On to more of my thoughts about the outstanding Wellnhofer pterosaur meeting, held at the Bayerische Staatssammlung für Paläontologie und Geologie (Bavarian State Palaeontological Collection – BSPG) in Munich last week. For part I go here, and for initial thoughts on what a neat conference it was go here. For those of you wondering (and for those of you that know that the pterosaur meeting was the third of three conferences I attended during August and September 2007), my plan is to talk about all three conferences, but in reverse order. Once I finish talking about pterosaurs, I’ll do SVPCA, and after that I’ll say at least something about the cryptozoology conference. Other things may well get in the way, of course.
Anyway: again, I’m not reviewing (or even mentioning) everything that happened, just covering the areas that caught my personal attention. Thanks to Mark, I got hold of additional photos from the meeting today, so expect lots of serious images; no more posing with cocktails (though you never know). Here we go…
Two talks – by Leon Claessens and Pat O’Connor – looked at pterosaur physiology and pneumaticity. Leon showed that, while pterosaurs exhibited a reduction of those skeletal components that permit costal ventilation of the lungs, the sternum and its associated structures must have been capable of a significant amount of dorsoventral excursion, and the prepubes probably helped with this too. The bottom line is that pterosaurs evolved an efficient avian-style flow-through pulmonary system long long prior to the appearance of birds. For those of you reading this that don’t know pterosaurs well, I should point out that the evidence for avian-style skeletal pneumatisation in pterosaurs is very robust and based on excellent evidence; for a review see Bonde & Christiansen (2001, 2003). Basal pterosaurs appear not to have been pneumatic, but rhamphorhynchids and pterodactyloids clearly were.
Pat O’Connor showed that skeletal pneumaticity in pterosaurs strongly parallels that present in birds, and that large taxa in both clades exhibit similar styles of skeletal pneumaticity not seen in their small relatives. Pterosaurs were probably able to pneumatise distal skeletal elements (such as wing finger phalanges) via the opportunistic invasion of diverticula from a subcutaneous air-sac that may have extended across part of the forelimb skeleton. As a very excited Dave Martill pointed out after Pat’s talk, there may indeed be direct evidence for the presence of this air-sac from some preserved pieces of pterosaur wing membrane: in addition to layers containing aktinofibrils and blood vessels, pterosaur wing membranes have also been reported to contain a ‘spongy’ layer that has been inferred to have been mostly air-filled (Frey et al. 2003). Might this ‘air-filled’ layer correspond to a subcutaneous air-sac? [I don't have any images showing Leon or Pat, so above is a pic of me giving my talk on European Lower Cretaceous pterosaurs].
One area of palaeobiological speculation that surprised me slightly was the inference from a few pterosaur workers that Tapejara (and related taxa) was frugivorous, or at least herbivorous: though don’t get me wrong, I’m familiar with the history of this idea in the literature (I just didn’t think that it was still taken that seriously). This idea was initially proposed by Wellnhofer & Kellner (1991) in their description of T. wellnhoferi, and later discussed (Fleming & Lipps 1991) by biologists interested in the role that Mesozoic vertebrates must have played in ancient endozoochory (the transport of plant seeds in an animal’s gut). It has also been viewed in a favourable light by Dave Unwin (2006). In an effort to determine the diet of Tapejara, Hanneke Meijer and colleagues used morphometric analyses to compare the inferred bite strength and other details of Tapejara with those of extant birds. I didn’t exactly follow the logic behind the application of these various tests (to be honest I never do), but the outcome was that Tapejara is about similar enough to extant herbivorous birds to have practised a similar lifestyle. Well, maybe.
