From fossil to microscope: Unraveling the tapestry of tissue anatomy through paleohistology

Abstract

Welcome to this special double issue of the Journal of Anatomy dedicated to advances in paleohistology, which is the anatomical study of ancient body tissues. The field of paleohistology has an unexpectedly long history that is comprehensively detailed by de Ricqlès (2021), and its many thin sections and fossils have been inherently linked since its inception. The first thin sections were produced about 210 years ago in 1815, by William Nicol in his studies of fossilized wood, including the “Great Tree” of Craigleith Quarry, Edinburgh, Scotland (de Ricqlès, 2021; Falcon-Lang & Digrius, 2014). This methodological advancement sparked a flurry of histological discovery, spurred on in tandem with refinements in microscopy by Joseph Jackson and his son, Joseph Lister, the founder of microbiology. To some degree around the turn of the 20th century, interest in paleohistology waned, albeit never completely fizzling out, with more sporadic research programs focused on foundational problems. Nevertheless, these built an important framework that allowed paleohistology, particularly vertebrate paleohistology, to be reinvigorated nearly 100 years after its birth, with the seminal works of Rodolfo Amprino (Amprino, 1947), Donald Enlow (Enlow & Brown, 1956), and Armand de Ricqlès (de Ricqlès, 1975, 1977, 1978). These works established critical links between skeletal tissues and development, growth rates, physiology, and phylogenetic evolution that entrenched vertebrate paleohistology as a core tool in the suite of paleontological investigation. Today, paleohistology is a thriving field, building on the shoulders of historical giants. Indeed, some aspects of Nicol's technique remain central to thin sectioning methods today, but as this volume highlights, two hundred years has not passed without major progress.

Our aim in this special issue is to showcase the wide range of techniques, questions, insights, and otherwise inaccessible paleobiological data that paleohistology encompasses. Among the articles included here are developments in our understanding of growth, development, and life history, with an emphasis on vertebrates, spanning across fishes, amphibians, saurians (“reptiles”), and mammals, including humans. Paleohistology continues to broaden our insight into extinct organisms by revealing the structural and developmental complexity of their hard tissues, and integrating them into the evolutionary perspective that only fossils can provide. Through a combination of methodological innovation and taxonomic diversity, the studies presented here demonstrate how fossilized microstructures can continue to uniquely illuminate the biological processes that shaped evolutionary history. This issue is arranged thematically, following a taxonomic thread beginning with fishes and progressing through younger clades from amphibians, through saurians, grading into archosaurs, through Dinosauria, and finally to eutherian mammals, including humans.

The first issue begins with two contributions that expand our understanding of early osteichthyan and amphibian histology, while highlighting the broad array of paleohistological techniques. Chen (2025) applies synchrotron tomography to examine tooth addition and replacement in the coronoid of a Devonian stem actinopterygian, likely Moythomasia. Leveraging advances in microtomographic techniques, the study reveals a lungfish-like pattern of antero-labial tooth addition and a transition from enameloid to enamel-capped replacement teeth, echoing features seen in larval amphibians. Particularly notable is the transformation from radial to linear tooth row organization and the observation of cross-position resorption and locus fusion or splitting. These features challenge conventional classifications of dental patterns and support greater developmental flexibility in early ray-finned fishes. Kalita et al. (2025) address long-standing challenges in identifying bone tissue types in the temnospondyl amphibian Metoposaurus krasiejowensis by applying circular cross-polarized light. This technique improves the visualization of collagen fiber orientation compared to traditional methods, allowing the authors to distinguish between interwoven structural fibered, parallel-fibered, and lamellar bone types. The study documents previously unrecognized looped fiber architectures and gradations between fine parallel-fibered and lamellar bone. These results demonstrate the value of refined imaging techniques for interpreting the histology of early tetrapods and call for a re-evaluation of tissue classifications based on less precise methods.

The next group of studies focuses on reptiles, beginning with marine taxa. Pereyra et al. (2025) investigate the small-bodied elasmosaurid Kawanectes lafquenianum from the Maastrichtian of Argentina. Histological analysis of limb bones from three individuals reveals mature bone tissues, including external fundamental systems and widespread secondary remodeling. These features indicate somatic maturity and support the hypothesis that Kawanectes represents a small-bodied adult form rather than a juvenile. The study also identifies interelemental variation in growth mark preservation and unusual fiber orientations that raise questions about the roles of Sharpey's fibers and intrinsic structural arrangement in plesiosaur bone microstructure.

