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

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.

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]