Already the article you’re reading is several weeks late, and I had to make a real effort to finish it before those weeks became months. Anyway, I present here the penultimate in the ‘month in dinosaurs (and pterosaurs)’ series (for the previous articles please see part I, part II, and part III). The whole point of this little series was to discuss what happened during January 2009 in dinosaur and pterosaur research. So, we’ve done the dinosaurs (or, at least, the theropods and ornithischians). This time: pterosaurs. Before launching into the new stuff, I must note that a single, exceptional event in the pterosaur literature overshadows everything else*: namely, the appearance of the special pterosaur volume of Zitteliana. This collection of papers results from the Peter Wellnhofer meeting held at Munich in 2007: there’s a lot of neat stuff in there, so much so that I’m not about to start talking about it now.
* Well, nearly everything else.
Pterosaur taxa come in thick and fast these days, and a great many (most) of them are from the Lower Cretaceous of China (usually Liaoning Province, the place where most of the feathered theropods come from)*. And yet another new Chinese pterosaur was named in January 2009: it’s Ningchengopterus liuae Lü, 2009 [shown above] from the Yixian Formation of Dashuangmiao, Nincheng County, Inner Mongolia. Ningchengopterus is reasonably well preserved and includes a complete skull and neck, an articulated wing, dorsal vertebrae and ribs, and a partial pelvis and hindlimbs. A portion of wing membrane is preserved and internal wing fibres are visible. Fibres are also preserved around the skull and neck; these presumably represent the integumentary hair-like structures now well known for pterosaurs. It’s clearly a pterodactyloid, and it might be a ctenochasmatoid (but read on).
* Barrett et al. (2008) listed 19 Liaoning pterosaur taxa (though a few of the species they listed are probably synonymous, some new ones have been published since (e.g., Shenzhoupterus, Elanodactylus), and a few others await publication).
Anyway, the big deal about Ningchengopterus is how tiny it is: its long-jawed, toothed skull is about 40 mm long and the bones in its wing add up to about 100 mm (minor point worth noting here: when people estimate pterosaur wingspans, they usually do so by adding the lengths of all the respective bones together. In life, however, the wing bones were not held in a straight line, so the ‘living’ wingspan would have been shorter. Chris Bennett has drawn attention to this but it’s rarely been mentioned elsewhere).
However, Lü (2009) admits throughout the paper that the only known specimen is a baby. This is supported not only by its small size, but also by the poor ossification present throughout the skeleton and by a lack of fusion between the skull bones (Dave Unwin has referred to pterosaur babies as ‘flaplings’, hence the word I used in the title for this article). Given the generally quite good evidence showing that morphology changes during ontogeny, is it possible then that this tiny baby is a tiny baby of a previously named taxon? Among Lower Cretaceous Chinese pterodactyloids, there are a few potential taxa to look at, including Eosipterus, Beipiaopterus and Gegepterus [shown here]. Lü notes that Ningchengopterus is different in various of its details from most of those taxa, but what’s suspicious is that its wing bone ratios are quite similar to those of Eosipterus. This is enough to at least suggest that Ningchengopterus might be an Eosipterus flapling, though Lü excludes this possibility by saying that Ningchengopterus lacks the elongate mid-cervical vertebrae of ctenochasmatids (the group to which Eosipterus belongs). Furthermore, the Eosipterus holotype lacks a skull, so we can’t say whether or not it was similar to Ningchengopterus in skull anatomy.
Of course, Ningchengopterus is far from the first, tiny baby pterosaur to be reported: there are tiny juvenile specimens of Pterodactylus from the Solnhofen Limestone, for example, that have wingspans of just 19 cm. We’ve previously looked at flaplings here, back when Nemicolopterus – also from Liaoning – was described in 2008.
Sailing the Cretaceous skies – – or not
Another, err, interesting pterosaur paper appeared in January. Among the most flamboyant and bizarre of pterosaurs are the ‘antlered’ nyctosaurs described in 2003 by Chris Bennett [his page on the fossils is here]. Referable to the nyctosaurid taxon Nyctosaurus, they differ from other nyctosaur specimens in possessing gigantic antler-like crests. The main spar of the crest projected mostly dorsally from the back of the skull, while an additional spar projected posteriorly from the main one (Bennett 2003).
Within the first few weeks of publication, people were already reconstructing nyctosaurs with wind-sails on their heads, and speculation abounded that these crests served some sort of aerodynamic function [there seems to be no end of sail-crested nyctosaur pics on the internet. The one shown here is from wikipedia]. While we know that the bony crests of various pterosaurs supported much larger soft-tissue crests, there is no evidence for this in nyctosaurs, nor indeed among ornithocheiroids in general. In fact Bennett (2003, p. 73) specifically considered the possibility of soft-tissue extensions on the nyctosaur crests and rejected it because the distinct, rounded edges of the crests are very different from the jagged, fibrous edges seen on those crests that did anchor soft tissue extensions. Bennett favoured the idea that the crests were visual display structures, and I agree (Naish & Martill 2003). In fact I personally have a strong dislike of the idea that weird crests and other such structures ‘must’ have evolved for functional reasons: in Mesozoic archosaurs, most of the evidence we have indicates that such things primarily functioned in display. This goes for the plates of stegosaurs, the frills and horns of ceratopsians, and the crests and horns of some theropods.
