Fibrolamellar Bone Research Articles

The postcranial anatomy and osteohistology of Terrestrisuchus gracilis (Archosauria, Crocodylomorpha) from the Late Triassic of Wales

AbstractThe earliest crocodylomorphs, known as non‐crocodyliform crocodylomorphs, first appeared during the Late Triassic. In contrast to extant crocodylians, which are all semi‐aquatic, early crocodylomorphs represent terrestrial taxa with a fully erect posture and in most cases a small body size. Their gracile skeletons suggest an active mode of life, possibly similar to contemporaneous, bipedal theropod dinosaurs. Despite this remarkable body plan, the postcranial morphology of early crocodylomorphs has rarely been documented in detail, restricting our ability to infer aspects of their functional morphology and evolution. Here, we provide a detailed description of the postcranium of Terrestrisuchus gracilis, a small‐bodied crocodylomorph from the Late Triassic of Pant‐y‐Ffynnon Quarry (southern Wales, UK), including a description of long bone tissues based on histological thin sections. Almost all elements of the postcranial skeleton have been preserved. The skeleton of Terrestrisuchus gracilis is highly gracile, even for a non‐crocodyliform crocodylomorph. Osteological correlates of the appendicular skeleton suggest that Terrestrisuchus gracilis had a digitigrade, quadrupedal posture. A quantitative analysis of limb robustness corroborates that Terrestrisuchus gracilis was a quadruped. Histological analysis suggests that all sampled specimens were skeletally immature and had fast growth at the time of death, as indicated by the lack of an external fundamental system and the predominance of fibrolamellar bone. The bone tissue is similar to that recently described for Saltoposuchus connectens and certain non‐crocodylomorph pseudosuchians, but differs from Hesperosuchus agilis and crocodyliforms, in which parallel‐fibred bone is more prevalent.

New information on paleopathologies in non-avian theropod dinosaurs: a case study on South American abelisaurids

Studies on pathological fossil bones have allowed improving the knowledge of physiology and ecology, and consequently the life history of extinct organisms. Among extinct vertebrates, non-avian dinosaurs have drawn attention in terms of pathological evidence, since a wide array of fossilized lesions and diseases were noticed in these ancient organisms. Here, we evaluate the pathological conditions observed in individuals of different brachyrostran (Theropoda, Abelisauridae) taxa, including Aucasaurus garridoi, Elemgasem nubilus, and Quilmesaurus curriei. For this, we use multiple methodological approaches such as histology and computed tomography, in addition to the macroscopic evaluation. The holotype of Aucasaurus shows several pathognomonic traits of a failure of the vertebral segmentation during development, causing the presence of two fused caudal vertebrae. The occurrence of this condition in Aucasaurus is the first case to be documented so far in non-tetanuran theropods. Regarding the holotype of Elemgasem, the histology of two fused vertebrae shows an intervertebral space between the centra, thus the fusion is limited to the distal rim of the articular surfaces. This pathology is here considered as spondyloarthropathy, the first evidence for a non-tetanuran theropod. The microstructural arrangement of the right tibia of Quilmesaurus shows a marked variation in a portion of the outer cortex, probably due to the presence of the radial fibrolamellar bone tissue. Although similar bone tissue is present in other extinct vertebrates and the cause of its formation is still debated, it could be a response to some kind of pathology. Among non-avian theropods, traumatic injuries are better represented than other maladies (e.g., infection, congenital or metabolic diseases, etc.). These pathologies are recovered mainly among large-sized theropods such as Abelisauridae, Allosauridae, Carcharodontosauridae, and Tyrannosauridae, and distributed principally among axial elements. Statistical tests on the distribution of injuries in these theropod clades show a strong association between taxa-pathologies, body regions-pathologies, and taxa-body regions, suggesting different life styles and behaviours may underlie the frequency of different injuries among theropod taxa.

A foundational description of Antilocapra americana pronghorn core osteohistology.

