How does the cellular‑level preservation of cutaneous spikes in an Early Cretaceous iguanodontian alter prevailing models of dinosaur integument evolution and inform phylogenetic reconstructions of defensive traits?
The discovery of Haolong dongi, a new iguanodontian dinosaur from the Lower Cretaceous of northeastern China, represents a paradigm-shifting find that fundamentally challenges established models of dinosaur integument evolution and necessitates substantial revisions to how defensive traits are coded and reconstructed in phylogenetic analysesCellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1.
The near-complete and articulated skeleton preserves exquisitely fossilized skin with integumentary structures fossilized at a resolution never before achieved in non-avian dinosaursCellular-level preservation of cutaneous spikes in an Early ... - Naturenature . Tomographic and histological analyses reveal that the cutaneous spikes possess a hollow, cylindrical structure composed of a cornified stratum corneum overlying a pluristratified epidermis with keratinocytes preserved to the level of nuclei, surrounding a porous central dermal pulpCellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1. This level of preservation—down to individual keratinocyte nuclei—provides an unprecedented window into the biology of dinosaurian integument at a cellular scale previously thought impossible“Spiny dragon” reveals hidden secrets of dinosaur skin ...naturalsciences .
The specimen also exhibits a complex integumentary mosaic: large overlapping scutate scales along the tail and tuberculate scales around the neck and thorax, markedly different from scale patterns described in other iguanodontiansCellular-level preservation of cutaneous spikes in an Early ... - Naturenature . This regional differentiation of scale types aligns with observations in other dinosaurs such as Psittacosaurus, where preserved scales are tuberculate (non-overlapping and non-polarized) and polygonal-to-rounded, typically 0.8–1.2 mm wide with interspersed larger feature scales of approximately 1.5–2 mm Cellular structure of dinosaur scales reveals retention of reptile-type skin during the evolutionary transition to feathers - PMC nih .
The cutaneous spikes of Haolong dongi represent a fundamentally different structure from the osteoderms (dermal bones) found in ankylosaurs, stegosaurs, and other armored dinosaursCellular-level preservation of cutaneous spikes in an Early ... - Naturenature . Osteoderms are bones that form within the skin, composed of an outer thin cortical bone layer and inner thick cancellous bone, covered externally by keratinous scutesScutes and osteoderms | Dave Hone's Archosaur Musingswordpress +1. All ankylosaur osteoderms possess extensive structural fibers and follow similar developmental pathways across spikes, plates, and clubsFunction and Evolution of Ankylosaur Dermal Armorbioone .
In contrast, the Haolong dongi spikes are purely cutaneous—epidermal-origin structures composed entirely of cornified tissue without bony coresCellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1. This distinction is critical because it demonstrates that dinosaurs evolved multiple independent pathways to develop defensive projections, a finding that fundamentally expands our understanding of integumentary diversity in the clade.
The spikes differ structurally from known protofeathers in non-avian dinosaurs and scaly spines in extant squamates, suggesting a distinct evolutionary originCellular-level preservation of cutaneous spikes in an Early ... - Naturenature . Previous debates about dinosaur integumentary structures have focused on whether filamentous coverings represent true protofeathers or degraded collagenous fiber meshworks from the dermisDo Feathered Dinosaurs Exist? Testing the Hypothesis on ...unc . The Haolong dongi spikes represent an entirely different category: cornified hollow cylinders with organized epidermal cellular architecture, clearly distinct from both filamentous structures and reptilian scaly spinesCellular-level preservation of cutaneous spikes in an Early ... - Naturenature .
Some researchers have suggested that ornithischian integumentary structures like Psittacosaurus quills, given their presence alongside scales, are more likely scaly derivatives than feathery onesprotofeathersreptilis . The Haolong dongi discovery supports this interpretation by demonstrating that ornithischians independently evolved novel epidermal defensive structures through pathways distinct from theropod feather evolution.
Prior to this discovery, extensive phylogenetic analyses of dinosaur integumentary structures using maximum-likelihood methods found that scales were most likely the plesiomorphic (ancestral) state for Dinosauria, with scaled likelihoods ranging from 0.92–1.00 for Dinosauria and 0.79–0.97 for Ornithischia when assuming a scaled pterosaur ancestor Evolution of dinosaur epidermal structures - PMC nih . These analyses concluded that protofeathers were probably not synapomorphic for dinosaurs, and scaly skin, unadorned by feathers or their precursors, was primitive for Dinosauria and retained in the majority of ornithischians Evolution of dinosaur epidermal structures - PMC nih .
