Osteoderms as calcium reservoirs: Insights from the lizard Ouroborus cataphractus

First report of desmatosuchine aetosaur (Pseudosuchia, Aetosauriformes) osteoderms from the Upper Triassic Tiki Formation of India: Their complex internal vascular system, functional significance and biostratigraphy

The Late Triassic Tiki fauna from India represents one of the richest and most diverse vertebrate faunal assemblages, essentially known for archosauromorphs, temnospondyls, fishes, lizards, and trace fossils like coprolites. In this paper, we provide a detailed morphological description of isolated lateral osteoderms from a multitaxic bonebed of the Tiki Formation. The osteoderms bear a spike-like eminence formed by two asymmetric flanges, radial ornamentation, and a thick, indented medial margin suggesting that these pertained to a pseudosuchian archosaur. With the purpose of demonstrating the internal anatomy, X-ray microCT was used to reveal a compact diploe structure with a highly vascularised core characterising most archosaurian osteoderms. Such a high degree of compactness with an internal vascular network and open vascular canals can be hypothesised to serve as protection from predation and for thermoregulation. A comparative study with different Late Triassic armoured archosauromorphs revealed morphological resemblance of the osteoderms to lateral osteoderms of desmatosuchine aetosaurs and the aetosauriform Acaenasuchus geoffreyi. A phylogenetic analysis comprising all the Late Triassic archosauromorphs placed the Tiki taxon within Aetosauriformes. However, an additional analysis including all aetosaurs positioned the new taxon deeply nested within Desmatosuchini as sister taxon to Desmatosuchus. Better understanding of the phylogenetic position can only be achieved by recovering more cranial and post-cranial materials. This contribution demonstrates the first occurrence of a Desmatosuchus-like taxon from the Tiki Formation and bolsters the significance of the horizon in global correlation and also contributes to our understanding of the dispersal of aetosaurs or similar taxa in different parts of Pangaea during the Late Triassic.

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.

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.

Intraspecific competition: A missing link in dermal armour evolution?

Predation is widely regarded as an important selective force in the evolution and maintenance of dermal armour; yet, the basic premise that predation and armour are strongly linked to each other has proven to be difficult to assess. In this concept, I put forward the fighting-advantage hypothesis, the view that aggressive interactions with conspecifics, not predation, might have been a key selective pressure in the evolution of dermal armour. Considering intraspecific competition as a potential explanation could not only reveal previously overlooked aspects of the functional and evolutionary significance of dermal armour, but also advance the emerging field of biomimetics in which such knowledge forms the starting point of technological innovation.