What happens when you find a feathered dinosaur that really isn't meant to have feathers? That's the question set by a spectacular new fossil that adds a confusing dimension to the origin of feathers.
The concept of dinosaurs with feathers is no longer surprising. Birds certainly have them and they are now considered to be living dinosaurs. The infamous Velociraptor and its relatives were covered in plumes, which ranged from the simple quills of Sinosauropteryx to the flight-capable plumes on Microraptor's four wings. We know about these prehistoric feathers through the beautiful fossil impressions they have left behind, but a new set of impressions may be the most impressive yet.
They were discovered by Chinese scientists led by Xiao-Ting Zheng, who named their new discovery Tianyulongconfuciusi, after the museum that Zheng works in and the famous Chinese philosopher. Its small, agile body, about the size of a cat, was covered in long, hollow filaments that closely resemble the primitive "proto-feathers" (or colloquially, "dinofuzz") of other dinosaurs. What makes Tianyulong unique is that it is a very distant relative of all these other feathered species.
So far, all feathered dinosaurs are theropods, a group of two-legged and (mostly) carnivorous animals that included Tyrannosaurus and Velociraptor, and indeed, modern birds. The theropods belong one of the two major groups of dinosaurs, the Saurischia. Tianyulong, however, is a clear member of the other dinosaur lineage, the Ornithischia, which include the various armoured, horned, spiked and duck-billed species. This is the first time that anyone has discovered an ornithischian with feather-like structures all over its body.
More specifically, Tianyulong is a heterodontosaur, a group of small plant-eaters that are the most primitive of the ornithischians. Its position in the dinosaur family tree raises big questions about the origins of feathers. If its filaments are related to the proto-feathers of the theropods (which is possible but not certain), they either evolved independently or were derived from filaments that covered the very earliest of dinosaurs.
Judging by the state of its bones, Zheng's specimen was probably a young adult that measured 70cm in length (and most of that in its tail). Even without the potential feathers, it would be an interesting find, for most other heterodontosaurs lived in Africa during the Jurassic period. Tianyulong, on the other hand, hailed from Cretaceous China, making it a "living fossil" that was removed from other members of its family in both time and space.
On its skeleton, Zheng found three patches of filaments on the underside of the neck, the back and the tail. Unlike the more complex structures of birds and some other dinosaurs, Tianyulong's filaments were single structures that never branched. They were very gently curved but otherwise rigid - no bends or waves were found. Those on the tail were especially prominent, measuring about 7cm in length (about a tenth of the creature's entire body) and about half a millimetre in width. Some of them also had a dark stripe down their middle, a classic sign that they were hollow tubes.
Some scientists have argued that other dinosaur proto-feathers are actually fibres of collagen that have come loose from the animals' skins. That would certainly make them less interesting, but collagen fibres are solid structures; based on the long, hollow nature of Tianyulong's filaments, Zheng rejects this explanation. To him, they clearly stuck out from the animal's skin.
The big question is whether Tianyulong's filaments were actually related to the proto-feathers of the theropods. Zheng can't be sure based on a single specimen, but he notes that there are definitely similarities. Among the theropods, the proto-feathers of Sinosauropteryx were most similar to those of Tianyulong - they were shorter and more slender, but they also didn't branch. They also have similarities to the feathers recently found on Beipaiosaurus, which were hailed as the simplest yet discovered.
It's possible that Tianyulong's filaments evolved independently from those of theropods. Indeed, no one has found evidence of proto-feathers in the earliest species of theropods, which suggests that the last common ancestor of this group didn't have them.
The more intriguing idea is that Tianyulong's filaments were a direct part of the evolutionary lineage that led to true feathers, which would mean that the common ancestor of saurisichians and ornithischians was fuzzy. It could have had simple filaments that were retained by Tianyulong, developed into true feathers by the theropods, and lost in many other lineages. Zheng thinks that the similarities between Tianyulong's filaments and those of Beipaiosaurus supports this idea.
Only one other ornithischian, an early horned dinosaur called Psittacosaurus, had similar structures but its filaments were sparser, more rigid and only found on its tail. Perhaps these too were elaborate versions of some ancestral filament, borne by the earliest dinosaurs some 230 million years ago.
In a related editorial, Lawrence Witmer says:
"Perhaps the only clear conclusion that can be drawn... is that little Tianyulong has made an already confusing picture of feather origins even fuzzier. Such an outcome is common in palaeontology. But the prospects of new fossils, new molecular and imaging techniques, and even new ideas, offer the hope of bringing the evolutionary picture into sharper focus -- and that picture may well end up being of fuzzy dinosaurs."
Reference: Zheng, X., You, H., Xu, X., & Dong, Z. (2009). An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures Nature, 458 (7236), 333-336 DOI: 10.1038/nature07856
Image: Reconstruction by Li-Da Xing
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