This may be old news for those who attended last years SVP meeting, but news of this is (to my knowledge) breaking for the first time online. Matthew Herne has finished a complete osteology of the Australian ornithischian Leaellynasaura, abstract here: http://www.vertpaleo.org/meetings/SVPProgramAbstracts09WEB.pdf.pdf
A few surprising things here. First, Leaellynasaura is traditionally called a hypsilophodontid, or at least basal ornithopod. This study finds that it's even more basal among ornithischians, even sharing some characters with thyreophorans, so it's best placed as a basal genasaurian. Next, the tail lacks the distinctive lattice of ossified, stiffening tendons found in members of many ornithischian clades. Instead, the postzygapophyses of the tail are greatly expanded relative to other members of this order, which may have helped stiffen the back half of the tail.
Most surprisingly, the tail itself is apparently ridonkulously (technical term) long. Leaellynasaurua has over 70 tail vertebrae, more than any other ornithischians save some hadrosaurs, but more astounding is the total length of the tail, which made up 75% the total body length, being three times longer than the torso, head and neck combined. Why such a long tail? One idea floated by Dann Pigdon on the DML today is that if Leaellynasaura had a covering of filamentous feather or fur-like integument (as seen in Psittacosaurus and Tianyulong), it may have been able to use its tail for warmth during cold antarctic nights, wrapping the tail around the body like an arctic fox. It may have also been useful for territorial signaling or mating displays, especially if (as in most animals with filamentous or feathery coats) it could puff the tail up to an apparently larger size by raising its hackles.
I couldn't help taking a break from my Yixian field guide series to try restoring this hypothesis, and the results are above. Can't wait to see this paper officially in print!
The new Leaellynasaura looks amazing. I suppose the fuzz makes more sense now that it could be closer to the base of Ornithischia, near the heterodontosaurids. (On the other hand, heterodontosaurids are being bounced around Ornithischia quite a lot.)
ReplyDeleteYeah, I tend to agree with Richard Prum that the fuzz of ornithischians is probably not homologous with feathers. The whole situation would be easier to swallow if heterodontosaurids are close to marginocephalians. Normally I'd really hesitate to put fuzz on something like Leaellynasaura but the concept was too good to pass up illustrating!
ReplyDeleteFurther proof that Leaellynasaura would make one heck of a cute pet.
ReplyDelete"I tend to agree with Richard Prum that the fuzz of ornithischians is probably not homologous with feathers."
ReplyDeleteInteresting. Recently, Norell (in Brusatte et al.'s review of early dinosaurs) considered very likely that the ornithischian fuzzs and the proto-feathers are homologous.
@ Andrea
ReplyDeleteI think the main contention many people have is the apparent lack of filaments in numerous lineages basal to both feathered theropods and "feathered" ornithischians. We have good, extensive skin impressions from ceratosaurs, carnosaurs (undescribed), sauropods, and hadrosaurs that show no trace of any filamentous integument. At this point we can either hypothesize that that filaments were gained once and lost at least 5 times among dinosaurs, or were gained two or three times. Parsimony leans slightly towards non-homology. It's not a huge difference, but enough that it's not nearly a solid case. We were happy to assume the filaments of pterosaurs and mammals were not homologous with feathers for years, so it's odd that finding one more filamented lineage of amniotes should suddenly start us arguing for homology.
Matt, awesome post.
ReplyDeleteI just want to mention that there may be some difficulties for a highly integumented Leaellynasaura. The first is the seeming similarity and potential relationship within Thyreophora, noted "shield bearers." Along with long-tailed Scutellosaurus, it may have been somewhat armored!
The second is that polar animals are rather unlikely to try to find ways to increase their body surface area by a great amount, as this contributes more to heat loss than heat retention due to increase in how much of the body and tissue where blood flows (if, like avian feathers, blood vessels are linked to the raches) is exposed to the air. Many polar animals have short ears, short tails, short and rounded feet, etc., diminishing their relative exposure regardless of size. While this may seem antithetic to a very long tailed animal, and coating the guys in "fuzz" is just not too much beyond what's already elaborate, it seems perhaps unlikely to be as long as you depict, although very cool looking.
@qilong
ReplyDeleteGood points, though I don't know if we can necessarily make some of those assumptions yet. Even if Leaellynasaura is "Scutellosaurus-grade", would this necessarily rule out filaments? Armored mammals are still generally furry, for example, even (presumably) up to Megatherium sizes. Also, even if (big if) ornithischian filamentous integument is hologous with birds and/or pterosaurs, there's no reason to suspect they'd be more like vaned feathers with rachides rather than plumaceous, hair-like or pycnofibre-like structures unconnected to blood vessels. The quill-like structures of Psittacosaurus and Tianyulong may be (are porcupine quills connected to blood vessels?) but Tianyulong has standard plumaceous integument as well, apparently based on undescribed specimens. Either way, with such a long extremity, it's hard to imagine how a furry covering could be anything but a net benefit in a cold environment (as long as, as you note, the fibers aren't extremities in themselves, but I don't think there's a reason to assume they are).
I wonder, though, would Scutellosaurus-like armor provide some insulation?
Yeah, I saw this at SVP '09 and was very excited. I'm looking forward to the paper, and I'm surprised at how basal the little guy is!
ReplyDelete