Recent attention as risen in regards to a brief abstract submitted at the 69th Annual Meeting of the Society of Vertebrate Paleontology, held in September of 2009 in Bristol, England. There, William Matthew Herne presented a preliminary report of material previously referred to one of the classical austral ornithischians, Leaellynasaura amicagraphica , which is available here. Therein, Herne describes this specimen has having a tail of over 70 caudal vertebrae (unheard of before save in hadrosaurs), and comprising around 75% of the total body length. A fantastically long tail.
Leaellynasaura amicagraphica is a small ornithischian taxon based on a few specimens, including a juvenile with a nearly complete (although mostly undescribed skull), and several referred specimens including a partial skull with an exposed endocast of the braincase , as well as some femora . Further referred specimens have included partial skeletal material (as Leaellynasaura sp.) including one particularly interesting specimen including a nearly complete tail [1,3]. The latter is referred to in Herne’s work describing the full osteology of these specimens , and has been known for over two decades. It finally sees the light soon!
One of two claims made recently about this taxon concerns the incredibly long tail, and the potential of austral dinosaurs, as non-avian archosaurs, to self-regulate their body temperature below the Antarctic Circle. Some have proposed, based on recent data on integumental structures in ornithischians [4,5] that they might have possessed filamentous integument, “dinofuzz,” providing insulation and they would have hibernated . But secondary to this is that at larger sizes, adaptation to cold weather with increase in surface area becomes more difficult, and as such animals that can reduce their overall surface area are better suited to cold environments. What use then a very long tail in an antarctic, austral dinosaur like Leaellynasaura amicagraphica? Mysteries, mysteries.
I am going to focus on that tail, although other questions are certainly interesting and deserving of their own time. I would like to say that extreme tail length is not unique to this taxon, or even among ornithischians, but for this point I will stick to ornithischians. This in no way invalidates Herne’s work, of course; even based on below, the caudal length and count exceed what we know in non-hadrosaur ornithischians. Nonetheless, it bears some comment to remark of the ridiculously long tail in some ornithischians.
First up is Scutellosaurus lawleri , one of the most basal members of Thyreophora. Based primarily on the holotype, MNA PL 175, Scutellosaurus lawleri has a caudal series broken in two portions, and these I will discuss below.
The first section is composed of 36 elements and probably articulated with the intact sacral series, and measures 47.6 cm, and the last caudal preserved in the series is incomplete but represents over half of its vertebra’s length; the second section is composed of 21 vertebrae, the first of which is incomplete but also represents over half of a complete centrum and thus does not correspond to the last element of the first section, and measures 27.6 cm. Total, the caudal series represents a combined preserved length of 55.2 centimeters, and the last centrum is hardly terminal; projecting only that the two intermediate partial vertebrae are adjacent, and complete, one can arrive at a preserved caudal length of ~56 cm, which compares to the precaudal vertebral series of 34.6 cm, with estimated cranial extension to 40 cm.
It should then be noted that, because the distal end of the caudal series can include more vertebrae, it is not complete, and projecting a round 60 cm (likely short of the full length, but round numbers are nice and aesthetic) gives us a potential 100 cm body length, of which 60% (or more) is tail (projecting a less well-rounded number and a greater gap between the two sections of caudals only increases the percentage). With nearly 60 elements in the tail preserved, and likely more, places this within the range predicted for one of the specimens referred to Leaellynasaura amicagraphica . As noted by Herne , hadrosaurs (by which I restrict myself to Hadrosauridae, as I assume does Herne) also have high caudal counts, but these numbers are mitigated by their shortness, where the brevity causes the caudal series to be shorter than the precaudal series in several hadrosaurs of note.
This also agrees well with Agilisaurus louderbacki , a very basal member of Ornithopoda. Based on ZDM 6011, where the preserved complete caudal series is 113 cm, combined with the precaudal series of 69 cm for a body length of 182 cm (tail is 62% of body length, and nearly twice precaudal length). And like Scutellosaurus lawleri above, the caudal series lacks the terminal vertebrae, and so was a bit longer.
Finally, it would be important to point out that under some projections (and the abstract of Herne ), Leaellynasaura amicagraphica is very mobile when it comes to ornithischian phylogeny, although inclusion of the total skeletal material in a matrix has never been done. If it were a basal ornithopod, a cerapodan of some sort, or a basal genasaurian, it would be very similar to the basal ornithopod Agilisaurus louderbacki and the basal thyreophoran (basal genasaurian) Scutellosaurus lawleri, which may indicate a basal trend in long-tailed ornithischians where each classic branch of Ornithischia (Thyreophora, Marginocephalia, and Ornithopoda) secondarily reversed the trend.
Other extremely long-tailed ornithischians also include Heterodontosaurus tucki, itself a systematically mobile taxon. Of course, none of these taxa seem to have tails that exceed ~65% of total body length, which Herne  proposes for Leaellynasaura amicagraphica, but they may have come pretty close.
Update (1): Andrea Cau reminds me (in comments) about Tenontosaurus tillettorum, which has been reconstructed by Paul (in various sources) to have a tail roughly 70% of the total vertebral length (slightly less when dealing with the skull). This is based on reconstructions which are collected from multiple specimens, so I may be more cautious about using this animal in support of such an argument, but it does suggest that extremely long tails are fairly pervasive in Ornithischia, even if they are not common.
 Herne, M. 2009. Postcranial osteology of Leaellynasaura amicagraphica (Dinosauria; Ornithischia) from the Early Cretaceous of southeastern Australia. Society of Vertebrate Paleontology, Abstracts. Journal of Vertebrate Paleontology 29(supp. to 3):113A.
 Rich, T. H. & Vickers-Rich, P. 1989. Polar dinosaurs and biotas of the Early Cretaceous of southeastern Australia. National Geographic Research 5:15-53.
 Rich, T. H. & Vickers-Rich, P. 1999. The Hypsilophodontidae from southeastern Australia. In Tomada, Rich & Vickers-Rich (eds) Proceedings of the Second Gondwana Dinosaur Symposium. National Science Museum Monographs 15:167-180.
 Mayr, G. Peters, D. S., Plodowski, G. & Vogel, O. 2002. Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus. Naturwissenschaften 89:361–365.
 Zheng X.-t., You H.-l., Xu X. & Dong Z.-m. 2009. An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature 458:333–336.
 Colbert, E. H. 1981. A primitive ornithischian dinosaur from the Kayenta Formation of Arizona. Museum of Northern Arizona Bulletin 53:1-61.
[n1] Peng G. 1990. [A new small ornithopod (Agilisaurus louderbacki gen. et sp. nov.) from Zigong, China]. [Newsletter of the Zigong Dinosaur Museum] 2:19-27.
[n1] It would be helpful to know the Romanized transcription for both the paper’s title and the journal in which it appears, if anyone can supply it!