Cranial bony projections ("headgear") have diverse forms and functions, such as defense, species recognition, mate selection, and thermoregulation. Most commonly, they are associated with the artiodactyl infraorder, Pecora. All pecoran headgear-antlers, horns, ossicones, and pronghorns-are osseous protrusions of the frontal or parietal bone with an integumentary covering, although there is taxonomic, developmental, and compositional variation. However, compared with other pecorans, there is a dearth of literature addressing extant antilocaprids-Antilocapra americana. This study provides a foundational osteohistological description of A. americana pronghorn cores in order to start building a framework to better understand the complex interplay among microanatomy, development, behavior, environment, and phylogenetic history of pronghorn headgear. Osteohistological analysis of adult A. americana pronghorn cores reveal the inner medullary region is composed of trabecular bone. Based on similar studies in bovids, we propose that these trabeculae may function to reduce the effects of repeated loading incurred by intraspecific combat. The deep aspect of the outer region was found to be composed of compacted coarse cancellous bone and primary bone remodeled to dense Haversian bone, in both male and female specimens, respectively, and superficially composed of highly vascularized fibrolamellar bone. The presence of fibrolamellar bone may indicate that the bone is fast-growing, and its presence at the periosteal surface suggests protracted growth of the pronghorn core beyond sexual maturity.

Bone histology supports gregarious behavior and an early ontogenetic stage to Decuriasuchus quartacolonia (Pseudosuchia: Loricata) from the Middle-Late Triassic of Brazil.

Decuriasuchus quartacolonia is a middle-sized basal "rauisuchian" (Pseudosuchia, Loricata) from the Triassic beds of Brazil, whose original description was based on 10 specimens of equivalent size found in aggregation. In this contribution, we explorethe osteohistology of its appendicular bones and a rib, aiming to infer growth patterns and ontogeny. In all analyzed bones (except in the rib) from three individuals, we found fibrolamellar bone (FLB) throughout the cortices, a pattern shared with other histologically sampled basal loricatans. We found evidence that suggests an early ontogenetic stage for all studied specimens: absence of secondary osteons, lack of transition from an inner highly vascularized FLB matrix to a poor vascularized parallel-fibred bone, one line of arrested growth, open vascular canals in the external surface of the cortex and absence of an external fundamental system. In addition, we observed that the neurocentral sutures in the caudal to cervical vertebrae of the holotype are unfused, strongly suggesting that these individuals were not skeletally mature, as had been previously assumed. In addition, our data support the prior hypothesis that Decuriasuchus is the oldest-known archosaur to show evidence of gregarious behavior and adds a probable cause of this as a strategy to obtain food and avoid predation until becoming independent as adults. Furthermore, our results open the possibility that Decuriasuchus may represent an earlier growth stage of the larger Prestosuchus chiniquensis, with two specimens recovered from the same stratigraphic level and paleontological site. Since the adult form of Decuriasuchus remains unknown, a deeper anatomical study is needed to discuss the validity of the species. This study case exemplifies the importance of paleohistology as an essential complementary tool to describe a new morphospecies in vertebrate paleontology.

Comparative bone histology of two thalattosaurians (Diapsida: Thalattosauria): Askeptosaurus italicus from the Alpine Triassic (Middle Triassic) and a Thalattosauroidea indet. from the Carnian of Oregon (Late Triassic)