The Haolong dongi discovery does not directly contradict this finding but significantly complicates it by demonstrating that ornithischian integumentary evolution was far more experimentally diverse than previously recognized“Spiny dragon” reveals hidden secrets of dinosaur skin ...naturalsciences . As one researcher noted, "The complexity of dinosaur skin is far greater than we imagined"“Spiny dragon” reveals hidden secrets of dinosaur skin ...naturalsciences .
The filamentous structures found in ornithischians like Tianyulong, Psittacosaurus, and Kulindadromeus were already interpreted as evolutionary experiments independent of true feather originsEvolution of Dinosaur Skin and Feathers: A Complex Story | David Evans Labwordpress . Kulindadromeus from the Jurassic of Siberia displayed simple filamentous fuzz alongside scales, plates, and bizarre tufted plates, demonstrating that primitive ornithischians possessed a wide variety of integumental featuresGEOL 104 Thyreophora: Defense! Defense! Defense!umd +1.
The Haolong dongi cutaneous spikes now add another distinct integumentary innovation to this list—one never previously observed in any dinosaur“Spiny dragon” reveals hidden secrets of dinosaur skin ...naturalsciences +1. This supports the emerging model that dinosaurs "experimented extensively with their 'outer look' and potentially independently along separate evolutionary lineages"Origins of feathered dinosaurs more complex than first thought - Phys.orgphys .
Recent evidence from Psittacosaurus demonstrates that early feathered animals had a mix of skin types, with avian-like skin only in feathered regions and reptile-style scaly skin in non-feathered regions'Hidden Gem' Dinosaur Skin Fossil Reveals Surprises About Feather Evolution : ScienceAlertsciencealert +1. The fossil skin cells share features with modern reptile skin cells, including similar cell size and shape and fused cell boundaries—a feature known only in modern reptiles'Hidden Gem' Dinosaur Skin Fossil Reveals Surprises About Feather Evolution : ScienceAlertsciencealert .
The Haolong dongi discovery extends this zoned development model by showing that different body regions could develop entirely different specialized structures: scutate scales on the tail, tuberculate scales on the neck and thorax, and cutaneous spikes interspersed throughoutCellular-level preservation of cutaneous spikes in an Early ... - Naturenature . This regional specialization indicates sophisticated developmental control over integumentary differentiation in ornithischian dinosaurs.
The discovery presents significant challenges for phylogenetic character coding. Traditional analyses of ornithischian defensive structures have focused on osteoderms—their presence, arrangement, and morphologyA new phylogeny of Stegosauria (Dinosauria, Ornithischia) | Request PDFresearchgate +1. Character state optimization has been conducted under maximum-parsimony criteria to infer the most parsimonious distribution of defensive traits across ornithischian phylogenyEvolutionary Trends in the Jaw Adductor Mechanics of Ornithischian Dinosaurs | Request PDFresearchgate .
The cutaneous spikes of Haolong dongi represent an entirely novel character state that must be incorporated into existing matrices. Because these structures are developmentally and structurally distinct from both osteoderms and filamentous integument, they cannot simply be coded as a variant of existing characters but require new character definitionsCellular-level preservation of cutaneous spikes in an Early ... - Naturenature .
Phylogenetic analysis suggests Haolong dongi is positioned within Iguanodontia, with some evidence indicating a clade relationship with Thescelosaurus and HypsilophodonPhylogeny of iguanodontian dinosaurs and the evolution of ...palaeo-electronica . This phylogenetic placement has significant implications for understanding when and how often cutaneous spikes evolved within Ornithischia.
The methodology of phylogenetic descent with modification analysis (PDA) requires identifying diagnostic features of an adaptation, performing phylogenetic analyses using all available character data, identifying the sequence of acquisition of each diagnostic feature, and analyzing the apparent roles of characters at each successive stageThe evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? - Padian - 2011 - Journal of Zoology - Wiley Online Librarywiley . For cutaneous spikes, this framework must now account for a structure type that appears only in Haolong dongi among known iguanodontians, raising questions about whether this represents an autapomorphy (unique derived feature) or a more widespread trait lost or not preserved in other lineages.
