The Skin of an Early Cretaceous Iguanodontian: Cellular-Level Insights from Haolong dongi

Haolong dongi is an iguanodontian dinosaur discovered in the Lower Cretaceous Yixian Formation of western Liaoning, China. The generic name means "spiny dragon," while the species name honors Dong Zhiming, a pioneering figure in Chinese dinosaur research. The holotype specimen, AGM 16793, is housed in the Anhui Geological Museum. The skeleton measures approximately 2.45 meters in length. The absence of fusion between the vertebral centra and neural arches, along with incomplete articulation between caudal vertebrae and ribs, indicates that this individual was osteologically immature at the time of death.

Reconstruction of Haolong dongi（Courtesy of Fabio Manucci）

Phylogenetically, Haolong represents a relatively basal member of Hadrosauroidea within Iguanodontia, occupying a position more primitive than contemporaneous taxa from the same formation such as Jinzhousaurus yangi and Bolong yixianensis. It retains plesiomorphic cranial features, including a visible antorbital fenestra between the maxilla and lacrimal in lateral view.

Previous discoveries of skin impressions in Late Cretaceous hadrosaurids have shown that their integument typically consisted of a complex mosaic of small, tubercle-like, non-imbricating scales, often accompanied by enlarged feature scales along the dorsal midline. By contrast, integumentary data from more basal iguanodontians have remained sparse. The specimen of Haolong dongi preserves extensive skin impressions from the tail, neck, thorax, hips, and base of the tail. These structures were examined using a combination of natural light observation, laser-stimulated fluorescence (LSF), X-ray computed tomography, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and histological thin sections, allowing detailed reconstruction of their morphology and composition.

Along the left side of the tail, at least nine longitudinal rows of large scutate scales are present. These scales are arranged in an imbricating pattern, partially overlapping one another like roof tiles, extending from the base of the tail to its distal end. Proximal scales reach several centimeters in size and gradually decrease in size both distally and ventrally. Such overlapping scales are common in extant lizards and snakes, where they function to reduce water loss and provide mechanical protection in terrestrial environments. This tail scalation differs markedly from that of adult hadrosaurids, which typically exhibit small, polygonal, non-overlapping scales.

Reconstruction of Haolong dongi and corresponding fossil structures: (c) small, non-overlapping tuberculate scales on the neck; (d) small spikes on the neck; (e) large scutate scales along the dorsal region of the proximal tail (non-overlapping in this area); (f) small tuberculate scales and spikes in the sternal region; (g) light blue areas in the sternal region also represent small tuberculate scales and spikes; (h) close-up view of (g)（Courtesy of Huang J et al. (2026)）

In addition to these large scutate scales, the neck, thorax, hips, and proximal tail region are covered with numerous small tuberculate scales. These are predominantly rounded, with some ovoid or polygonal forms, measuring approximately 0.6 to 1.3 mm in diameter. Tomographic data reveal a dense outer layer with an average thickness of about 90 µm, comparable to the stratum corneum of ventral scales in modern crocodilians or the scales on avian feet.

Interspersed among these small tuberculate scales are numerous cutaneous spikes. Most of these are small, measuring 2–3 mm in length and about 0.1 mm in maximum width, and are generally aligned parallel to one another with their tips directed posteriorly. Medium-sized spikes, 5–7 mm in length, are also present, and several larger spikes are preserved, albeit incompletely. The largest of these reaches 44.2 mm in length with a basal width of 7.8 mm. Together, the mixture of scales and spikes produces a rough, spinose, and partially armored integument.

Tomographic and histological analyses demonstrate that these spikes are not bony structures. Instead, they are hollow and cylindrical, consisting of a cornified epidermal cortex surrounding a porous central dermal pulp. The outer layer includes a stratum corneum overlying a pluristratified epidermis composed of keratinocytes, some of which retain preserved nuclei. This exceptional preservation allows detailed reconstruction of the skin's microanatomy. Within the central pulp, spherical structures approximately 10 µm in diameter are present; their size and morphology do not match fibroblasts and are instead interpreted as mineralized traces associated with microbial activity during decomposition.

At first glance, these hollow spikes might be compared to early stages of feather evolution, as earlier models proposed tubular structures as the initial phase of feather development. However, current understanding suggests that the earliest feathers were solid monofilaments containing melanosomes rather than hollow tubes. The spikes in Haolong lack observable melanosomes and differ in both morphology and distribution from protofeathers in non-avian theropods. They also differ from the scaly spines of extant squamates, which typically arise from broadened scale bases, whereas the spikes in Haolong form distinct tubular structures with a central pulp. These observations indicate that the spikes represent a unique type of integumentary appendage with no clear homology to feathers or elongated scales.

Several functional interpretations have been proposed. The paleoenvironment of the Yixian Formation was relatively cool, and the presence of large feathered theropods such as Yutyrannus huali has been interpreted as an adaptation to lower mean annual temperatures. The spikes in Haolong may have contributed to thermoregulation, although their density appears insufficient to form an effective insulating layer. A sensory function is also conceivable, as microscopic spinules on the scales of modern lizards and snakes can detect tactile stimuli and vibrations, and in some cases aid in water absorption. However, the spikes of Haolong are substantially larger and structurally distinct, making such a function uncertain.

Reconstruction of Yutyrannus huali（Image source：TotalDino， CC BY 4.0 ）

The most plausible primary function is defensive. The Yixian ecosystem included not only large predators but also numerous small theropods with limited gape size. In such environments, prey species often evolve spines, osteoderms, or other protrusions that make them more difficult to capture and ingest. In Haolong, vulnerable regions of the body are covered with keratinous spikes, while the tail is protected by large overlapping scutate scales. These structures would not have rendered the animal invulnerable, but they likely increased the time and effort required for predators to subdue and consume it—an advantage particularly important for a juvenile individual.

The discovery of Haolong dongi provides an unprecedented view of dinosaur skin at the cellular level. It reveals that iguanodontian dinosaurs possessed a far greater diversity of integumentary structures than previously recognized. Rather than being covered solely by simple scales, they developed a range of skin appendages that may have served multiple roles, including defense, thermoregulation, and sensory function. This specimen offers a rare window into the complexity of dinosaur integument, preserving subtle and unexpected variations along the thin boundary between organism and environment in Early Cretaceous ecosystems.

Author: Shui-Ye You

Reference:

Huang J et al. (2026). Cellular-level preservation of cutaneous spikes in an Early Cretaceous iguanodontian dinosaur. Nature Ecology & Evolution.