Tianyulong, meaning “Tianyu dragon,” is a genus of heterodontosaurid ornithischian dinosaur from the Late Jurassic Period (155 million years ago). It’s fossils were recovered from the Tiaojishan Formation in Jianchang County, Western Liaoning Province, China. The type and only species is T. confuciusi. It is named for the Shandong Tianyu Museum of Nature, where the fossils are housed.

Tianyulong is known from an incomplete skeleton preserving a partial skull and mandible (jaw), partial presacral vertebrae, middle caudal (tail) vertebrae, nearly complete right scapula (shoulder), and several other fossil fragments. The individual discovered is possibly a sub-adult probably measuring about 27.5 inches in length based on the proportions of the related species
Heterodontosaurus tucki.

Tianyulong is characterized by a slender body, long tail and a pair of enlarged canine-like tusks. This dinosaur was herbivorous or possibly omnivorous. Until the discovery of Tianyulong, known members of the group were restricted to the Early Jurassic of South Africa and North America, with possibly one additional genus from the Early Cretaceous of England.

Tianyulong is notable for a row of long, filamentous structures apparent on the back, tail and neck. The similarity between these structures and those found on some other theropods suggest a common link with feathers and raises the possibility that the earliest known dinosaurs and their ancestors were covered with filamentous structures considered to be primitive feathers.

The filamentous structures in Tianyulong are most similar to the singular un-branched proto-feathers of Sinosauropteryx and Beipiaosaurus. The estimated length of the structures on the tail is about 2.35 inches which is seven times the height of a caudal vertebra. Such dermal structures have previously been reported only in derived theropods and ornithischians, and the discovery in Tianyulong extends the existence of such structures further down in the phylogenetic tree.

If the link is supported, the consequence is that the common ancestor of both saurischians and ornithischians were covered by feather-like structures, and that groups for which skin impression are known such as thesauropods were only secondarily featherless. But if it is not supported, it would indicate that the filamentous structures evolved independently in saurischians and ornithischians.

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