Introduction: A Discovery That Overturns Existing Knowledge
The research findings published in the academic journal "Nature Ecology & Evolution" on February 6, 2026, fundamentally overturned existing knowledge regarding the integumentary system of dinosaurs.
High-resolution structural analysis at the cellular level revealed that "Haolong dongi," a newly discovered species of iguanodontian found in the Lower Cretaceous strata (approximately 125 million years ago) of northeastern China, possessed "hollow cutaneous spikes"—a feature never before confirmed in dinosaur fossils.
This report provides a comprehensive explanation of the details of this innovative discovery, the preservation state of the specimen, its histological properties, and its significance in the evolutionary history of dinosaurs.
Geological Background and Specimen Overview
Gemini Generated Image
The holotype specimen (AGM 16793) of Haolong dongi was excavated from the Yixian Formation of the Jehol Group, distributed in Liaoning Province, China. This area is globally renowned as the origin of the exceptionally well-preserved "Jehol Biota," where numerous significant fossils, including feathered dinosaurs, early birds, mammals, and angiosperms, have been found.
Depositional Environment and Taphonomy of the Yixian Formation
Northeastern China during the Early Cretaceous, where Haolong lived, was a lake region accompanied by active volcanic activity. The sediments consisted of fine volcanic ash and silt, creating an ideal preservation environment where carcasses were rapidly buried, protecting them from oxygen decomposition and scavenger damage.
What is authigenic mineralization?
A notable aspect of the Haolong specimen is the preservation mode of its soft tissues. Usually, dinosaur skin is often left as an impression (mold), but in Haolong's case, a phenomenon called "authigenic mineralization" occurred, where the fine structures down to the cell nucleus were replaced and mineralized by calcium phosphate. This rare state of preservation has made it possible to directly observe biological information at the cellular level from 125 million years ago.
Morphological Characteristics of the Holotype Specimen
The discovered individual is an almost complete skeleton with a total length of about 2.45 meters (8 feet), unearthed in an articulated state. Histological analysis of the bones and the closure status of the neural arches indicate that this individual was still a juvenile at the time of its death.
| Item | Detailed Data |
|---|---|
| Scientific Name | Haolong dongi |
| Specimen Number | AGM 16793 (Holotype) |
| Discovery Site | Liaoning Province, China (Yixian Formation) |
| Era | Early Cretaceous (approx. 125 Ma) |
| Total Length | Approx. 2.45 m |
| Developmental Stage | Juvenile |
| Classification | Ornithischia, Iguanodontia (Basal hadrosauroid) |
Taxonomic Status and Phylogeny
The name Haolong dongi was given in honor of Dr. Dong Zhiming, a pioneer in Chinese paleontology. The genus name "Haolong" means "Spiny Dragon" in Chinese, symbolizing its unique integumentary structure.
Position within Iguanodontia
As a result of phylogenetic analysis, Haolong was found to be positioned as a basal hadrosauroid, close to the lineage of the Hadrosauridae (commonly known as "duck-billed dinosaurs") that thrived later in the Late Cretaceous, among the Iguanodontia.
Previously, based on studies of "mummified" specimens, the skin of hadrosauroids was thought to consist primarily of non-overlapping, polygon-shaped tubercle-like scales. However, the discovery of Haolong suggests that in the early evolutionary stages of this group, there existed species equipped with extremely complex and aggressive defensive armor, reminiscent of modern mammals like porcupines.
Detailed Analysis of the Integumentary System: Three Types of Appendages
Haolong's entire body was covered in three main integumentary structures that varied by region. These structures demonstrate a diversity that overturns the conventional image of dinosaurs having "monotonous scaly skin."
1. Overlapping Scales (Scutate Scales) on the Tail
Large, overlapping "scutate scales" were confirmed at the end of the tail. These scales take an imbricating arrangement, much like the scales of modern lizards and snakes. Because overlapping scales tend to disappear in later hadrosaurs, Haolong's tail section exhibits a more ancestral or independently evolved form of armor, believed to have served the role of increasing physical defense without hindering the tail's movement.
2. Tuberculate Scales on the Neck and Torso
From the neck to the trunk, small, non-overlapping "tuberculate scales" formed the basal layer. These scales were arranged irregularly, functioning as "basement scales" that protected the whole body while maintaining skin flexibility. The thickness of the stratum corneum was measured at about 90 μm, a value close to the stratum corneum of modern crocodilians and bird scales.
3. Hollow Cutaneous Spikes
The "cutaneous spikes," which are the most prominent feature of this species, grew scattered among the scales on the neck, back, and sides. They varied in size, ranging from a minimum of about 2 mm to a maximum of 44.2 mm (about 1.7 inches). Cross-sectional analysis revealed that these spikes were "cylindrical" with a hollow or porous internal structure. Furthermore, they were composed of skin-derived keratin rather than bone tissue (osteoderms), making them entirely different embryologically from the armor plates of ankylosaurs.
| Type of Skin Appendage | Location | Structural Features | Estimated Function |
|---|---|---|---|
| Scutate Scales | Tail | Overlapping, shield-like | Physical defense, ensuring flexibility |
| Tuberculate Scales | Neck and Torso | Small, non-overlapping, irregular | Basic skin protection |
| Cutaneous Spikes | Whole body (among scales) | Hollow, cylindrical, keratinous | Defense, thermoregulation, mechanoreception |
Histological and Cellular-Level Findings
The most shocking data provided by the Haolong fossil is its microscopic histological structure. Through high-resolution X-ray micro-CT scanning and observation of tissue sections, an "anatomical map of the skin" of an organism from over 100 million years ago was recreated.
