dense capping tissue, like that reported for Heloderma, was found in only one of the three species examined, B. major. Osteoderm structure can be surprisingly complex and variable, both among related taxa, and across the body of individual animals. This raises many questions about OD function but also about the genetic and developmental factors controlling OD shape.

Compound osteoderms preserved in amber reveal the oldest known skink

Scincidae is one of the most species-rich and cosmopolitan clades of squamate reptiles. Abundant disarticulated fossil material has also been attributed to this group, however, no complete pre-Cenozoic crown-scincid specimens have been found. A specimen in Burmite (99 MYA) is the first fossil that can be unambiguously referred to this clade. Our analyses place it as nested within extant skinks, supported by the presence of compound osteoderms formed by articulated small ostedermites. The specimen has a combination of dorsal and ventral compound osteoderms and overlapping cycloid scales that is limited to skinks. We propose that this type of osteoderm evolved as a response to an increased overlap of scales, and to reduced stiffness of the dermal armour. Compound osteoderms could be a key innovation that facilitated diversification in this megadiverse family.

Decoupled evolution of ventral and dorsal scales in agamid lizards: ventral keels are associated with arboreality

Arboreality has evolved in all major vertebrate lineages and is often associated with morphological adaptations and increased diversification concomitant with accessing novel niche space. In squamate reptiles, foot, claw, and tail morphology are well-studied adaptations shown to be associated with transitions to arboreality. Here, we examined a less well understood trait-the keeled scale-in relation to microhabitat, climate, and diversification dynamics across a diverse lizard radiation, Agamidae. We found that the ancestral agamid had keeled dorsal but not ventral scales; further, dorsal and ventral keels are evolutionarily decoupled. Ventral keeled scales evolved repeatedly in association with arboreality and may be advantageous in reducing wear or by promoting interlocking when climbing. We did not find an association between keeled scales and diversification, suggesting keels do not allow finer-scale microhabitat partitioning observed in other arboreal-associated traits. We additionally found a relationship between keeled ventral scales and precipitation in terrestrial species where we posit that the keels may function to reduce scale degradation. Our results suggest that keeled ventral scales facilitated transitions to arboreality across agamid lizards, and highlight a need for future studies that explore their biomechanical function in relation to microhabitat and climate.

Living in the mountains: Thermal ecology and freezing tolerance of the lizard Abronia taeniata (Squamata: Anguidae)

The impact of daily and seasonal variation in environmental temperature on lizards is important, since their physiological processes are body temperature dependent. Lizards that occupy mountainous areas must have been favoured to colonize such habitats through selection on thermal biology traits to thermoregulate effectively. Moreover, mountain lizards may be able to maintain their activity near their minimum critical temperature and even have antifreeze mechanisms. Tolerance of freezing is related to the biosynthesis of cryoprotective molecules, such as glucose, whose concentration may increase after freezing. The aims of the present work were: (1) study the thermoregulation of the viviparous lizard Abronia taeniata, and (2) determine its survival and/or tolerance to freezing. This species occurs in pine forests, pine-oak forests, and mountain mesophilic forests in areas that reach freezing temperatures. In the field, we recorded air, substrate, and body temperatures at capture time of the lizards, and registered operative temperatures at the study area. In the laboratory, we determined thermal preferences, crystallization point, and blood glucose levels of individuals before and after freezing. We found out that A. taeniata sustains activity in a wide range of temperatures, actively avoids thermally favourable microhabitats in spring, and is a moderate thermoregulator during autumn and winter. In A. taeniata, the body temperatures are tightly linked to air and substrate temperatures. Seasonality had an effect over body temperature, preferred temperatures and thermoregulatory effectiveness indices. When exposed to temperatures below zero, A. taeniata showed an increase in blood glucose levels, which aided them in surviving freezing. Taken together, our results suggest that A. taeniata may sustain activity at low environmental temperatures, due to an effective behavioural thermoregulation, and in case temperatures of its habitat go below zero, is also capable of tolerate freezing.