Prevailing interpretations of dinosaur defensive structures have increasingly emphasized display and species recognition functions alongside or instead of pure anti-predator defenseThe evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? - Padian - 2011 - Journal of Zoology - Wiley Online Librarywiley +1. The variation in ankylosaur scute form and distribution, for instance, suggests that there was no "optimal" defensive pattern and that display may have been more important than defense in shaping armor evolutionThe evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? - Padian - 2011 - Journal of Zoology - Wiley Online Librarywiley +1.
The researchers interpreting Haolong dongi propose that the spikes likely served as a deterrent against predators, comparable in function to porcupine quills, making the animal harder to swallow for smaller theropods in the same ecosystem“Spiny dragon” reveals hidden secrets of dinosaur skin ...naturalsciences +1. The hollow, cornified nature of the spikes with a porous dermal pulp also suggests potential secondary functions in thermoregulation or mechanoreceptionCellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1.
The porous central dermal pulp structure described in the Haolong dongi spikes may have facilitated thermoregulationCellular-level preservation of cutaneous spikes in an Early ... - Naturenature . In extant vertebrates, epidermal appendages including scales, feathers, and hairs all contribute to thermoregulation through various mechanismsComparison of vertebrate skin structure at class level: A review - Akat - 2022 - The Anatomical Record - Wiley Online Librarywiley . Feathers are vital for thermoregulation in birds, while hair in mammals helps maintain internal body temperatureComparison of vertebrate skin structure at class level: A review - Akat - 2022 - The Anatomical Record - Wiley Online Librarywiley .
Stegosaur plates provide a comparison: CT scans reveal that plates have large internal vascular piping while spikes have a large axial channel, suggesting functional and evolutionary distinctions between cutaneous spikes and more extensive osteodermsDifferent Growth Patterns between the Skeleton and Osteoderms of Stegosaurus (Ornithischia: Thyreophora) | Request PDFresearchgate . The highly vascularized nature of stegosaur plates has been interpreted as evidence for thermoregulatory functionThe evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? - Padian - 2011 - Journal of Zoology - Wiley Online Librarywiley +1.
Sensory perception is an important function of vertebrate skin, with various receptors detecting touch, heat, cold, and painComparison of vertebrate skin structure at class level: A review - Akat - 2022 - The Anatomical Record - Wiley Online Librarywiley +1. In lepidosaurs, skin sense organs are found in the epidermis, including mechanoreceptors that detect mechanical stimuliComparison of vertebrate skin structure at class level: A review - Akat - 2022 - The Anatomical Record - Wiley Online Librarywiley . The epidermis of mammals contains Merkel cells that serve as mechanoreceptors for light touchAnatomy, Skin (Integument), Epidermis - StatPearls - NCBI Bookshelfnih .
The preservation of the dermal pulp in the Haolong dongi spikes raises the possibility that these structures housed sensory nerve endings, similar to how hair follicle plexuses detect movement of hair at the skin surfaceLayers of the Skin | Anatomy and Physiology I - Lumen Learninglumenlearning +1. However, determining whether sensory function was present requires additional taphonomic and comparative analysis.
The Early Cretaceous Jehol Biota has yielded numerous small-sized dinosaurs and documented predator-prey interactions, including gut contents showing predation on mammals, birds, and other dinosaurs(PDF) Inter-amphibian predation in the Early Cretaceous of China - ResearchGateresearchgate +1. Predators in this ecosystem included Sinosauropteryx (with mammal gut contents), Sinocalliopteryx (with Confuciusornis, Sinornithosaurus, and ornithischian dinosaur gut contents), and Microraptor (with enantiornithine bird gut contents)(PDF) Inter-amphibian predation in the Early Cretaceous of China - ResearchGateresearchgate .
