Haolong: Beast of the Week

 This week we'll be checking out a newly described dinosaur that is so unique, it completely changes what we think we knew about dinosaur skin!  Enter Haolong dongi!

Haolong was a plant-eating dinosaur that lived in what is now Liaoning, China, during the Early Cretaceous period, about 112.5 million years ago.  The only specimen on record measures about 8 feet (2.45m) from beak to tail but was a juvenile when it died so the species likely grew larger.  The genus name translates from Chinese to "Spiny Dragon" for a very good reason we are about to get into!

Life reconstruction of Haolong in watercolors by Christopher DiPiazza.

Haolong is an incredibly interesting find because it possesses a feature never before seen in any dinosaur.  like many dinosaur fossils found in this part of China, it includes more than bones, including much of its skin, which was scaly as you might expect for an ornithopod.  What nobody would have predicted, however, is the presence of pointed spines growing out from between the scales.  These spines are relatively short, extremely narrow, and tube shaped.  When examined with a microscope paleontologists were able to see that they were actually hollow with keratin (same material that the outer layer of claws and beaks are made of) outside and bony pulp on the inside.    These strange features are growing out from between the mosaic-like scales on much of the upper part of the dinosaur's body, stopping at the base of the tail.  It is a mystery as to exactly what these spines evolved for.  They very well could have been for protection against predators, similar to what modern hedgehogs have.  Others have suggested they may have also helped keep the dinosaur warm by providing extra insulation.  

Close up image of some of the spines found on Haolong.  These happen to be from the neck.  Image from the paper referenced below by Huang et al. 

In addition to the spines, the skin covering Haolong's tail was also preserved.  It possessed rows of broad overlapping shingle-like scales running down the top and sides of the tail, another feature never before seen in an ornithopod dinosaur.  

Haolong belonged to the iguanodontid family of ornithopod dinosaurs, so it was closely related to the famous Iguanodon, as well as dinosaurs like Mantellisaurus and Tenontosaurus.  Like them it had shorter front limbs than hind limbs and could have likely walked on two and four legs in life.  Each hand had five fingers.  The middle three were fused together into a mitt-like structure, and would have been for supporting its body when standing or walking on all fours.  The pinky jutted out to the side and would have been flexible, possibly to help grab plants.  The thumb claw was in the form of a cone-shaped spike, a trademark feature for iguanodontids.  The thumb spikes could have been for defense against predators but also very well could have been for combat within the species for dominance. (or both?)

Photograph of the complete fossilized skeleton of Haolong dongi. (image credit: Theirry Hubin, Institute of Natural Sciences)

On its skull Haolong possessed a large broad beak backed up by of relatively large blocky teeth in its jaws, which would have been ideal for chopping up plants.    

The environment Haolong lived in when alive would have been warm and humid based on the plant fossils that are found there, like horsetails and ferns, but thanks to paleontologists being able to examine growth rings on fossilized trees from there, we know it also was seasonally quite cold, down to 50 degrees Fahrenheit (10 degrees celsius).  Based on this its is would make sense if Yaolong's unusual covering was indeed a way for it to maintain warmth.  Other dinosaurs it would have coexisted with were the therizinosaur, Beipaosaurus, the hawk-sized four-winged dromaeosaur, Changyuraptor, and the large feathered tyrannosaur, Yutyrannus.  

References

Huang, J.; Wu, W.; Mao, L.; Bertozzo, F.; Dhouailly, D.; Robin, N.; Pittman, M.; Kaye, T. G.; Manucci, F.; He, X.; Wang, X.; Godefroit, P. (2026). "Cellular-level preservation of cutaneous spikes in an Early Cretaceous iguanodontian dinosaur". Nature Ecology & Evolution: 1–8.