Here, we present the first bone histological and microanatomical study of thalattosaurians, an enigmatic group among Triassic marine reptiles. Two taxa of thalattosaurians, the askeptosauroid Askeptosaurus italicus and one as yet undescribed thalattosauroid, are examined. Both taxa have a rather different microanatomy, tissue type, and growth pattern. Askeptosaurus italicus from the late Anisian middle Besano Formation of the southern Alpine Triassic shows very compact tissue in vertebrae, rib, a gastralium, and femora, and all bones are without medullary cavities. The tissue shows moderate to low vascularization, dominated by highly organized and very coarse parallel-fibred bone, resembling interwoven tissue. Vascularization is dominated by simple longitudinal vascular canals, except for the larger femur of Askeptosaurus, where simple vascular canals dominate in a radial arrangement. Growth marks stratify the cortex of femora. The vertebrae and humeri from the undescribed thalattosauroid from the late Carnian of Oregon have primary and secondary cancellous bone, resulting in an overall low bone compactness. Two dorsal vertebral centra show dominantly secondary trabeculae, whereas a caudal vertebral centrum shows much primary trabecular bone, globuli ossei, and cartilage, indicating an earlier ontogenetic stage of the specimens or paedomorphosis. The humeri of the thalattosauroid show large, simple vascular canals that are dominantly radially oriented in a scaffold of woven and loosely organized parallel-fibred tissue. Few of the simple vascular canals are thinly but only incompletely lined by parallel-fibered tissue. In the Oregon material, changes in growth rate are only indicated by changes in vascular organization but no distinct growth marks were identified. The compact bone of Askeptosaurus is best comparable to some pachypleurosaurs, whereas its combination of tissue and vascularity is similar to eosauropterygians in general, except for the coarse nature of its parallel-fibred tissue. The cancellous bone of the Oregon thalattosauroid resembles what is documented in ichthyosaurs and plesiosaurs. However, in contrast to these its tissue does not consist of fibro-lamellar bone type. Tissue types of both thalattosaurian taxa indicate rather different growth rates and growth patterns, associated with different life history strategies. The microanatomy reflects different life styles that fit to the different environments in which they had been found (intraplatform basin vs. open marine). Both thalattosaurian taxa differ from each other but in sum also from all other marine reptile taxa studied so far. Thalattosaurian bone histology documents once more that bone histology provides for certain groups (i.e., Triassic Diapsida) only a poor phylogenetic signal and is more influenced by exogenous factors. Differences in lifestyle, life history traits, and growth rate and pattern enabled all these Triassic marine reptiles to live contemporaneously in the same habitat managing to avoid substantial competition.

Palaeobiological inferences of "rauisuchians" Fasolasuchus tenax (Los Colorados Fm., Argentina) and Prestosuchus chiniquensis (Santa Maria Super sequence, Brazil) from the Middle-Upper Triassic of South America based on microstructural analyses.

"Rauisuchia" is a non-monophyletic group of quadrupedal and carnivorous pseudosuchians that inhabited the entire world during the Middle-Upper Triassic period (Anisian/Ladinian-Rhaetian). In South America, "rauisuchians" reached the largest sizes among continental carnivores. Despite their important ecological role, some aspects of their palaeobiology have been poorly examined. Here, we study appendicular bones, dorsal ribs and osteoderms of two genera, the Argentinean Fasolasuchus tenax (PVL 3850, holotype) and the Brazilian Prestosuchus chiniquensis (SNSB-BSPG AS XXV) respectively. The femur of F. tenax is formed by laminar fibrolamellar bone, which is composed of non-fully monorefringent woven-fibred matrix and primary osteons; the dorsal rib has a Haversian bone composition with an external fundamental system recorded and the osteoderm is formed by well-organised parallel-fibred bone. The femur, humerus and fibula of P. chiniquensis are mostly composed of strongly arranged parallel-fibred bone and a laminar vascularisation. The minimal ages obtained correspond to 9 years for F. tenax (based on the maximum number of growth marks in the osteoderm) and 4 years for P. chiniquensis (obtained from the highest count of growth marks in the femur and in the humerus). F. tenax attained somatic and skeletal maturity, while P. chiniquensis was near to reaching skeletal and sexual maturity, but it was somatically immature. The overall rapid growth rate and the high and uniform vascularisation seems to imply that these features are common in most of "rauisuchians", except in P. chiniquensis.

