The bird-hipped dinosaurs finally get feathers… or not…

15 08 2007

One of the most important taxonomic humps that anyone starting to learn about dinosaurs must overcome is that “bird-hipped” doesn’t mean “ancestral to birds.” In what seems like a great evolutionary joke, dinosaurs like Triceratops, Iguanodon, Polacanthus, and Stegosaurus, dinosaurs that are certainly not ancestral to birds, have hips that appear to very superficially resemble those of living birds. Because of this resemblance, in 1887 paleontologist Harry Seeley regarded dinosaurs with such hips as Ornithischians (“bird-hipped”) and others like Allosaurus and Diplodocus as Saurischians (“lizard-hipped”). I can only imagine how scientific history would have been different if Seeley had some of the Cretaceous dinosaurs like Sinosauropteryx or Deinonychus at his disposal, but despite the oddly reversed names, Seeley’s system has held up. Indeed, today the evolution of birds from relatively small theropod dinosaurs has been well-established, but that doesn’t mean our friends the ornithischians can’t still surprise us.

Skull of Psittacosaurus, from Osborn. “Two Lower Cretaceous Dinosaurs of Mongolia,” American Museum Novitiates, October 19, 1923.

In 2002, a paper in the journal Naturwissenschaften turned a lot of heads. While known to the public as a 2nd-class dinosaur, a re-examination of old material and new fossils of the Mongolian ceratopsian Psittacosaurus* began to give scientists a closer look at the paleobiology of this genus, and the researchers Mayr, Peters, Plodowski, and Vogel reported finding long “filamentous integumentary structures” on the tail of an especially well-preserved specimen. At that time such “integumentary” structures were only known from theropod dinosaurs like Sinosauropteryx, so there would be no reason to expect them in ornithischians, dinosaurs that had been evolving in their own way for hundreds of millions of years. Still, the “bristles” were clearly on the slab, and (according to the study) they may have been made out of the same material as the rest of the skin, which appeared to be similar to skin impressions found from other ornithischian dinosaurs. Indeed, it’s likely that the bristles were modified scales, but modified for what purpose, we can’t be sure.

A reconstruction of the bristled Psittacosaurus by Pavel Riha.

One of the questions brought up by the presence of bristles in Psittacosaurus is how many other ornithischians had such structures. Other famous specimens of ornithischians preserving skin impressions show no sign of having such bristles or “integuments”, and given the split with saurischian dinosaurs long before ceratopsians even began to evolve, the structures are probably not homologous. If more specimens like the one reported in Naturwissenschaften could be found, however, it might give us some clues as to what the structures were used for and how they evolved. I don’t mean to use this hypothesis as a catch-all, but whenever I see such an eye-catching structure I think “sexual selection,” and I wonder if sexual dimorphism can be correlated with the presence/absence (or prominence) of the bristles as is suggested in Riha’s painting. More fossils will be needed to figure out whether such an idea has any weight behind it, but it is interesting to ponder if other ceratopsians had bristles as well and what such structures could have been used for.

Psittaco type
Type skeleton of Psittacosaurus mongoliensis. H.F. Osoborn, in his initial descriptions, hypothesized that Psittacosaurus was ancestral to some kind of ankylosaur. From Osborn. “Psittacosaurus and Protiguanadon: Two Lower Cretaceous Iguanodonts From Mongolia.” American Museum Novitiates, Sept. 4, 1924

What we’ve been able to learn about Psittacosaurus doesn’t begin and end with bristles, however, as in recent years we’ve been able to get a better idea about what the life of this dinosaur was like based upon some wonderful clues left for us through taphonomy. As reported in Nature by Meng, Liu, Varricchio, Huang, and Gao in 2004, an adult Psittacosaurus was found, articulated in an upright position, over the skeletons of at least 34 juveniles. This is strong evidence for parental care in these dinosaurs, especially if the young exhibited slow growth rates that have been reported for small ornithischian dinosaurs like Orodromeus. Parents would have good reason to mind their offspring as well, as another Nature article from 2005 reported a new dog-sized mammal named Repenomamus giganticus, found with the remains of a juvenile Psittacosaurus in its stomach contents. As the authors note in their abstract, this fossil runs counter to the view that all mammals were small insectivores during the Mesozoic, and I think there were likely more complex relationships between dinosaurs and mammals than we now understand.