The role of cranial osteoderms on the mechanics of the skull in scincid lizards

Osteoderms (ODs) are calcified organs formed directly within the skin of most major extant tetrapod lineages. Lizards possibly show the greatest diversity in ODs morphology and distribution. ODs are commonly hypothesized to function as a defensive armor. Here we tested the hypothesis that cranial osteoderms also contribute to the mechanics of the skull during biting. A series of in vivo experiments were carried out on three specimens of Tiliqua gigas. Animals were induced to bite a force plate while a single cranial OD was strain gauged. A finite element (FE) model of a related species, Tiliqua scincoides, was developed and used to estimate the level of strain across the same OD as instrumented in the in vivo experiments. FE results were compared to the in vivo data and the FE model was modified to test two hypothetical scenarios in which all ODs were (i) removed from, and (ii) fused to, the skull. In vivo data demonstrated that the ODs were carrying load during biting. The hypothetical FE models showed that when cranial ODs were fused to the skull, the overall strain across the skull arising from biting was reduced. Removing the ODs showed an opposite effect. In summary, our findings suggest that cranial ODs contribute to the mechanics of the skull, even when they are loosely attached.

General aspects on skin development in vertebrates with emphasis on sauropsids epidermis

General cellular aspects of skin development in vertebrates are presented with emphasis on the epidermis of sauropsids. Anamniote skin develops into a multilayered mucogenic and soft keratinized epidermis made of Intermediate Filament Keratins (IFKs) that is reinforced in most fish and few anurans by dermal bony and fibrous scales. In amniotes, the developing epidermis in contact with the amniotic fluid initially transits through a mucogenic phase recalling that of their anamniotes progenitors. A new gene cluster termed EDC (Epidermal Differentiation Complex) evolved in amniotes contributing to the origin of the stratum corneum. The EDC contains numerous genes coding for over 100 types of corneous proteins (CPs). In sauropsids 2-8 layers of embryonic epidermis accumulate soft keratins (IFKs) but do not form a compact corneous layer. The embryonic epidermis of reptiles and birds produces small amount of other, poorly known proteins in addition to IFKs and mucins. In the following development, a resistant corneous layer is formed underneath the embryonic epidermis that is shed before hatching. The definitive corneous epidermis of sauropsids is mainly composed of CBPs (Corneous beta proteins, formerly indicated as beta-keratins) derived from the EDC. CBPs belong to a gene sub-family of CPs unique for sauropsids, contain an inner amino acid region formed by beta-sheets, are rich in cysteine and glycine, and make most of the protein composition of scales, claws, beaks and feathers. In mammalian epidermis CPs missing the beta-sheet region are instead produced, and include loricrin, involucrin, filaggrin and various cornulins. Small amount of CPs accumulate in the 2-3 layers of mammalian embryonic epidermis and their appendages, that is replaced with the definitive corneous layers before birth. Differently from sauropsids, mammals utilize KAPs (keratin associated proteins) rich in cysteine and glycine for making the hard corneous material of hairs, claws, hooves, horns, and occasionally also scales.

Pangolin-inspired untethered magnetic robot for on-demand biomedical heating applications

Untethered magnetic miniature soft robots capable of accessing hard-to-reach regions can enable safe, disruptive, and minimally invasive medical procedures. However, the soft body limits the integration of non-magnetic external stimuli sources on the robot, thereby restricting the functionalities of such robots. One such functionality is localised heat generation, which requires solid metallic materials for increased efficiency. Yet, using these materials compromises the compliance and safety of using soft robots. To overcome these competing requirements, we propose a pangolin-inspired bi-layered soft robot design. We show that the reported design achieves heating > 70 °C at large distances > 5 cm within a short period of time <30 s, allowing users to realise on-demand localised heating in tandem with shape-morphing capabilities. We demonstrate advanced robotic functionalities, such as selective cargo release, in situ demagnetisation, hyperthermia and mitigation of bleeding, on tissue phantoms and ex vivo tissues.

Osteoderms in a mammal the spiny mouse Acomys and the independent evolution of dermal armor

Osteoderms are bony plates found in the skin of vertebrates, mostly commonly in reptiles where they have evolved independently multiple times, suggesting the presence of a gene regulatory network that is readily activated and inactivated. They are absent in birds and mammals except for the armadillo. However, we have discovered that in one subfamily of rodents, the Deomyinae, there are osteoderms in the skin of their tails. Osteoderm development begins in the proximal tail skin and is complete 6 weeks after birth. RNA sequencing has identified the gene networks involved in their differentiation. There is a widespread down-regulation of keratin genes and an up-regulation of osteoblast genes and a finely balanced expression of signaling pathways as the osteoderms differentiate. Future comparisons with reptilian osteoderms may allow us to understand how these structures have evolved and why they are so rare in mammals.