Three studies on modern and fossil turtles provide further insight into their growth strategies and extinction dynamics. Ong et al. (2025b) study the extant soft-shelled turtle Apalone spinifera and demonstrate that growth mark counts vary significantly between elements, with costal width serving as a reliable proxy for body size. Latitudinal effects on growth rate are present but relatively weak, and the study highlights the limited utility of suture fusion as a maturity indicator. A companion study by Ong et al. (2025a) investigates over 250 fossil shell sections from pan-trionychian turtles spanning the Cretaceous–Paleogene boundary. Taxa with highly vascularized cortices associated with cutaneous respiration were disproportionately lost at the extinction event, while survivors displayed greater remodeling capacity and lower physiological specialization. These results suggest that adaptability, rather than size or armor thickness, played a central role in survival. Bhat and Cullen (2024) conduct a multielemental histological study of modern chelydrid turtles, identifying the tibia as the most informative element for skeletochronology. They document cyclical growth marked by parallel-fibered bone and numerous growth lines, as well as functional differences in vascular patterns between fore- and hindlimbs. The presence of compacted coarse cancellous bone in certain limb bones and its absence in others also suggests developmental and biomechanical differentiation. These findings provide a valuable modern baseline for interpreting growth in fossil cryptodires.

Bringing us to squamates, Schlief et al. (2025) use fluorescent bone labeling in captive leopard geckos to track growth dynamics across ontogeny. Their data reveal substantial variability in growth rate and remodeling across elements, with embryonic labels persisting in some bones for over three years. Other elements lose early growth signals due to remodeling, underscoring the need for multielement sampling in fossil studies. The results offer a detailed experimental framework for understanding the reliability of growth mark preservation and interpreting squamate life history.

Moving into Triassic archosaurs, Goldsmith et al. (2024) examine the smallest known phytosaur femur and find extremely slow growth rates marked by parallel-fibered bone and a lack of growth marks. These findings challenge assumptions of rapid juvenile growth in early archosauriforms and suggest that some basal taxa grew at rates comparable to lepidosaurs and turtles. Ponce et al. (2025) contrast this by identifying rapid early growth in Trialestes romeri, a Jurassic early crocodylomorph, based on fibrolamellar and woven-fibered bone and extensive vascularization. Despite fused neurocentral sutures, the absence of somatic maturity indicators suggests a decoupling of skeletal and reproductive development. These findings support rapid early growth as the ancestral condition in Crocodylomorpha.

Crocodyliforms comprise a major portion of the special issue. Weiss et al. (2024) employ synchrotron microcomputed tomography to study the osteohistology of Orthosuchus stormbergi. Their non-destructive virtual histology reveals that Orthosuchus reached skeletal maturity within four to five years and possessed primarily lamellar bone with localized woven and parallel-fibered regions. Microanatomical metrics such as cortical thickness and compactness suggest a semi-aquatic or fossorial lifestyle, despite a lack of overt morphological adaptations. This study highlights how virtual imaging expands access to delicate fossil material and enhances interpretations of growth and ecology. Two studies address growth and osteoderm development in notosuchian crocodyliforms. Navarro et al. (2025) provide the first multielement histological analysis of a peirosaurid, revealing moderate cyclic growth, intraskeletal variation in growth mark counts, and the presence of mid-cortical rings of rapidly deposited tissue. These features suggest a complex growth history and emphasize the importance of multielement sampling for accurate life history reconstruction. Cajado et al. (2025) examine osteoderm histology across multiple notosuchian clades, documenting broadly conservative tissue structure but substantial variation in vascularization, remodeling, and ornamentation. They also identify ontogenetic shifts in crest morphology and surface texture, suggesting a dynamic interplay between dermal development, body region, and individual age.

The second installment of this Journal of Anatomy special double issue continues the momentum, and focuses our attention toward dinosaurs and mammals, with a rich array of paleohistological studies that illuminate growth patterns, developmental variation, and pathological conditions across a range of Mesozoic and Cenozoic taxa.

Beginning with Ornithischia, Maíllo et al. (2025) launch the dinosaur section with a detailed multielement histological study of an Early Cretaceous iguanodontian from Spain. By applying the three-front model to ornithopod material for the first time, they reveal developmental variation in growth mark counts and remodeling intensity across ribs, ischium, tibia, and fibula. This study highlights the difficulty in selecting a universally reliable element for skeletochronology and supports a growth model characterized by early rapid deposition followed by a plateau phase, offering refined insight into styracosternan life history. Expanding the scope of histological inquiry into dermal ossification, Sanchez et al. (2024) explore the development of dermal ossicles in the Antarctic nodosaurid Antarctopelta oliveroi. Their use of synchrotron tomography uncovers a two-layered microstructure and supports a dual-mode formation process involving both metaplastic ossification and neoplastic-like differentiation of new fibers. The identification of osteodermine-like tissue in these ossicles parallels dermal tissues in squamates and challenges traditional assumptions about ankylosaur osteoderm development.