Osteohistology of the large-sized Cretaceous crocodylomorph Stratiotosuchus maxhechti (Notosuchia, Baurusuchidae) indicates fast growth and niche partitioning with medium-sized theropods

Among the wide diversity of notosuchian crocodyliforms, Baurusuchidae is remarkable by their morphology, indicating a terrestrial hypercarnivory habit during the Late Cretaceous of Gondwana. Commonly, amongst baurusuchids, the anatomy of the large-sized Stratiotosuchus maxhechti shows theropod mimic features in its skull and limb bones. Such similarities supported the hypothesis of competitive exclusion of medium to large-sized theropods by baurusuchids. However, a recent taphonomical approach claims that other lines of evidence should be considered (e.g., biomechanics); due to the biased fossil record found in the Bauru Group. This approach supported niche partitioning between theropods and baurusuchids instead of competitive exclusion. Here we performed a paleohistological analysis on a baurusuchid (S. maxhechti) to evaluate if the terrestrial ecology implies fast growth and if the microstructure is similar to what is found in theropods, with insights into a niche competition hypothesis. The samples were taken from one specimen with two femora and another one comprising the left ulna and tibia. All the four samples show the presence of fibrolamellar bone complex arranged in a cyclical growth. The growth zones are composed of woven matrix that gradually turns into a parallel-fibered bone, and is followed by lines of arrested growth (LAGs). There is also the presence of simple and anastomosed vascular canals. Compared to living crocodylians the growth shows a higher depositional rate and a more complex organization pattern. In this respect, the growth dynamics are like medium-to-large theropods. Stratiotosuchus lacks an External Fundamental System (EFS), which indicates that the material studied here represents individuals that died before reaching their maximum size, suggesting that they could take more than twelve years to to be fully grown. Comparing the body mass estimates by femoral circumference (∼178–180 kg) and the bone microstructure with theropods, they share a fast growth pattern by showing fibrolamellar bone complex. However, the body size diversity in both groups favors the niche partitioning hypothesis over the suggestion of competitive exclusion between baurusuchids and theropods.

New data on high-latitude hadrosaurid dinosaurs from the Upper Cretaceous Kakanaut Formation of Chukotka, Russia

Fossils from high-latitude Mesozoic vertebrate localities provide a unique opportunity to reveal specific biological features and reconstruct adaptations to months of twilight and relatively low annual temperatures of polar dinosaurs. Here we provide morphological description of new hadrosaurid specimens (vertebrae, femora) and results of the histological analysis of hadrosaurid femora, proximal fragments of a humerus and a scapula from the Upper Cretaceous high-latitude (palaeolatitude estimate of 75°N) Kakanaut locality (Chukotka, Russia). The two studied hadrosaurid femora are different in size and form, but both are histologically mature and belong to fully grown adult individuals. This difference may indicate the presence of two different hadrosaurid taxa in the Kakanaut fauna. The bone histology of the Kakanaut dinosaurs does not differ from that of dinosaurs from lower latitudes and does not show specific histological features (namely, cyclic change of vascularization of the fibrolamellar bone, with sharply defined borders between cycles) found in polar Edmontosaurus from Alaska. This suggests that the Kakanaut hadrosaurid dinosaurs were not significantly stressed, and this may be the result of the local mild and weakly seasonal climate reconstructed from palaeobotanical data. We suggest that Arctic dinosaurs of Chukotka, like other polar dinosaurs, were year-round residents of paleopolar ecosystems.

Bone histology of Neogene angulate tortoises (Testudines: Testudinidae) from South Africa: palaeobiological and skeletochronological implications.

