For years we told ourselves a simple story: dinosaurs with scales, dinosaurs with feathers. A reassuring, almost scholastic division. Then paleontology began to uncover uncomfortable, complex, wonderfully out-of-the-box details. And today a new discovery from north-eastern China calls everything into question: a dinosaur that lived about 125 million years ago possessed hollow spines integrated into the skinstructures never before documented in an ornithischian.
His name is Haolong dongiwhich means “thorny dragon”, and this alone would be enough to evoke powerful images. It was an iguanodontian from the Early Cretaceous, a group of herbivorous dinosaurs that we know almost exclusively through bones. Soft tissues, as we know, have very little chance of surviving time. Yet, in this case, the fossilization was so extraordinary to preserve very detailed skin impressionscomplete with microstructures visible under the microscope.
The fossil, on display at the Anhui Geological Museum in Hefei, tells a story that goes far beyond the skeleton. On the tail you can distinguish large overlapping scales, almost forming shield armour; Small rounded scales appear on the neck and trunk, distributed evenly. And then, among these, they emerge: thin, cylindrical spines, some a few millimeters long, others over four centimeters.
Hollow spines: an evolutionary experiment never seen before
The specimen studied measured approximately 2.4 meters and had not yet completed growth, as demonstrated by the unfused vertebrae. Adults of the same evolutionary line could reach about five meters, while other iguanodontians of the period reached even greater dimensions. This detail already opens up a fascinating question: were those spines a juvenile characteristic or did they accompany the animal throughout its life?
Microscopic analysis revealed the nature of these structures: hollow cylinders composed of layers of keratinized skin enclosing a porous inner core. They are not bones, they are not horns, they are not proto-feathers. They represent something different, an autonomous anatomical solution, a true parallel branch in the evolution of skin appendages.
Thanks to advanced techniques such as laser-stimulated fluorescence, X-ray imaging and very thin histological sections, the researchers were able to observe details at the cellular level. An absolute rarity for a 125 million year old fossil. It is plausible that similar structures also existed in other dinosaurs, but only in this case was the preservation so exceptional as to give them back to us with this clarity.
According to Pascal Godefroit, paleontologist and co-author of the study, this discovery suggests that the diversity of dinosaur skin was extraordinaryfar beyond the simplistic contrast between scales and feathers that we have learned from books.
What were those thorns for?
Discovering a structure is the first step, understanding its function is the real test. The researchers hypothesize that the spines of Haolong dongi had primarily a defensive role. The animal lived in the ecosystems of the Yixian Formation in China, where many predators were relatively small and had limited jaw opening.
An irregular and pointed surface makes attack more difficult, complicates ingestion and increases the time needed to kill prey. The thorns did not guarantee invulnerability against teeth and claws, but they represented an effective deterrent, a survival strategy in a world where every second could make the difference.
There is also another track, linked to the climate of the time, which had average temperatures around 10 degrees. The thorns could have contributed to the thermoregulationeven without constituting a real insulating covering. However, the hypotheses of a communicative or sensorial function remain less solid, also because no evidence of associated pigmentation has emerged.
In recent decades we have discovered feathered dinosaurs, ornithischians covered in filaments, skin structures that resemble primitive hair. Now these hollow spines are added, different from the spines of mammals, from the spines of current reptiles and from the first feathers. The skin of dinosaurs increasingly reveals itself as an evolutionary laboratory full of alternative paths, attempts, adaptations.
And perhaps this is precisely the most powerful aspect of the discovery: it reminds us that evolution does not proceed in a straight line, but explores possibilities, experiments with solutions, creates combinations that escape our mental categories.
The results of the study were published in the journal Nature Ecology & Evolutionadding a new chapter to a narrative that continues to surprise us. Because every time we think we understand how dinosaurs were made, a fossil comes along to remind us how much the Earth has been – and continues to be – infinitely more creative than us.