Osteoderm Development during the Regeneration Process in Eurylepis taeniolata Blyth, 1854 (Scincidae, Sauria, Squamata)

Osteoderms are bony structures that develop within the dermal layer of the skin in vertebrates and are very often found in different lizard families. Lizard osteoderms are diverse in topography, morphology, and microstructure. Of particular interest are the compound osteoderms of skinks, which are a complex of several bone elements known as osteodermites. We present new data on the development and regeneration of compound osteoderms based on the results of a histological and Computed Microtomography (micro-CT) study of a scincid lizard: Eurylepis taeniolata. The specimens studied are stored in the herpetological collections of the Saint-Petersburg State University and Zoological Institute of the Russian Academy of Sciences located in St. Petersburg, Russia. The topography of osteoderms in the integuments of the original tail area and its regenerated part was studied. A comparative histological description of the original and regenerated osteoderms of Eurylepis taeniolata is presented for the first time. The first description of the development of compound osteoderm microstructure in the process of caudal regeneration is also presented.

Female-female aggression in the Gila monster (Heloderma suspectum)

Historically, the role of aggression in the social lives of animals overwhelmingly focused on males. In recent years, however, female-female aggression in vertebrates, particularly lizards, has received increasing attention. This growing body of literature shows both similarities and differences to aggressive behaviours between males. Here, we document female-female aggression in captive Gila monsters (Heloderma suspectum). Based on four unique dyadic trials (eight adult female subjects), we developed a qualitative ethogram. Unexpected and most intriguing were the prevalence and intensity of aggressive acts that included brief and sustained biting, envenomation, and lateral rotation (i.e. rolling of body while holding onto opponent with closed jaws). Given specific behavioural acts (i.e. biting) and the results of bite-force experiments, we postulate that osteoderms (bony deposits in the skin) offer some degree of protection and reduce the likelihood of serious injury during female-female fights. Male-male contests in H. suspectum, in contrast, are more ritualized, and biting is rarely reported. Female-female aggression in other lizards has a role in territoriality, courtship tactics, and nest and offspring guarding. Future behavioural research on aggression in female Gila monsters is warranted to test these and other hypotheses in the laboratory and field.

The first description of dermal armour in snakes

Osteoderms, also called dermal armour, often play a role in predator defence. The presence of osteoderms is highly irregularly distributed across the squamate phylogeny and they have not been found in snakes. In this study, we searched for candidate snake species that would benefit from such armour to protect their body, focusing primarily on fossorial species with defensive tail displays. We examined the tail morphology of 27 snake species from different families using micro-computed tomography (µCT) and micro- radiography. We discovered dermal armour in four species of sand boas (Erycidae) that also feature enlarged and highly modified caudal vertebrae. This is the first description of dermal armour in snakes. Ancestral state reconstructions revealed that osteoderms likely evolved once or multiple times in Erycidae. We have not found osteoderms in any other examined snake species. Nevertheless, similar structures are known from unrelated squamate clades, such as gerrhosaurids and geckos. This supports the idea of underlying deep developmental homology. We propose the hypothesis that osteoderms protect sand boas like the "brigandine armour" of medieval warriors. We interpret it as another component of the sand boas' rich defence strategy.

A survey of osteoderm histology and ornamentation among Crocodylomorpha: A new proxy to infer lifestyle?

Osteoderms of eight extant and extinct species of crocodylomorphs are studied histologically and morphologically. Most osteoderms display the typical "crocodilian" structure with a woven-fibered matrix surrounded by an upper and a lower parallel fibered matrix. The dorsal ornamentation of those specimens consists of a pit-and-ridge structure, with corresponding remodeling mechanisms. However, an osteoderm of Iberosuchus, studied here for the first time, differs in being nearly devoid of ornamentation; moreover, it shows strong bundles of straight Sharpey's fibers perpendicular to the surface in its lateral and dorsal walls, along with a rough plywood-like structure in its basal plate. This suggests that this osteoderm was more deeply anchored within the dermis than the other osteoderms studied hitherto. This peculiar structure might have been linked to a terrestrial ecology and a specific thermoregulation strategy. Some other notosuchians in our sample do not exhibit ornamentation on their osteoderms, as opposed to neosuchians. Considering current interpretations of osteoderm function(s) in crocodilians, our observations are discussed in reference to possible ecophysiological peculiarities of Notosuchia in general, and Iberosuchus in particular.