Here we examine the tibial microstructure of modern and fossil angulate tortoises to assess the histology and growth from the late Miocene-early Pliocene, Pleistocene through to modern forms. The cross-sections of all the tibiae sampled revealed highly vascularized, uninterrupted, fibrolamellar bone tissue during early ontogeny, which suggests that early growth was fast. However, later in ontogeny, growth was slower, as indicated by the deposition of parallel-fibred bone tissue in the outer cortex, and even ceased periodically, as indicated by lines of arrested growth. Comparative analyses of the growth rates of the tortoises from different time periods showed that the tortoises from the late Miocene-early Pliocene Langebaanweg locality and from Diepkloof Rock Shelter had relatively slower growth rates under less optimal growth conditions. Additionally, these prehistoric specimens show extensive remodelling, and several generations of secondary osteons further suggest functional and/or metabolic stresses on the skeleton. Palaeoenvironmental reconstructions suggest that it was mostly cooler and drier with seasonal fluctuations in late Miocene-early Pliocene, and it is likely that Chersina responded to these conditions by having a lower rate of growth as compared with their modern counterparts, which thrive in the current prevailing more favourable Mediterranean type of climate.

Diversity of cortical bone morphology in anuran amphibians.

The cortical bones of mammals, birds, and reptiles are composed of a complex of woven bone and lamellar bone (fibrolamellar bone) organized into a variety of different patterns; however, it remains unclear whether amphibians possess similar structures. Importantly, to understand the evolutionary process of limb bones in tetrapods, it is necessary to compare the bone structure of amphibians (aquatic to terrestrial) with that of amniotes (mostly terrestrial). Therefore, this study compared the cortical bones in the long bones of several frog species before and after metamorphosis. Using micro-computed tomography (CT), we found that the cortical bones in the fibrolamellar bone of Xenopus tropicalis (Pipoidea superfamily) and Lithobates catesbeianus (Ranoidea superfamily) froglets are dense, whereas those of Ceratophrys cranwelli (Hyloidea superfamily) are porous. To clarify whether these features are common to their superfamily or sister group, four other frog species were examined. Histochemical analyses revealed porous cortical bones in C. ornata and Lepidobatrachus laevis (belonging to the same family, Ceratophryidae, as C. cranwelli). However, the cortical bones of Dryophytes japonicus (Hylidae, a sister group of Ceratophryidae in the Hyloidea superfamily), Microhyla okinavensis (Microhylidae, independent of the Hyloidea superfamily), and Pleurodeles waltl, a newt as an outgroup of anurans, are dense with no observed cavities. Our findings demonstrate that at least three members of the Ceratophryidae family have porous cortical bones similar to those of reptiles, birds, and mammals, suggesting that the process of fibrolamellar bone formation arose evolutionarily in amphibians and is conserved in the common ancestor of amniotes.

An insight into cancer palaeobiology: does the Mesozoic neoplasm support tissue organization field theory of tumorigenesis?

BackgroundNeoplasms are common across the animal kingdom and seem to be a feature plesiomorphic for metazoans, related with an increase in somatic complexity. The fossil record of cancer complements our knowledge of the origin of neoplasms and vulnerability of various vertebrate taxa. Here, we document the first undoubted record of primary malignant bone tumour in a Mesozoic non-amniote. The diagnosed osteosarcoma developed in the vertebral intercentrum of a temnospondyl amphibian, Metoposaurus krasiejowensis from the Krasiejów locality, southern Poland.ResultsA wide array of data collected from gross anatomy, histology, and microstructure of the affected intercentrum reveals the tumour growth dynamics and pathophysiological aspects of the neoplasm formation on the histological level. The pathological process almost exclusively pertains to the periosteal part of the bone composed from a highly vascularised tissue with lamellar matrix. The unorganised arrangement of osteocyte lacunae observed in the tissue is characteristic for bone tissue types connected with static osteogenesis, and not for lamellar bone. The neoplastic bone mimics on the structural level the fast growing fibrolamellar bone, but on the histological level develops through a novel ossification type. The physiological process of bone remodelling inside the endochondral domain continued uninterrupted across the pathology of the periosteal part.ConclusionsBased on the results, we discuss our case study’s consistence with the Tissue Organization Field Theory of tumorigenesis, which locates the causes of neoplastic transformations in disorders of tissue architecture.