Articulated skeleton of “Protiguanodon” (=Psittacosaurus), which contained evidence of gastroliths (“stomach stones”). From From Osborn. “Psittacosaurus and Protiguanadon: Two Lower Cretaceous Iguanodonts From Mongolia.” American Museum Novitiates, Sept. 4, 1924

While perhaps not as “sexy” as the feathered Cretaceous theropods, there is a lot to say for further study of Psittacosaurus. Not only does the presence of bristles open up new questions about convergent evolution and sexual selection, but the depositional environments in which this dinosaur is found preserve fossils so well that we are able to reconstruct far more of its lifestyle than is possible for many other dinosaurs. I would love to see a reconstruction of the ecology that Psittacosaurus found itself in, from what it was eating to what dangers faced its young. Much could be learned from such a study, and while representations of this dinosaur in popular books are rather plain, they have quickly become some of the most interesting and surprising dinosaurs available to scientists for study.

*This was one of the hardest dinosaurs names for me to learn to pronounce, and I still trip over it sometimes. I actually didn’t get it until I heard a paleontologist pronounce it Ce-tack-o-sore-us (although I will defer to Zach’s “Si-TAK-o-SORE-us” due to my status as a greenhorn 😉 ), although the first syllable still sometimes jumbles in my mouth when I try to say the word.

[A much belated hat-tip to Dr. Thomas Holtz for brining up this topic in a personal communication many months ago]



12 responses

15 08 2007
Zach Miller

Si-TAK-o-SORE-us. 🙂

Anyway, nice little post summarizing the famous parrot dinosaur’s recent upheavels. Personally, I think that the bristles are analogous to porcupine barbs. The structures are round and hollow, after all, and banded. They could also be (and this is a long shot) modified Stage 1 Protofeathers, which would mean that the common ancestor of Psittacosaurus and Sinosauropteryx had Stage 1 Protofeathers. I think that Psittacosaurus’ bristles evolved independantly of feathers, but that’s a thought.

Also, head over to Acta Palaeontologica Polonica for a new Sereno paper detailing a new species of parrot lizard–Psittacosaurus major, whose head is 30% larger than other Psittacosaurus species, yet the body size is basically the same as its sister species. So we’re talking a bit ol’ head, here.

15 08 2007

Thanks Zach. I actually just downloaded that paper, and I was going to include it although I felt it was a bit extraneous for this post. As for the bristles, I think they evolved independently as well, although I would love to know if other ceratopsians had such structures (if they were used for some defensive purpose, maybe not, at least not the big one). Short of Doc Brown showing up I don’t think I’ll get a chance to find out though…

15 08 2007

I remember first seeing a reconstruction of a psittacosaurus with bristles and, having not kept up with dinosaur science as I should, waved it off as an artist’s odd interpretation. Then I saw a second reconstruction by a different artist and realized I had missed something.

For a layman like myself, it’s fascinating how much our views about dinosaur appearance have changed in recent years as new discoveries have come to light. I have several non-fiction books about dinosaurs on my bookshelf, the oldest about 12 or 13 years old, where the illustrations seem as dated as a Charles R. Knight painting given what we now know (no offense to Knight).

15 08 2007

I remember first seeing it on my 2nd edition copy of The Dinosauria last year and not knowing what was going on (I didn’t recognize Confusciousornis either, though). It really is amazing how much our ideas of dinosaurs have changed; I just received a copy of the book/comic All New Dinosaurs and Their Friends from 1975, and although the dinosaurs are fairly advanced they’re still very different from the ones gracing magazine articles today. Even in Knight’s work, there’s a sort of evolution that goes on. I’m not sure of the dates (which I should look into), but one of his Tyrannosaurus paintings has a very lizard-like predator, looking like little more than a big lizard. The Tyrannosaurus/Triceratops face-off, however, has a very different carnivore than the earlier one. Then, of course, there’s the “Leapin’ Laelaps” painting that’s still one of my all-time favorites.

And not to give too much away, but I’ve heard from at least one paleo artist who might be sending in some goodies for my to put up here from time to time, so hopefully I’ll be able to share some things with you all in the near future.

15 08 2007
Zach Miller

Hmmm…if that includes papers, I’m gonna need copies, Brian. 😀

16 08 2007
Christopher Taylor

I believe there’s a Charles R. Knight picture out there where he hedged his bets – a sluggish, upright Tyrannosaurus slumbers in the foreground, while in the background you can see two more active reconstructions of Tyrannosaurus zipping past.

I’ve been unsuccessful in finding a copy online, but the best double reconstruction of this kind is of the segmented Ediacaran animal Spriggina, which has been interpreted by some as a mobile worm, and by others as a sessile frond-like animal (with the ‘head’ representing a holdfast). In the illustration I’m thinking of, a worm-Spriggina is crawling past a patch of quite different-looking frond-Spriggina.

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The bird-hipped dinosaurs finally get feathers… or not… | Laelaps

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