The following layer structures were confirmed from histological sections of the cutaneous spikes:
- Stratum Corneum: A highly keratinized layer forming the outermost part.
- Pluristratified Epidermis: The layer of living cells beneath the stratum corneum. Here, "keratinocytes," the cells that produce keratin, are preserved along with their cell nuclei.
- Dermal Pulp: A porous core in the center of the spike. It is inferred that blood vessels and nerves passed through here during life, supporting the growth of the spike.
Such preservation at the cellular level has opened an unprecedented window for understanding the physiology, metabolism, and growth processes of dinosaurs. In particular, it has been pointed out that by analyzing the shape and density of cell nuclei, it may be possible to estimate Haolong's growth rate and genome size in the future.
Functional Morphology and Adaptive Strategies
Why did Haolong dongi evolve such highly peculiar armor? The research team has proposed hypotheses from three perspectives: defense, thermoregulation, and mechanoreception.
Defense Mechanism: Countermeasures Against Predators
The most plausible theory is adaptation to predation pressure in the ecosystem of the time. The Yixian Formation was home to many small to medium-sized theropods (e.g., therizinosaurs, dromaeosaurs, and early tyrannosaurs). Many small predators employ tactics of swallowing prey whole or biting off specific parts. Having sharp, hollow spikes scattered over the body, much like a porcupine, would have acted as a powerful deterrent, preventing predators from carelessly biting Haolong. Additionally, the fact that the Haolong specimen was a juvenile suggests the possibility that these spikes were "juvenile-specific armor" designed to increase survival rates during a particularly vulnerable growth stage.
Physiological Function: Assistance in Thermoregulation
The second hypothesis involves thermoregulation by expanding surface area. It is known that this region during the Early Cretaceous was relatively cool, with an average temperature of about 10°C. A hollow structure is lightweight yet can gain surface area, potentially contributing to heat absorption from sunlight and the release of excess body heat. While Yutyrannus, a large dinosaur from the same period, insulated itself with thick feathers, Haolong may have adapted to the environment using a different method—spikes.
Sensory Function: Potential as Mechanoreceptors
The research team also mentioned the possibility that these spikes functioned as sensory organs (mechanoreception). Some modern snakes sense surrounding vibrations using bristle-like structures on their body surface. It is speculated that Haolong's spikes might have also served as "sensors" to detect air vibrations caused by moving through dense forests or vegetation, or by the approach of predators.
Evolutionary Significance and Conclusion: Beyond the Scale-vs-Feather Dichotomy
The discovery of Haolong dongi has fundamentally changed how we view "skin evolution" across the entire dinosaur phylogenetic tree. Until now, the body covering of dinosaurs has often been discussed in a simple dichotomy: ornithischians with scales and some theropods with feathers. However, Haolong's spikes are the result of a unique evolutionary path, structurally different from both feathers and the spikes of modern lizards.
| Subject of Comparison | Difference from Haolong's Cutaneous Spikes |
|---|---|
| Feathers (Protofeathers) | Feathers have a branching structure, whereas Haolong's spikes are simple cylinders. The internal structures are also different. |
| Ankylosaurus armor plates | They are bony tissues (dermal bone), whereas Haolong's spikes are epidermal keratin. |
| Extant lizard spikes | They are simply pointed skin scales and lack the complex hollow structure and internal pulp layer seen in Haolong. |
Filamentous or bristle-like structures in ornithischians have been previously reported in Kulindadromeus and Psittacosaurus. While sharing some similarities with these, Haolong is decisively unique in having "hollow spikes scattered over the entire body." This confirms that even among ornithischians, integumentary evolution proceeded independently and extremely complexly across multiple lineages.
Conclusion and Future Research Challenges
The discovery of Haolong dongi has once again demonstrated the importance of "soft tissue preservation" in paleontology. The fact that such a surprise lay hidden in the 200-year history of iguanodontian research tells us that we have still only captured fragments of the true appearance of dinosaurs.
Future research awaits the discovery of adult specimens to determine whether these spikes, seen in the juvenile, fall off with growth or evolve into more robust armor. Furthermore, if the analysis of melanosomes progresses, the body color and pattern will likely be revealed.
Haolong dongi has pushed the boundaries of the biological diversity and adaptive capabilities of dinosaurs. Going forward, through chemical analysis and further high-resolution imaging, the 125-million-year-old story told by this "Spiny Dragon" will undoubtedly become even clearer.
References and Bibliography
- A dinosaur with spikes exhibiting unprecedented properties ... - CNRS, https://www.cnrs.fr/en/press/dinosaur-spikes-exhibiting-unprecedented-properties-discovered-china
- Paleontologists Unearth New Dinosaur Species with Never-Before ..., https://www.sci.news/paleontology/haolong-dongi-14545.html
- Haolong dongi: Newly unearthed fossil gives scientists unprecedented look at dinosaur skin, https://www.independent.co.uk/news/science/dinosaur-fossil-skin-haolong-dongi-b2916538.html
- Cellular-level preservation of cutaneous spikes in an Early Cretaceous iguanodontian dinosaur | Request PDF - ResearchGate
- This 125 Million-Year-Old Dinosaur Had Spikes Like a Porcupine - SciTechDaily, https://scitechdaily.com/this-125-million-year-old-dinosaur-had-spikes-like-a-porcupine/