Fossil bone histology reveals ancient origins for rapid juvenile growth in tetrapods

Patterns of growth throughout the lifetime of an animal reflect critical life history traits such as reproductive timing, physiology, and ecological interactions. The ancestral growth pattern for tetrapods has traditionally been described as slow-to-moderately paced, akin to modern amphibians, with fast growth and high metabolic rates considered a specialized physiological trait of amniotes. Here, we present bone histology from an ontogenetic series of the Early Carboniferous stem tetrapod Whatcheeria deltae, and document evidence of fibrolamellar bone—primary bone tissue associated with fast growth. Our data indicate that Whatcheeria juveniles grew rapidly and reached skeletal maturity quickly, allowing them to occupy a large-bodied predator niche in their paleoenvironment. This life history strategy contrasts with those described for other stem tetrapods and indicates that a diversity of growth patterns existed at the origins of tetrapod diversification. Importantly, Whatcheeria marks an unexpectedly early occurrence of fibrolamellar bone in Tetrapoda, both temporally and phylogenetically. These findings reveal that elevated juvenile growth is not limited to amniotes, but has a deep history in the tetrapod clade and may have played a previously unrecognized role in the tetrapod invasion of land.

Osteological and taxonomic reassessments of Sahaliyania elunchunorum (Dinosauria, Hadrosauridae) from the Upper Cretaceous Yuliangzi Formation, northeast China

ABSTRACT The osteology and taxonomy of the lambeosaurine Sahaliyania elunchunorum from the upper Yuliangzi Formation (middle Maastrichtian) are reevaluated herein. Based on detailed morphological comparisons among hadrosauroids, we argue that Wulaga lambeosaurine specimens, most of which were previously ascribed to S. elunchunorum, display a combination of features that is typical of Amurosaurus riabinini from the upper Udurchukan Formation (middle Maastrichtian) at the Blagoveschensk locality. This combination includes several anterodorsally-posteroventrally oriented lateral foramina of the maxilla with a very large last one partially obscured by the ventral extremity of the jugal contact, a narrow infratemporal region of the jugal that is ∼70% as wide as the orbit, a relatively low anterior third of the strongly dorsally concave sagittal crest of the parietal, mesiodistally wide maxillary tooth crowns (∼1.2–1.3 teeth per cm), the sternal posterolateral process slightly shorter than the anteromedial plate, and a gently ventrally deflected preacetabular process of the ilium. Considering the osteological similarities and the fact that the Yuliangzi and Udurchukan formations were deposited synchronously in the same basin, S. elunchunorum is regarded here as a junior synonym of A. riabinini. Approximately 70% of the complete Amurosaurus fibulae from the Wulaga bonebed exhibit a total length of more than 85 cm that probably corresponds to the adult stages. Histological sections from these fibulae are moderately remodeled by dense secondary osteons, with poorly developed vascularization of primary osteons along fibrolamellar bone in the outer cortex. The phylogenetic analysis of Lambeosaurinae recovers a sister-taxon relationship between Amurosaurus and Lambeosaurus.

Osteohistology of Aucasaurus garridoi (Dinosauria, Theropoda, Abelisauridae): inferences on lifestyle and growth strategy

ABSTRACT Although osteohistology has proven to be useful to discern several palaeobiological traits and systematics in non-avian dinosaurs, the bone microstructure of several groups (e.g., abelisaurid theropods) is still poorly documented. Here, we provide a microstructural description of different bones (a cervical epipophysis, three dorsal ribs, three grastralia, and the left femur) of the type specimen of the abelisaurid Aucasaurus garridoi, its ontogenetic stage and compare its growth with other theropods, especially abelisauroid taxa. The cortical bone is mostly composed by fibrolamellar bone tissue composed of a woven-fibred matrix. Secondary remodelling varies among the different elements. The compacta of the rib shows eleven growth marks (with an additional seven in the external fundamental system), implying that the specimen was at least eleven years old and had reached somatic maturity when it died. The presence of lines of arrested growth/annuli supports a cyclical growth strategy, as in Niebla, Quilmesaurus, and Abelisauridae indet. MMCh-PV 69. All these abelisaurids indicate that the maturity based on cyclical growth marks spacing is not reliable, due to considerable variability of this parameter within a single element. This study represents the first intensive osteohistological investigation for an Abelisauridae member, providing new information on palaeobiological traits of this clade.

