HAOLONG

Haolong dongi was found in the Early Cretaceous rocks of northeastern China and is represented by a nearly complete and beautifully articulated skeleton, which in itself is not unusual. But it adds an unexpected twist to the story of iguanodontian dinosaurs by preserving something extraordinarily rare: skin so exquisitely fossilised that its scales and even its cellular structures can be studied in detail.

The preserved skin reveals a mosaic of textures. Along the tail lie large, overlapping shield-like scales, forming a kind of natural armour. Around the neck and chest, the scales shift to small, raised bumps, arranged in a pattern unlike anything described in other iguanodontians. This alone would make Haolong noteworthy. But the fossil holds a far more surprising feature.

Interspersed among the scales are skin spikes—slender, hollow tubes of keratinized tissue, preserved down to the level of individual cell nuclei. 3D-scan and thin-section studies show that each spike consists of a tough, horn-like outer layer overlying multiple layers of epidermal cells, all wrapped around a porous, spongy core. These structures are not protofeathers, nor are they comparable to the scaly spines of modern lizards. They represent a distinct evolutionary experiment, a unique integumentary invention within Ornithopoda.

Although their construction is unlike the quills of porcupines or the spines of hedgehogs, their function appears strikingly similar. The spikes likely served as a deterrent to predators, making Haolong a far less appealing target for any carnivore considering a bite. Their placement and form also hint at possible secondary roles in helping control body temperature or aiding touch, adding layers of ecological nuance to this otherwise familiar lineage of herbivorous dinosaur.