Ok, I am actually really rather excited by this development but as ever I don’t want to go down the route of simply reviewing the paper as that is what I suspect every other blogger and news outlet is doing right now. For those who are already lost, today in Nature a new paper came out showing an ornithischian dinosaur – Tianyulong – with large integumentary structures (protofeathers is definitely pushing it at this point and is a term I’ll be avoiding with respect to this taxon at least for a long time to come) which superficially at least look very similar to those of theropod dinosaurs that certainly are precursors to feathers in birds. The big deal of course is that now we have these kinds of structures in both ornithischian dinosaurs and saurischian dinosaurs (the really fundamental split at the base of Dinosauria) and not just in saurischians. This opens up the possibility that these kinds of structures were ancestral to dinosaurs and thus inherited by both clades and did not just evolve in the derived theropods. However there is much more to this than meets the eye, most notably the fact that this is manifestly not the first ornithischian to have been suggested to have such structures.
I don’t want to belabour the details, I suspect the vast majority of my readers will have been inundated with this kind of information elsewhere and in far greater detail, so let’s rattle off the basics on this. If some of it does come as a surprise, do let me know!
1. This is a heterodontosaurid. These are now considered to be one of the most basal of ornithischians which fits nicely with the idea that this condition might be basal for dinosaurs and inherited in both major lineages.
2. Despite being Cretaceous in age (yes, it’s yet another Jehol taxon) this means the taxon itself was probably quite derived, but the lineage as a whole is basal (i.e. evolved early on – modern lizards are obviously derived animals, but in terms of reptiles, lizards as a whole evolved early on).
3. Ornithischians are incredibly rare in the Jehol fossil record so in some ways it’s not a surprise that these things have simply not been recoded before. Such soft tissues are incredibly rare and this is pretty much our only source for them, so coupled with a lack of ornithischians it’s not a surprise we had not found one sooner.
4. We cannot say for certain that these structures are homologous with protofeathers or similar structures in derived theropods (i.e. the same structure, modified in the same way). I would say they are almost certainly an example of parallelism however (i.e. the same original structure modified in a similar way), but until the distribution of these structures across Ornithischia and indeed in basal dinosaurs on both sides and their composition and distribution on individual taxa, I would not want to call them homologues, and certainly not protofeathers or feathers, though they clearly are similar and in a fairly closely related group.
5. Despite a general lack of such fossils with exceptional preservation I think it is problematic that all we have is just one definitive ornithischian with such covering and no evidence of such things in basal dinosaurs, dinosauromorphs, sauropods or basal theropods in the sense that “all / many / most dinosaurs had this”. It could still be that this is convergence, parallelism, or was present in basal ornithischians and then lost in later clades etc. Certainly it opens up that possibility, but it does not make it so, or, to my mind, even likely at this point.
6. Their distribution on the body, especially the elongate set on the base of the tail is especially interesting in the light of what we know of the other alleged ‘proto-feathered’ ornithischian which is what I want to turn to now.
Yes, despite all the (justified) palaver, Tianyulong is the second, and not the first ornithischain to be reported with such features. That, perhaps dubious, honour goes to a Psittacosaurus specimen described in 2002 and the reason that this new dinosaur has had so much attention is that problematic work done on the aforementioned animal (for those who have not seen it, you can see a look at a decent image of it here).
First off, the integumentary structures in this one look very little like those seen in theropods (and indeed in Tianyulong, they are longer and stiffer in Psittacosaurus) which made researchers suspicious that perhaps they were not genuine (genuine bits of skin that is, they certainly are biological structures) especially in the light of at least some people wanting to interpret them as homologues to protofeathers. Their narrow distribution on the body (only really on part of the tail, and sparsely at that) hardly looked like a covering for insulation, display or anything else obviously functional, and several researchers have confidently asserted that these are in fact merely plant pieces that are simply in association with the skeleton and not part of the dinosaur itself (and the carbonisation process that seems to work in the Jehol certainly produces fossils of plants and other things that look all but identical to this). Certainly the structure of them is very fine and it would be hard to make any meaningful comparison to other feathered dinosaurs without better specimens. There are other Psittacosaurus specimens known from the Jehol which do have soft tissues preserved (and hundreds if not thousands of skeletons from outside) and none of which show such structures which again implies that these were not real. In addition, ceratopsians such as Psittacosaurus are derived ornithischians and again with such a big evolutionary and temporal gap between them and the derived theropods (of which there were far fewer feathered examples back them) it was difficult to draw good parallels. Finally the specimen is of, let’s say ‘questionable legality’ with regard to its export from China, and a large number of researchers have refused to work on it as a result meaning it has not been examined by some key researchers and relatively little work in general has been carried out on it. they are longer and stiffer in
When you add all that up you can see why the specimens was and has been treated with quite a bit of caution, and as a result why the new one has got so much acclaim. It remained a possible example of a derived ornithischian with some kinds of modified integument, but equally it was very problematic for practical (making comparisons and a lack of corroboration) and philosophical reasons (both in terms of evolutionary theory and the legality of the material). However, in the light of the Tianyulong it seems that more attention will have to be paid to this specimen.
The fact that we do now have something that shows with far greater clarity and certainty that such structures are in the Ornithischia, and with a heterodontosaurid showing them the evolutionary gap between derived theropods and ceratopsians has been narrowed considerably. It certainly gives more credence to the idea that other ornithischians had such structures and corroboration from other specimens of these two and other taxa will be eagerly sought.
What interest me particularly is that in both cases there is a concentration of these things on the base of the tail which certainly supports the case for the Psittacosaurus specimen being genuine. However it still does not explain the odd pattern of structures in the ceratopsian or why they might be present at all. At the risk of leaving a lamentably obvious and unhelpful closing line, this really is the next twist in a long saga over feather evolution and indeed skin evolution in general in dinosaurs and reptiles as a whole and certainly opens up new possibilities and provides new evidence, but, as ever, more reseach is needed.