However… having said all that, I suppose it would be wrong not to at least test the possibility of an aerodynamic function for the crest. Actually, Elgin et al. (2008) did exactly this for Pteranodon: they put various Pteranodon skull models in a wind tunnel and recorded the side forces, yaw, and drag acting on the skull. They concluded that “the overall aerodynamc effect is modest” and that “the crest most probably evolved independently of any aerodynamic function” (p. 167). Like Bennett (2003) and Naish & Martill (2003), Elgin et al. (2008) favoured the hypothesis that the crest’s primary role was one of display, noting in the conclusions that “Animals today often sport astonishing structures for display purposes (e.g. peacock, lyrebird, red deer): providing the structure does not represent an overwhelming encumberance, the riotous functioning of sexual selection may far outweight the common sense of natural selection” (p. 174). Actually, however, some display structrures do “represent an overwhelming encumberance”, and exact a heavy cost on their owners. Isn’t that the whole point? This is known as ‘peacock’s tail syndrome’.
Incidentally, I know from a poster display I once saw that Ross Elgin and colleagues also tested the aerodynamic performance of various other crested pterosaur species too. Does anyone know if and when these results will be published? I need to know as I’d like to see a published smack-down of Frey et al.’s (2003) proposal that sail-crested tapejarids were wind-surfers.
Anyway, I digress. Xing et al. (2009) looked anew at the possibility of sail-crested nyctosaurs. In contradition to what Bennett wrote, they reconstructed Nyctosaurus with soft-tissue extensions on the crest, and after doing some mathemetical analysis (I will skillfully gloss over that section of the paper), they concluded that a sail-crested nyctosaur may indeed have been able to control thrust and movement by changing the pitch of its crest. I cannot assess the mathematics involved, but there are some problems with the analysis worth noting [adjacent CG model from Xing et al. (2009)].
As mentioned above, the idea of a huge head-sail is flat out not supported for these animals. Secondly, the authors probably have the shape of the nyctosaur crest all wrong. They show it as being shaped like a wind-surfer’s or yacht’s sail: very tall, short anteroposteriorly, and with a high aspect ratio. However, as is clear from Bennett’s descriptions, the posterior spar of the nyctosaur crest is not short as assumed by Xing et al. (2009), but very long: Bennett (2003, p. 70), stated that, in one of the specimens, the posterior spar is at least 32 cm long while the superior part of the crest is at least 42 cm long. Given the correct configuration [see Mark Witton's restoration below, from here on his site], a hypothetical nyctosaur head-sail must have been long anteroposteriorly, and with an absurdly low aspect ratio. This would have radically altered the properties of any hypothetical head-sail.
One final comment on Xing et al. (2009): they also tested the aerodynamic properties of a nyctosaur crest that lacked the hypothetical sail-like extension, and in this case they found that the crest had no special aerodynamic properties. Assuming that their analysis is sound (and, as mentioned, I am not able to conclude whether or not it is), this is interesting as it mirrors the conclusion reached by Elgin et al. (2008).
More pterosaurs next – – to be published tomorrow, probably.
Refs – –
Barrett, P. M., Butler, R. J., Edwards, N. P. & Milner, A. R. 2008. Pterosaur distribution in time and space: an atlas. Zitteliana B 28, 61-107.
Bennett, S. C. 2003. New crested specimens of the Late Cretaceous pterosaur Nyctosaurus. Paläontologische Zeitschrift 77, 61-75.
Elgin, R. A., Grau, C. A., Palmer, C., Hone, D. W. E., Greenwell, D. & Benton, M. J. 2008. Aerodynamic characters of the cranial crest in Pteranodon. Zitteliana B 28, 167-174.
Frey, E., Martill, D. M. & Buchy, M.-C. 2003. A new species of tapejarid pterosaur with soft-tissue head crest. In Buffetaut, E. & Mazin, J.-M. (eds) Evolution and Palaeobiology of Pterosaurs. Geological Society Special Publication 217. The Geological Society of London, pp. 65-72.
Lü, J. 2009. A baby pterodactyloid pterosaur from the Yixian Formation of Ningcheng, Inner Mongolia, China. Acta Geological Sinica 83, 1-8.
Naish, D. & Martill, D. M. 2003. Pterosaurs – a successful invasion of prehistoric skies. Biologist 50 (5), 213-216.
Xing, L., Wu, J., Lu, Y., Lü, J. & Ji, Q. 2009. Aerodynamic characteristics of the crest with membrane attachment on Cretaceous pterodactyloid Nyctosaurus. Acta Geological Sinica 83, 25-32.