Microstructure of axial bones of lithostrotian titanosaurs (Neosauropoda: Sauropodomorpha) shows extended fast-growing phase

Titanosaurs reached extremes in size and diversity, but many paleobiological aspects of these sauropod dinosaurs are still unknown. Here we infer life history traits based on the microstructure of axial bones of three lithostrotian titanosaur species: Austroposeidon magnificus, Gondwanatitan faustoi and Maxakalisaurus topai — Campanian–Maastrichtian of Bauru Group, Brazil. The present study is the first wide characterization of the microstructure of axial bones comparing distinct lithostrotian lineages with different body sizes. We analyzed the cross sections of cervical ribs, dorsal ribs and haemal arches. The medullary cavity in samples of A. magnificus are pneumatic, with the primary bone rich in fibrolamellar bone, with scarce growth marks, which is different from the Lines of Arrested Growth (LAGs)-dominated pattern of G. faustoi and M. topai, with parallel fibered bone. In general, the inner cortex exhibits fibrolamellar bone with longitudinal canals. The outer cortex is rich in parallel fibered bone rows of longitudinal canals and growth marks. Based on dense remodeling, decrease of LAGs distances and presence of an External Fundamental System in some samples; we concluded that all individuals reached at least the sexual maturity. These titanosaurs shared with basal neosauropods two main growth phases: modular (inner cortex) and LAG-dominated (outer cortex). Distinct from basal neosauropods, titanosaurs presented extended fast modular period (rather than LAG-dominated). This pattern supported that titanosaurs adopted a distinct strategy of bone mass gain in dorsal ribs, alternating the timing in rib growth.

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.

Palaeobiological inferences of the aetosaur Aetosauroides scagliai (Archosauria: Pseudosuchia) based on microstructural analyses of its appendicular bones

ABSTRACT Aetosaurs were a group of armoured pseudosuchians, recorded in most of the Upper Triassic continental deposits worldwide. Several osteohistological contributions of aetosaurs focused on their osteoderms, but rarely on appendicular bones. Here, we analyse the microstructure of the humerus, femur and tibia of Aetosauroides scagliai (specimens PVL 2073 [holotype] and PVL 2052). These exhibit cortical bone formed by highly vascularised fibrolamellar bone present in the inner portion of the cortex, mixed with scarce parallel-fibred bone. Also, they show parallel-fibred bone in the outermost portion of the cortex. A general growth pattern that includes a first rapid stage followed by a slow stage is reported. Nevertheless, the growth rate and the presence of parallel-fibred bone embedded in fibrolamellar bone layers recognise more variation within Aetosauria. The value of appendicular bones and osteoderms as age estimators is variable, the first being useful in early stages, and in late stages the osteoderms are better (based on the particular growth of osteoderms). Through morphological (neurocentral sutures) and histological (EFS absent) information, the holotype (PVL 2073) was recovered between juvenile and subadult stages. Using a statistical model that combines microanatomical and morphological data, a terrestrial lifestyle is inferred for Aetosauroides, which concur with previous analyses.