Osteohistological characterization of notosuchian osteoderms: Evidence for an overlying thick leathery layer of skin

Osteoderms are mineralized structures embedded in the dermis, known for nonavian archosaurs, squamates, xenarthrans, and amphibians. Herein, we compared the osteoderm histology of Brazilian Notosuchia of Cretaceous age using three neosuchians for comparative purposes. Microanatomical analyses showed that most of them present a diploe structure similar to those of other pseudosuchians, lizards, and turtles. This structure contains two cortices (the external cortex composed of an outer and an inner layers, and the basal cortex) and a core in-between them. Notosuchian osteoderms show high bone compactness (>0.85) with varying degrees of cancellous bone in the core. The neosuchian Guarinisuchus shows the lowest bone compactness with a well-developed cancellous layer. From an ontogenetic perspective, most tissues are formed through periosteal ossification, although the mineralized tissues observed in baurusuchid LPRP/USP 0634 suggest a late metaplastic development. Histology suggests that the ossification center of notosuchian osteoderm is located at the keel. Interestingly, we identified Sharpey's fibers running perpendicularly to the outer layer of the external cortex in Armadillosuchus arrudai, Itasuchus jesuinoi, and Baurusuchidae (LPRP/USP 0642). This feature indicates a tight attachment within the dermis, and it is evidence for the presence of an overlying thick leathery layer of skin over these osteoderms. These data allow a better understanding of the osteohistological structure of crocodylomorph dermal bones, and highlight their structural diversity. We suggest that the vascular canals present in some sampled osteoderms connecting the inner layer of the external cortex and the core with the external surface may increase osteoderm surface and the capacity of heat transfer in terrestrial notosuchians.

Unravelling the structural variation of lizard osteoderms

Vertebrate skin is a remarkable organ that supports and protects the body. It consists of two layers, the epidermis and the underlying dermis. In some tetrapods, the dermis includes mineralised organs known as osteoderms (OD). Lizards, with over 7,000 species, show the greatest diversity in OD morphology and distribution, yet we barely understand what drives this diversity. This multiscale analysis of five species of lizards, whose lineages diverged ∼100-150 million years ago, compared the micro- and macrostructure, material properties, and bending rigidity of their ODs, and examined the underlying bones of the skull roof and jaw (including teeth when possible). Unsurprisingly, OD shape, taken alone, impacts bending rigidity, with the ODs of Corucia zebrata being most flexible and those of Timon lepidus being most rigid. Macroscopic variation is also reflected in microstructural diversity, with differences in tissue composition and arrangement. However, the properties of the core bony tissues, in both ODs and cranial bones, were found to be similar across taxa, although the hard, capping tissue on the ODs of Heloderma and Pseudopus had material properties similar to those of tooth enamel. The results offer evidence on the functional adaptations of cranial ODs, but questions remain regarding the factors driving their diversity. STATEMENT OF SIGNIFICANCE: Understanding nature has always been a significant source of inspiration for various areas of the physical and biological sciences. Here we unravelled a novel biomineralization, i.e. calcified tissue, OD, forming within the skin of lizards which show significant diversity across the group. A range of techniques were used to provide an insight into these exceptionally diverse natural structures, in an integrated, whole system fashion. Our results offer some suggestions into the functional and biomechanical adaptations of OD and their hierarchical structure. This knowledge can provide a potential source of inspiration for biomimetic and bioinspired designs, applicable to the manufacturing of light-weight, damage-tolerant and multifunctional materials for areas such as tissue engineering.

