One of the few things that everybody knows about dromaeosaurs – the sickle-clawed maniraptoran theropods best represented by Velociraptor from Mongolia and Deinonychus from Montana – is that they possessed a peculiar tail. Super-long zygapophyses and chevrons formed a bizarre, inter-twined array of body rods that ran the length of the tail and apparently assisted in its function as a dynamic stabiliser [image, © Greg Paul, shows Velociraptor versus two troodontids].
In describing this remarkable conformation in Deinonychus, Ostrom (1969) suggested that these bundles of bony rods would have inhibited flexibility: he imagined that the tail was stiffened and functioned ‘as a single unified member’ (p. 79). Ever since, people have been depicting dromaeosaurs as stiff-tailed, at most capable of bending the entire tail laterally or dorsally as a unit, but not with any of the degree of flexibility present in other long-tailed theropods.
In view of all this it was very surprising when Norell & Makovicky (1999) described a new Velociraptor specimen in which the near-complete, articulated tail was preserved in a gentle sinuous S-curve (with the curves being in the lateral plane, not the vertical) [tail of specimen shown here, from Norell & Makovicky 1999]. They noted that ‘this conformation of the tail seems not to disturb the arrangement of the elongate prezygapophyses, suggesting that there may have been substantial lateral mobility of the tail in life’ (Norell & Makovicky 1999, p. 24). It’s true that, in the specimen they described (IGM 100/986), the curving of the tail does look natural and ‘life-like’, but if this really was possible in life it raises the question as to why these animals had the super-long zygapophyses and chevrons in the first place. Elongate, cylindrical bones (like these zygapophyses and chevrons, and like the cervical ribs on Tanystropheus and brachiosaurs for example) must have had some flexibility, but were they really this flexible?
Judging from the recent description of the Mongolian dromaeosaurid Tsaagan mangas Norell et al., 2006 from the Djadokhta Formation, Mark Norell and colleagues are indeed still thinking that the dromaeosaur tail had a reasonable degree of lateral flexibility. While the tail of Tsaagan is unknown, a life restoration in the descriptive paper (produced by Nicholas Frankfurt and reproduced below) shows Tsaagan bending its tail laterally in a manner that does not seem too constrained by those super-long zygapophyses and chevrons. Incidentally, you might not be familiar with Tsaagan, but you’ve probably seen pictures of it, as photos of the excellently preserved holotype specimen have been used in countless magazine articles about Mongolian dinosaurs (typically labelled as Velociraptor).
Could these rod-like bones really be bent, in life, as much as IGM 100/986 indicates? I find this really hard to believe, but as usual my incredulity is nothing more than an opinion and shouldn’t count for much. I can imagine that a long, rod-like bone with a sub-circular cross-section has as much play in it as does, say, a long, slim rod of flexible wood (like a fishing rod), but bending it into an S-shape seems like a real stretch. The problem is compounded by the fact that the in-life flexibility of these structures is so difficult to test: the fossils are no use, and there just aren’t living animals with rod-like bones at all like dromaeosaur zygapophyses and chevrons.
Could the twisty shape of IGM 100/986 be the result of post-mortem distortion? We know from many, many fossil bones that structures that were straight in life can become bent and distorted into wiggly or curved shapes when subjected to hundreds of thousands of years of compression. A good example is provided by the neural spines of Spinosaurus aegyptiacus which, when viewed anteriorly or posteriorly, could be seen to be wiggly (Stromer 1915) and not vertically erect as they must have been in life. In the case of Spinosaurus, this happened, presumably, because the tall neural spines were lying on top of ribs or other structures, and were slowly forced to take on dips and wiggles as they were compressed by sediment. But, having said all that, the tail of IGM 100/986 really doesn’t look distorted and in fact looks in pretty good shape. Plus, the rod-like parts of the zygapophyses and chevrons of these animals are only 2 mm wide or less, perhaps making flexibility plausible.
So, I’m confused. I don’t have any answers and would be interested in seeing what people think. Can long, rod-like bones really be this flexible? And, if so, what’s the point of having them in the first place? And should we all stop drawing our dromaeosaurs with stiff, rod-like tails?
Refs – –
Norell, M. A., Clark, J. M., Turner, A. H., Makovicky, P. J., Barsbold, R. & Rowe, T. 2006. A new dromaeosaurid theropod from Ukhaa Tolgod (Ömnögov, Mongolia). American Museum Novitates 3545, 1-51.
– . & Makovicky, P. J. 1999. Important features of the dromaeosaurid skeleton II: Information from newly collected specimens of Velociraptor mongoliensis. American Museum Novitates 3282, 1-27.
Ostrom, J. H. 1969. Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana. Bulletin of the Peabody Museum of Natural History 30, 1-165.
Stromer, E. 1915. Ergebnisse der Forschungreisen Prof. E. Stromers in den Wüsten Ägyptens. II. Wirbeltier-Reste der Baharîje-Stufe (unterstes Cenoman). 3. Das Original des Theropoden Spinosaurus aegyptiacus nov. gen., nov. spec. Abhandlungen der Königlich Bayerischen Akademie der Wissenschaften Mathematisch-physikalische Klasse 28 (3), 1-32.