Osteohistology of non-mammalian therapsids from Puesto Viejo Group, Argentina

ABSTRACT The Puesto Viejo Group is an important unit for the knowledge Triassic faunas especially non-mammalian therapsids. This group crops out in the San Rafael Block, southwest Mendoza, Argentina. It is composed of the basal Quebrada de los Fósiles Formation (QF) which is overlaid by the Río Seco de la Quebrada Formation (RSQ). This report comprises tetrapod remains from the RSQ Formation at the Quebrada del Durazno area that includes, until now, exclusively therapsids. The aim of this study is to analyse the bone microstructure of cynodonts and dicynodonts to elucidate the histological features and growth patterns. Histological thin sections made in ribs and appendicular bones reveal a predominance of fibrolamellar bone tissue suggesting an overall fast bone growth, however, the presence of parallel-fibred bone suggest periods of slowing down of the growth. The existence of growth marks indicates periodic interruptions of growth, which is likely the expression of their ability to rapid respond to environmental changes. This contribution offers new insights into the bone microstructure of tetrapod assemblages from RSQ Formation during the Upper Triassic span.

Palaeohistological inferences of resting metabolic rates inConcornisandIberomesornis(Enantiornithes, Ornithothoraces) from the Lower Cretaceous of Las Hoyas (Spain)

AbstractThe bone histology of non‐avian theropods such asTroodon, early pygostylians such asConfuciusornis, and neornithines, is characterized by the post‐hatching formation of fibrolamellar complex. In contrast, the cortex of enantiornithine birds, likeConcornisandIberomesornis, is made of poorly vascularized parallel‐fibred tissue. The cortex of metatarsals ofConcornis lacustrisMCCM‐LH21006 is composed of a thin layer of lamellar endosteal bone and a thick layer of almost avascular parallel‐fibred periosteal bone organized into an inner part containing large spherical osteocyte lacunae and a few secondary osteons, and an outer part containing flattened osteocyte lacunae and lines of arrested growth. The cortex of metatarsals ofIberomesornissp. MCCM‐LH09681 is similar to the inner layer described inConcornis. Endosteal bone, and the outer layer of periosteal bone containing flattened osteocytes and lines of arrested growth, are missing inIberomesornis, suggesting that this specimen is a sub‐adult. Here we test statistically a hypothesis suggesting that the absence of post‐hatching formation of fibrolamellar bone in Enantiornithes may be a side effect of their low resting metabolic rates (RMR). For this, we construct a palaeobiological inference model including osteocyte lacunar density (explaining a significant fraction of variation in RMR) and phylogenetic eigenvectors as predictor variables. RMR inferred forConcornisandIberomesornisare significantly higher than those measured in extant ectotherms, but they are not significantly different from the RMR measured in a sample of extant arboreal perching Neornithes of similar body size. We conclude that the ‘intermediate physiological condition’ hypothesis is rejected.

Bone histology of dinocephalians (Therapsida, Dinocephalia): palaeobiological and palaeoecological inferences

AbstractWe investigate the bone histology of multiple skeletal elements of dinocephalian taxa from the middle PermianTapinocephalusAssemblage Zone of the Karoo Basin of South Africa. The results show that the cortex is predominantly composed of fibrolamellar bone, suggesting rapid osteogenesis for these basal non‐mammalian therapsids. However, in a few skeletal elements, growth marks interrupt the deposition of the fibrolamellar bone tissue, indicating periodic arrests in growth. Ontogenetic differences are observed among the specimens studied: the majority have fibrolamellar bone deposited up to the peripheral margin of the bone wall, indicating continuous fast growth right up to the time of death, while a few specimens have avascular lamellar bone tissue with multiple, closely spaced rest lines, indicating maturity and a slowing down of growth prior to death. Several taxon‐specific histological variations in terms of orientation of vascular canals and primary osteons, incidence of growth marks, and extent of secondary medullary reconstruction suggest slight differences in growth trajectories between the different taxa. The skeletal elements of the herbivorous taxa (Keratocephalus,MoschopsandStruthiocephalus) and the omnivorousJonkeria, previously studied, are characterized by a thick cortex, and extensively developed medullary spongiosa, suggestive of semi‐aquatic habits. In contrast, the femoral histology of the carnivorousAnteosaurussuggests that it was more terrestrial.