A new Early Cretaceous lizard in Myanmar amber with exceptionally preserved integument

We here report on a well-preserved juvenile lizard specimen in Albian amber (ca. 110 mya) from the Hkamti site (Myanmar). This new taxon is represented by an articulated skull and the anterior portion of the trunk, including the pectoral girdle and forelimbs. The scleral ossicles and eyelid are also visible, and the specimen exhibits pristine detail of the integument (of both head and body). In a combined molecular and morphological analysis, it was consistently recovered as a scincoid lizard (Scinciformata), as sister to Tepexisaurus + Xantusiidae. However, the phylogenetic position of the new taxon should be interpreted with caution as the holotype is an immature individual. We explored the possibility of miscoding ontogenetically variable characters by running alternative analyses in which these characters were scored as missing data for our taxon. With the exception of one tree, in which it was sister to Amphisbaenia, the specimen was recovered as a Pan-xantusiid. Moreover, we cannot rule out the possibility that it represents a separate lineage of uncertain phylogenetic position, as it is the case for many Jurassic and Cretaceous taxa. Nonetheless, this fossil offers a rare opportunity to glimpse the external appearance of one group of lizards during the Early Cretaceous.

Lizard osteoderms - Morphological characterisation, biomimetic design and manufacturing based on three species

Osteoderms (OD) are mineralised dermal structures consisting mainly of calcium phosphate and collagen. The sheer diversity of OD morphologies and their distribution within the skin of lizards makes these reptiles an ideal group in which to study ODs. Nonetheless, our understanding of the structure, development, and function of lizard ODs remains limited. The specific aims of this study were: (1) to carry out a detailed morphological characterisation of ODs in three lizard species; (2) to design and manufacture biomimetic sheets of ODs corresponding to the OD arrangement in each species; and (3) to evaluate the impact resistance of the manufactured biomimetic sheets under a drop weight test. Skin samples of the anguimorphsH. suspectumandO. ventralis, and the skinkC. zebratawere obtained from frozen lab specimens. Following a series of imaging and image characterisations, 3D biomimetic models of the ODs were developed. 3D models were then printed using additive manufacturing techniques and subjected to drop weight impact tests. The results suggest that a 3D printed compound of overlapping ODs as observed inCoruciacan potentially offers a higher energy absorption by comparison with the overlapping ODs ofOphisaurusand the non-overlapping ODs ofHeloderma.Compound overlapping ODs need to be further tested and explored as a biomimetic concept to increase the shock absorption capabilities of devices and structures.

Cranial anatomy of the Galápagos marine iguana Amblyrhynchus cristatus (Squamata: Iguanidae)

Amblyrhynchus cristatus, the marine iguana, is unique among the ~7,000 species of living limbed lizards as it has successfully evolved adaptations that allow it to live in both terrestrial and marine environments. This species is endemic to the Galápagos Archipelago and has evolved a specialized feeding behavior, consuming primarily the algae that grow on the rocky seafloor. The intriguing questions arising around the evolution of the marine iguana concerns the use of exaptations of terrestrial features for aquatic and specifically marine adaptations. However, the lack of fundamental information about its anatomy currently prevents us from understanding how it became adapted to such a peculiar lifestyle in comparison to all other iguanids. The goal of this study is to provide the first ever description of the skull, mandible, and hyoid of Amblyrhynchus. We examined several specimens of marine iguana, including skeletal, wet, and ct-scanned material, and individuals at different ontogenetic stages. We also analyzed specimens of all other modern iguanid genera (Conolophus, Iguana, Ctenosaura, Cyclura, Dipsosaurus, Brachylophus, Sauromalus) in order to make comparisons between Amblyrhynchus and its closest relatives. We were able to identify several autapomorphic features that distinguish the marine iguana from all other iguanids. These unique morphologies are mostly associated with the modified configuration of the snout (nasal chamber), increased muscle attachments in the temporal-postorbital region of the skull, and dentition. Since Amblyrhynchus is the only nonophidian squamate currently able to exploit the ocean at least for some vital functions (i.e., feeding), we used comparisons to fossil marine lizards (e.g., mosasaurids) to discuss some of these unique traits. The new cranial features described for Amblyrhynchus may represent a source of novel morphological characters for use in future phylogenetic analyses of iguanian (or squamate) relationships, which will then serve as the foundation for the exploration of evolutionary patterns and processes that led to the development of such unique adaptations.