Somewhere in the lakebeds of northeastern China, a baby dinosaur died 125 million years ago and took a secret to its grave. The secret stayed hidden until a research team peeled back the rock and found something no one had seen on this kind of dinosaur: rows of hollow, hedgehog-like spikes running across the animal’s body, preserved down to the individual cells.
The specimen, described in a study published in Nature Ecology & Evolution in May 2026, belongs to a newly named species called Haolong dongi. It is an iguanodontian, part of the broad family of plant-eating dinosaurs that eventually gave rise to the duck-billed hadrosaurs. Until now, no member of that lineage had ever been found wearing anything like these spikes.
A juvenile with an unexpected coat
The fossil, catalogued as holotype AGM 16793, came from the Lower Cretaceous Yixian Formation, a stretch of ancient lakebed sediments famous for preserving feathered theropods, early birds, and mammals with fur impressions. The fine volcanic ash that buried these creatures acts almost like a natural casting mold, capturing soft-tissue details that vanish in coarser rock.
Bone analysis confirmed the specimen was a juvenile. Key skeletal elements were incompletely hardened, and the animal’s proportions matched early growth stages rather than adult form. The research team has not disclosed an exact body length, but the overall size is small, consistent with a young animal that had not yet reached anything close to full iguanodontian bulk.
What makes Haolong dongi remarkable is not just that it had spikes, but what those spikes look like on the inside. Using a combination of high-resolution CT scanning, thin-section histology, and Laser-Stimulated Fluorescence (LSF), the researchers showed that each spike is hollow. That internal architecture has no parallel in any previously documented dinosaur skin structure. Ankylosaurs carried heavy, solid osteoderms. Some ceratopsians had horn-like projections reinforced with bone. The hollow tubes on this baby iguanodontian are something else entirely.
Seeing what normal light cannot
LSF, a technique formally described in 2015, works by bathing a fossil in laser light and capturing the fluorescence that different minerals and organic residues emit. Soft-tissue outlines that are invisible under normal illumination suddenly pop into view. In Haolong dongi, LSF helped the team trace the boundaries of each spike and separate them from the surrounding matrix.
The imaging went deeper than surface mapping. X-ray scans and histological cross-sections revealed cellular-level preservation inside the spikes: individual cell outlines and internal structures that survived 125 million years of burial. That level of detail is extraordinary but not without precedent in the Yixian Formation. A 2021 study on the small feathered dinosaur Caudipteryx reported possible nuclear material preserved in cartilage cells from the same geological setting, though that finding remains debated among specialists. What the Caudipteryx work established, and what Haolong dongi reinforces, is that the Yixian’s burial chemistry is capable of freezing biological detail at a scale most fossil sites cannot match.
Placing the species on the family tree
Beyond the spikes, the skeleton carries enough diagnostic features to anchor Haolong dongi within Iguanodontia. The skull shape, forelimb proportions, and pelvic girdle details show a mix of primitive and more derived traits. When coded into a phylogenetic matrix, those characters consistently pull the species into the iguanodontian radiation rather than into more distantly related ornithischian groups.
That placement matters because iguanodontians are generally reconstructed as scale-covered animals without elaborate skin defenses. Their integument is, frankly, poorly known; skin impressions from the group are rare and tend to show simple, pebbly scales. Finding hollow spikes on a juvenile iguanodontian is like discovering that a familiar family of animals had a whole wardrobe no one suspected.
Some readers may think of Psittacosaurus, the small ceratopsian from the same Jehol Biota whose tail bore long, bristle-like structures. But Psittacosaurus sits on a different branch of the ornithischian tree, and its bristles are structurally distinct from the hollow tubes on Haolong dongi. The two discoveries share a lesson, though: the Jehol fossils keep revealing that dinosaur skin was far more varied than textbooks have shown.
What scientists still do not know
The study describes a single specimen, and that limits how far anyone can generalize. The most pressing question is whether adult Haolong dongi also carried hollow spikes or whether the structures were shed during growth, the way some modern reptiles lose juvenile color patterns or crests. Without adult material, the full developmental arc of the spikes is unknown.
Function is another open question. Hollow spikes could have served as lightweight armor against the small predators that shared the Jehol ecosystem, including dromaeosaurids and early mammals. They might have worked as sensory organs, detecting vibrations in dense vegetation. Or they could have been display structures, signaling to other juveniles or deterring rivals. The hollow interior argues against pure blunt-force defense, since solid bone or keratin would absorb impacts better. But a hollow tube can still look threatening while saving weight, and in some living animals, hollow structures play roles in thermoregulation or even sound production. No biomechanical modeling has been published yet to test which explanation best fits the spike geometry, so all functional ideas remain informed speculation.
There is also a preservation-bias problem. The Yixian Formation is one of the few places on Earth where delicate skin features survive fossilization. It is entirely possible that iguanodontians elsewhere carried similar structures that simply rotted away before burial could preserve them. Until comparable finds turn up from other regions or time periods, researchers cannot say whether Haolong dongi was an outlier or part of a broader pattern of spiky juveniles among plant-eating dinosaurs.
Finally, the chemistry of the preserved tissue has limits. The study documents the mineralization pathways that stabilized the spikes at the cellular level, but it cannot reconstruct original colors, precise mechanical properties, or full biochemical composition. Any claims about bright warning pigments or specific textures remain hypothesis, not evidence.
Why one small fossil reshapes a big picture
Paleontology has a long history of revising what dinosaurs looked like, often because new imaging technology catches details that earlier generations of scientists walked right past. LSF, high-resolution CT, and advanced histology now let researchers revisit long-known specimens and extract information about soft tissues, growth, and physiology that was always locked inside the rock.
Haolong dongi is a case study in what that toolkit can reveal. A single, well-preserved juvenile has challenged assumptions about an entire lineage’s appearance and early-life survival strategies. The hollow spikes are genuinely unprecedented in iguanodontians, but “unprecedented” should not be mistaken for “impossible everywhere else.” It may simply mean that the right fossil, in the right rock, under the right laser, had not been found until now.
For the broader story of dinosaur diversity, the takeaway is straightforward: baby dinosaurs did not all look like miniature versions of their parents, and the defenses they carried in their most vulnerable months may have been stranger, and more creative, than anyone guessed.
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*This article was researched with the help of AI, with human editors creating the final content.