Pterosaur skull evolution was also looked at by Michael Fastnacht and Jesús Marugan-Lobón. Michael’s talk employed the principles of construction morphology, and among other things he argued that certain phylogenetic pathways were not available once a certain construction had evolved. I have a minor comment on this, which is that phylogenies indicate that ‘impossible’ reversals can indeed happen in evolution: certain marsupial tree frogs have apparently re-evolved both a tadpole phase and a lower jaw dentition, biphasic life history has re-evolved in some plethodontid salamanders from direct-developing ancestors, some stick insects have re-evolved wings from wingless ancestors, egg-laying in some squamates has evolved from viviparous ancestors, and some people think that limbed snakes re-evolved their limbs from limbless ancestors, for example. I am therefore cautious about claims that such events as, for example, the re-evolution of teeth from edentulous ancestors cannot happen. They probably can, actually [adjacent photo is me with Ursula Göhlich, and the Wellnhofer Pteranodon model in the background. God I look awful].
The immense, sometimes bizarre sail-like cranial crests of some pterosaurs (see, for example, Tapejara navigans shown at the very top of the article) have led to suggestions from the start that these organs might – or even must – have had some sort of aerodynamic effect. Ross Elgin and colleagues have actually tested this, using scale models and wind tunnels. A paper on this is due to appear soon, and the bottom line is that crests have no significant aerodynamic effect, either positive or negative. Dave Unwin provided a good review of what we know about pterosaur wing membranes based on the fossil evidence. As Tet Zoo regulars will know (see the ver 1 article here), fossils show – like it or not – that pterosaur patagia, preserved across a wide and representative diversity of taxa, were mostly extensive, incorporating the hindlimb down to the shin or ankle.
However… (moving now away from Dave’s talk).. there are apparently good aerodynamic reasons to think that this configuration didn’t apply to all pterosaurs, and that oceanic soarers in particular (like some ornithocheiroids) may have benefited from a narrow-chord wing. Pterosaurs are highly controversial animals, and it’s interesting that, even now, the extent of the patagia remains one of the most debated issues in the field. Certainly it was the most debated thing I heard about, and toward the end of the conference I had lost count of how many times I’d heard the same discussion (albeit going on among different people). Maybe I spent too much time hanging out with Mike Habib and John Conway, both of whom generally favour narrow-chord wings [adjacent Conway pic of Anhanguera from his excellent palaeo.pterus.net site], and Mike’s argument in particular comes from his understanding of avian wing aerodynamics. I know nothing of aerodynamics and am sceptical of the idea of favouring a theoretical model over fossil evidence – I also harbour the naïve suspicion that organisms are not necessarily optimally ‘designed’, nor do they have to work in a way that makes most sense to us – but I can see that these arguments have merit and that we should remain open-minded.
And that’s not all, lots more to come in part III. It will be posted soonish.
Refs – –
Bonde, N. & Christiansen, P. 2001. Aspects of the detailed anatomy of Rhamphorhynchus. In Two Hundred Years of Pterosaurs, A Symposium on the Anatomy, Evolution, Palaeobiology and Environments of Mesozoic Flying Reptiles. Strata Série 1 11, 29-31.
– . & Christiansen, P. 2003. The detailed anatomy of Rhamphorhynchus: axial pneumaticity and its implications. In Buffetaut, E. & Mazin, J.-M. (eds) Evolution and Palaeobiology of Pterosaurs. Geological Society Special Publication 217. The Geological Society of London, pp. 217-232.
Fleming, T. H. & Lipps, K. R. 1991. Angiosperm endozoochory: were pterosaurs Cretaceous seed dispersers? The American Naturalist 138, 1058-1065.
Frey, E., Tischlinger, H., Buchy, M.-C. & Martill, D. M. 2003. New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. In Buffetaut, E. & Mazin, J.-M. (eds) Evolution and Palaeobiology of Pterosaurs. Geological Society Special Publication 217. The Geological Society of London, pp. 233-266.
Unwin, D. 2006. The Pterosaurs from Deep Time. Pi Press, New York.
Wellnhofer, P. & Kellner, A. W. A. 1991. The skull of Tapejara wellnhoferi Kellner (Reptilia, Pterosauria) from the Lower Cretaceous Santana Formation of the Araripe Basin, northeastern Brazil. Mitt. Bayer. Staatsslg. Paläont. Hist. Geol. 31, 89-106.