The high predation pressure in this ecosystem, particularly from numerous smaller theropods, provides ecological context for interpreting the defensive function of the Haolong dongi spikes“Spiny dragon” reveals hidden secrets of dinosaur skin ...naturalsciences . The Jehol environment was predominantly closed and forested, though evidence from Sinosauropteryx countershading patterns indicates more heterogeneous habitat availabilityCountershading and Stripes in the Theropod Dinosaur Sinosauropteryx Reveal Heterogeneous Habitats in the Early Cretaceous Jehol Biota - ScienceDirectsciencedirect .
The Haolong dongi specimen represents a juvenile individualCellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1. This raises important questions about whether the cutaneous spikes represent a juvenile-specific defensive adaptation or would have persisted into adulthood.
Ontogenetic studies in other dinosaurs demonstrate that integumentary structures can change dramatically during growth. In pachycephalosaurs, for example, dome size increases while spike size decreases during development, with Dracorex, Stygimoloch, and Pachycephalosaurus now interpreted as growth stages of a single taxonShape-shifting dinosaurs - Creation Ministries Internationalcreation . Similarly, in the oviraptorosaur Similicaudipteryx, radical morphological changes occurred in feather development between early and late juvenile stages[PDF] Exceptional dinosaur fossils show ontogenetic development of early featherscas .
Whether juvenile Haolong dongi spikes would have been retained, modified, or lost in adults remains unknown. In tyrannosauroids, the presence of scales in large adults does not rule out the possibility that younger individuals possessed feathers, though such a developmental switchover would be unprecedentedTyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution | Biology Letters | The Royal Societyroyalsocietypublishing .
The exceptional preservation of Haolong dongi raises questions about whether similar cutaneous spikes existed in other ornithischians but were not preserved. Beta-keratins, the structural proteins comprising reptilian and avian epidermal structures, demonstrate high preservation potential due to disulfide bonds conferring rigidity and hydrophobic residues excluding waterKeratin Durability Has Implications for the Fossil Record: Results from a 10 Year Feather Degradation Experiment | PLOS One - Research journalsplos +1.
However, keratin preservation remains contentious, with some researchers arguing that keratinous structures fossilize primarily as pigment and/or calcium phosphate remnants rather than as preserved polypeptidesCommentary: Keratin protein preservationpalaeo-electronica +1. Future interpretations of fossils must consider that calcium phosphate and pigments may be the only components of keratinous structures known to survive fossilization in mature sedimentsLow fossilization potential of keratin protein revealed by experimental taphonomy - Saitta - 2017 - Palaeontology - Wiley Online Librarywiley .
The Jehol Biota's exceptional preservation conditions—fine-grained lacustrine sediments with acidic water from active volcanism—favor soft tissue preservationThe dinosaur boom in the Cretaceous | Geological Society, London, Special Publicationslyellcollection . The absence of similar cutaneous spikes in other ornithischian fossils may therefore reflect taphonomic bias rather than true absence, and targeted examination of other well-preserved specimens from similar depositional environments may reveal additional examples.
The Haolong dongi discovery necessitates a revised framework for understanding ornithischian integumentary evolution:
Multiple independent origins of defensive projections: Ornithischians evolved at least three distinct types of defensive projections—bony osteoderms (in thyreophorans), filamentous structures (in basal neornithischians), and purely cutaneous spikes (in at least one iguanodontian)Cellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1.
Greater integumentary complexity than previously recognized: The combination of scutate scales, tuberculate scales, and cutaneous spikes in a single individual demonstrates sophisticated regional differentiation of integumentary developmentCellular-level preservation of cutaneous spikes in an Early ... - Naturenature .
Novel defensive solutions outside Thyreophora: Previously, elaborated defensive integumentary structures in ornithischians were primarily associated with Thyreophora (ankylosaurs and stegosaurs). The Haolong dongi discovery shows that ornithopods independently evolved comparable defensive adaptations through entirely different developmental pathwaysCellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1.
Multifunctional integumentary structures: The morphology and internal architecture of the spikes suggest they served multiple functions beyond defense, potentially including thermoregulation and mechanoreceptionCellular-level preservation of cutaneous spikes in an Early ... - Naturenature +1.
The discovery of Haolong dongi demonstrates that even extensively studied groups like iguanodontian dinosaurs continue to yield surprises, fundamentally expanding our understanding of the diversity and complexity of dinosaurian integumentary evolution“Spiny dragon” reveals hidden secrets of dinosaur skin ...naturalsciences .