Sauropoda forms a major emphasis of the dinosaur section. Woodruff et al. (2024) investigate the histology of some of the largest Morrison Formation sauropods, Diplodocus hallorum and Supersaurus vivianae. Their findings demonstrate skeletal maturity in both taxa, with age estimates indicating extreme longevity and extensive remodeling in the oldest individuals. These data suggest that large body size may have been linked to survivorship bias and challenge simplistic notions that gigantism was purely phylogenetic. Pathological insights are provided by Kaikaew et al. (2025), who document an osteogenic tumor in the ulna of a Late Jurassic mamenchisaurid from Thailand. Through a combined approach of CT imaging and histological sectioning, they identify a likely neoplastic lesion with reactive bone features, making this the first such diagnosis in a basal eusauropod. This study underscores the diagnostic power of paleohistology for detecting disease in the fossil record. Toefy et al. (2025) contribute a phylogenetically informed study of South African sauropodomorphs, identifying fibrolamellar bone in both transitional sauropodiforms and basal sauropods. Variation in growth mark timing and pathology across taxa suggests that sauropod gigantism evolved via flexible growth strategies rather than a single developmental template. D'Emic et al. (2024) further this line of inquiry by examining tooth replacement rates across sauropods. Their expanded dataset shows evolutionary decoupling between replacement rate and crown morphology, with independent increases in replacement rate among diplodocoids and titanosaurs. Notably, Abydosaurus exhibits unusually slow tooth formation despite a high replacement rate, challenging assumptions about sauropod feeding ecology.

Theropod paleohistology is addressed by a pair of studies. Garros et al. (2025) shift attention to small-bodied theropods, presenting a histological study of troodontid metatarsals from the Campanian Dinosaur Park Formation. Their results reveal divergent growth trajectories and pathological alterations, providing rare insights into variation and disease in fragmentary North American theropod remains. Sombathy et al. (2025) focus on Ceratosaurus, using multielement histology and growth modeling to reconstruct a fast-growing life history. Their integration of osteoderm histology and application of sigmoidal growth models supports rapid growth rates in early ceratosaurians, expanding the known range of developmental strategies among non-avian theropods.

Finally, a single study analyzes a broad range of dinosaurs: Sharpe et al. (2025) offer a novel anatomical reconstruction of a hypothesized soft tissue structure, the “exoparia,” in non-avian dinosaurs. Using a new method, THLEEP, they reconstruct entheseal fiber orientation across cranial elements and propose a ligamentous or muscular connection between the jugal and surangular. This interdisciplinary approach offers new perspectives on soft tissue anatomy in extinct taxa.

The mammal portion of the special issue ranges from the earliest Cenozoic to the recent archaeological realm. Funston et al. (2025) open the mammal section with the first histological analysis of the Paleocene taeniodont Conoryctes comma. Their multielemental dataset suggests rapid juvenile growth, early sexual maturity, and the presence of a potential weaning mark. Limb-bone compactness and coarse cancellous bone distribution further support fossorial adaptation and a placental-like growth model. Cuccu et al. (2025) analyze incremental structures in teeth of the Miocene deer Procervulus ginsburgi, revealing faster crown formation and enamel extension than in modern roe deer. These traits suggest a more rapid life history strategy, likely influenced by the seasonally variable habitats of the Miocene Climatic Optimum. Chinsamy and Valenciano (2024) provide a paleopathological assessment of a Pliocene canid from South Africa, diagnosing multiple exostoses and an osteochondroma. Histological confirmation of cartilage-derived growth patterns supports the diagnosis, marking the first such identification in a fossil African carnivoran. Nacarino-Meneses et al. (2025) present the first dental histology study of the extant giraffe, Giraffa camelopardalis, providing key parameters such as DSR and crown formation time. Variability between molars suggests differences in somatic growth rates, offering a modern comparative dataset for extinct giraffids. Finally, Lozano-Bendicho et al. (2025) close this special issue with a quantitative study of occipital bone modeling in subadult humans. Their SEM-based analysis identifies age-specific patterns of resorption and deposition that challenge traditional assumptions about brain growth and developmental timing. The study provides a framework for interpreting cranial development in fossil hominins.

More than two hundred years after its birth, the field of paleohistology is thriving. The studies across both volumes of this special issue illustrate the remarkable breadth and maturity of paleohistological research today. Numerous methodological advances enable clearer insights and broader application of paleohistology, while consistently increasing the precision of our data. Among the articles here, there is cutting-edge imaging, refined analytical frameworks, and ever-expanding taxonomic coverage, and so these contributions reveal how microscopic traces of tissue can unlock macroscopic stories of evolution, development, ecology, and disease. In particular, continued sampling, from Devonian fishes to Paleogene mammals, expands our perspective of when and where major transitions in microanatomy, skeletal growth, and life history arose. Nevertheless, these volumes reinforce that continued paleohistological study will be fruitful and necessary to better understand the paleobiology of extinct species; the evolutionary stories of development, growth, and life history are a rich tapestry that perpetually produces unexpected surprises.

From methodological innovations to unexpected anatomical discoveries, this collection exemplifies the integrative power of paleohistology in reconstructing the biology of extinct life. We thank all contributing authors and the numerous reviewers for their rigorous and thoughtful work, and we hope this issue inspires continued exploration into the hidden histories preserved in fossilized tissues.