Paleontologists Identify Likely First Case of an Infected Skin Ulcer in a Dinosaur

Key facts

The identification of the ancient lesion occurred during a detailed re-examination of fossilized *Iguanodon* hindlimb elements. Paleontologists examining the tibia—the primary weight-bearing shinbone—noticed an irregular, localized mass of reactive bone protruding from the shaft surface. In a healthy animal, cortical bone presents a smooth, uniform outer texture. In contrast, the affected region on this specimen displayed a roughened, elevated mass characteristic of periostitis or localized osteomyelitis, biological conditions in which bone tissue expands in response to persistent inflammation or infectious agents.

By evaluating the morphology and structural distribution of the abnormal bone, researchers determined that the injury was unlikely to have stemmed from a simple mechanical fracture or an internal bone tumor. Instead, the pathology corresponds closely to medical cases in modern animals where a deep skin ulcer—caused by physical trauma, environmental abrasions, or parasitic damage—becomes chronically contaminated with environmental pathogens. As the localized infection broke down the skin, subcutaneous fascia, and underlying muscle tissue, infectious microorganisms eventually made contact with the periosteum, the dense membrane of vascular connective tissue that coats the exterior of bones.

In response to persistent bacterial irritation, the periosteum produced chaotic layers of new secondary bone tissue, forming the permanent swelling preserved in the fossilized skeleton. The thickness and structural density of the bony reaction indicate that the *Iguanodon* lived with the painful, open ulcer for weeks or even months before its death, providing sufficient time for its body to mount an osteogenic defense.

Why it matters

The discovery provides an extraordinary look into the soft-tissue vulnerability and immune capabilities of non-avian dinosaurs, demonstrating that these extinct reptiles suffered from dermatological conditions comparable to those found in modern animals and humans. Paleopathology has historically focused on dramatic skeletal injuries, such as healed compound fractures, severe osteoarthritis, or gouges left by predator teeth, primarily because soft tissue rarely fossilizes. Skin, fat, and muscle decay rapidly after death, leaving scientists largely unable to observe dermatological diseases in prehistoric animals unless they leave a direct imprint on the skeleton.

This finding establishes that deep soft-tissue infections can indeed leave unmistakable secondary markers on underlying bone when an external ulcer breaches structural boundaries. By defining a clear diagnostic pattern for ulcer-induced bone swelling, the research enables paleontologists to review thousands of dinosaur limb bones stored in museum collections around the world that may have previously been misidentified as ordinary healed fractures or non-specific bone tumors.

Additionally, the evidence that an *Iguanodon* survived a chronic, deep leg infection highlights the physical resilience and immune capacity of large herbivorous dinosaurs. As a massive animal reaching lengths of up to 10 meters and weighing several metric tons, *Iguanodon* relied heavily on the structural integrity of its hindlimbs for movement and defense. Operating with a severe, painful ulcer on its lower leg would have impaired its gait and speed, increasing its vulnerability to Early Cretaceous predators or isolating it from its herd. The fact that the animal maintained normal bone deposition processes around the site of infection confirms that its immune system was capable of isolating and surviving severe localized infections over extended periods.

*Iguanodon* holds a central place in the history of paleontology. First discovered in Sussex, England, in the early 1820s by Mary Ann Mantell and described by her husband, physician Gideon Mantell, *Iguanodon* was one of three original genera—alongside *Megalosaurus* and *Hylaeosaurus*—used by English anatomist Sir Richard Owen in 1842 to establish the formal scientific category Dinosauria. Existing during the Early Cretaceous Period between 125 million and 110 million years ago, *Iguanodon* species were among the dominant terrestrial herbivores across Europe, North America, and Asia.

Despite two centuries of academic study involving thousands of cataloged bones—including major fossil discoveries such as the 1878 Bernissart coal mine assemblage in Belgium, which yielded dozens of complete skeletons—most scientific research historically concentrated on skeletal anatomy, biomechanics, and evolutionary relationships. The systematic study of fossilized disease, known as paleopathology, developed more rigorous diagnostic standards in recent decades as researchers adapted modern medical tools to inspect fossil interiors.

In early paleontological literature, disease diagnoses were often subjective and based purely on external visual inspection. Over the past twenty years, the adoption of high-resolution computed tomography (CT), micro-CT scanning, and thin-section bone histology has allowed scientists to examine micro-vascular structures inside fossilized bones without destroying specimens. Previous paleopathological milestones have documented malignant bone cancer in a horned dinosaur (*Centrosaurus*), respiratory infections in long-necked sauropods, and septic arthritis in duck-billed hadrosaurs. However, unambiguous evidence of an external skin ulcer eroding into the skeleton had not been confirmed in dinosaur fossils until this analysis.

Reaction

Following the report, paleontologists and specialists in ancient disease are expected to evaluate the study's diagnostic framework within academic literature and conference presentations, such as the annual meeting of the Society of Vertebrate Paleontology. Researchers will closely examine the microscopic tissue arrangements and surface morphology to verify that alternative causes, such as localized bone trauma or fungal infections, can be conclusively ruled out.

Veterinary pathologists specializing in reptiles and birds are also expected to weigh in with comparative analyses. Because birds represent the living evolutionary lineage of theropod dinosaurs and crocodilians are their closest living archosaurian relatives, modern veterinary data regarding severe avian foot and leg ulcers—such as bumblefoot, or ulcerative pododermatitis—provide vital reference points for understanding how archosaurian soft tissue and periosteum react to persistent open wounds. Museum curators overseeing extensive ornithopod fossil collections may also launch targeted re-examinations of stored leg bones to identify similar pathological growths.

What we don't know yet

Several important questions about the *Iguanodon* specimen remain unanswered due to the physical limitations of fossilization. Because organic genetic material and soft tissues degrade over millions of years, scientists cannot identify the specific bacterial, fungal, or viral pathogen that infected the skin ulcer. It is also impossible to determine the exact initial cause of the skin wound; the puncture could have resulted from an attack by a theropod predator, a territorial clash with a rival herbivore, a sharp environmental puncture from vegetation, or a severe parasitic infestation.

Furthermore, the available evidence does not clarify whether the ulcer directly caused the animal's death. While the presence of secondary reactive bone proves the dinosaur survived the initial phase of infection, the open wound could have eventually led to fatal systemic blood poisoning (sepsis), or the associated lameness may have left the herbivore unable to forage effectively or escape predators. Finally, because the surrounding skin and muscle did not preserve, the precise exterior dimensions and visual appearance of the open ulcer on the living animal's leg can only be inferred from the footprint of the bony lesion beneath it.

What to watch

In the coming years, several specific developments will indicate how this finding shapes paleontological science:

This account is based on original reporting by *Discover Magazine*.