Posted by 
Long-time readers will know that I am an unashamed fan of both speculative zoology, and of Dougal Dixon’s hypothetical ‘alternative’ animals. Inspired by a comment made here in August by Jenny Islander, I have been having a re-think about the possible evolution of flightless pterosaurs: the fossil record gives no indication that such animals ever existed, and maybe they didn’t, but that hasn’t stopped people from speculating. The best known hypothetical pterosaurs are those from The New Dinosaurs (Dixon 1988), and among the most bizarre and memorable of them is the Lank Herbafagus longicollum, a long-necked, long-legged, long-faced flightless African pterosaur that eats grass and can swiftly out-run predators. Ridiculous, right? Well, the recent work on azhdarchid palaeobiology has led me to reconsider exactly how ‘ridiculous’ it was…
Like many of the animals from The New Dinosaurs, the Lank is suspiciously convergent with an extant animal, and in this case it’s the giraffe [painting by Steve Holden © Dougal Dixon 1988, used with permission]. It’s even patterned like a giraffe (and, specifically, like a Reticulated giraffe Giraffa reticulata [following the taxonomy of Brown et al. (2007)]). Yes, I will admit that, like others, I long found the Lank to be one of the daftest and most improbable of Dougal’s ‘alternative’ animals.
In his review of The New Dinosaurs, Greg Paul (1990) dismissed the Lank as one of the stupidest creatures in the book, writing ‘[T]he “lank” is perhaps the worst beast in the book. That pterosaurs would beat their dinosaurian cousins into evolving giraffes, of all things, complete with hooves and reticulated orange-brown colour pattern, is unbelievable’ (p. 311). David Unwin (1992) was similarly dismissive, writing ‘Bipedal pterosaurs with wings free of the hind limbs, unlikely as they seem, cannot be ruled out, but I doubt whether evolution, at its most inventive, could produce ‘lanks’ and ‘flarps’, fully erect, four legged, grass-grazing, flightless pterosaurs!’ (p. 63: note that this description actually applies only to the Lank as the Flarp Vexillala robusta is bipedal).
I would agree with Greg that the possibility of flightless pterosaurs successfully out-competing thriving and diverse herbivorous dinosaurs in adapting to plains-dwelling herbivory does not seem plausible, but as to whether the Lank is plausible in principle… well, I have, as so often happens, changed my mind. Now that the case for strong terrestrial adaptation in azhdarchids has been made (Witton & Naish 2008), we can reconsider the idea that, were the right conditions to arise, or were azhdarchids to hang on beyond the end of the Maastrichtian, they might well have reduced their already short wing-fingers and evolved flightlessness.
As you’ll know from Witton & Naish (2008), or from the Tet Zoo or Wittoniana or Azh Pal articles that accompanied it, azhdarchids combined their proportionally short wing-fingers with elongate hindlimbs and small, narrow, well-padded feet. They walked with a narrow, parasagittal gait, and their tremendously robust, well-muscled arms indicate that they were the strongest launchers and, more importantly here, most powerful quadrupedal runners among the Pterosauria (the data supporting these assertions has yet to be published but is in the works and has been alluded to here on Tet Zoo before. For the time being it can be regarded as M. Habib, pers. comm.).
In view of the ‘terrestrialization’ of azhdarchids, I would say that, if any pterosaurs were to become flightless, then azhdarchids are at the top of the list. The fossil record gives us no indication that this happened. But if it did, it would (probably) only happen in environments devoid of predators like sebecosuchians or big theropods, as even a giant galloping flightless azhdarchid is probably under pressure to retain a flight ability so long as such animals are around. Note that, if the development of flightlessness happened at giant size, then flightless azhdarchids would probably not have to worry too much about small predators, like troodontids or small dromaeosaurs.
As Witton & Naish (2008) noted, the proportions of a quadrupedally walking azhdarchid are most similar to those of cursorial ungulates, specifically artiodactyls like wildebeest and giraffes. This is coincidental: cursorial ungulates and azhdarchids have not converged in proportions because of similar lifestyles. So what is a flightless azhdarchid – one with strongly reduced or absent wing-fingers – going to look like? By now you’ve already seen my attempt at imagineering one. This animal, Shemhazai ptychocheirus, might be from the Upper Cretaceous, or it might be from the Paleogene in a world where the end-Maastrichtian event never happened (like Spec). It has lost the uropatagia and most of the brachiopatagia and has a strongly reduced wing-finger. Manual digits I-III have hoof-like unguals, have rotated on the metacarpus to face forwards, and are more robust and better at weight-bearing. But that’s about it, it really isn’t that different from its volant relatives. Given that this animal cannot fly away when resources are dwindling, it could only have evolved in a place where there is a year-round supply of diverse foodstuffs, and I imagine it inhabiting semi-tropical mixed woodland where it forages for arthropods, molluscs, worms, small vertebrates, fruits, seeds, carrion and anything else edible.
Azhdarchids of the Miocene
If we now imagine another 20-30 million years of evolution on top of that, in which time members of this lineage have become even more terrestrial, better at running, and better at exploiting terrestrial resources, what might they look like? Well, kind of like the Lank actually. My cursorial, grassland-inhabiting Miocene azhdarchid – Qilin parungulatus* – is shown here (with Toni Naish for scale). What’s happened now?
* Qilin was proposed by Alec T and parungulatus by Stevo Darkly. Thank you all again.
All traces of wings and wing-membranes are gone, and the even longer limbs are now more specialised for cursoriality and terrestriality. The body is far deeper and narrower than was the case in volant azhdarchids, and the forelimb and scapulocoracoid have become reoriented such that the humerus now projects posteroventrally. The distal end of metacarpal IV has become a heavily keratinised pseudo-hoof, and digits are entirely absent. Given that metacarpal IV is so large and robust in Cretaceous pterodactyloids, I thought it more likely that this – rather than digits I-III – would become the main organ of support in the forelimb. In the hindlimb, a need to become swift and cursorial has resulted in the development of digitigrady and of three hoofed digits on a super-long limb. The long neck is more flexible than that of its ancestors, and its serrated rhamphothecal tomia (= bill edges) make it an efficient browsing herbivore, capable of cropping and eating most plant material. It has voluminous guts, a big muscular gizzard, and is huge. And, yes, it looks like a giraffe, but that isn’t because I really wanted it to.
A few discrepancies make the Lank ‘inaccurate’ if we compare it to Qilin parungulatus. For starters, the Lank walks on a strongly reduced, hoof-tipped wing-finger (Dixon 1988). As a result, its remaining ‘free’ fingers (there are only two) are now located up off the ground. The animal uses them in grooming but they are otherwise non-functional. However, given that azhdarchids (and other pterosaurs) walked with their wing fingers folded up and out of the way (only the base of the first phalanx could have played a role in weight support) and with their three ‘free’ fingers contacting the ground (indeed, these fingers were probably retained because of their role in quadrupedal walking), we should really expect the Lank to have a pretty different forelimb morphology. It might still have hoof-tipped digits, but these would correspond to digits I, II and/or III, not IV (the wing-finger). The wing-finger should either be absent altogether or, if retained, form some weird little display organ perhaps (as it does in Shemhazai).
Unlike any azhdarchid (or azhdarchoid), lanks possess teeth, and that might seem strange if lanks are meant to be derived azhdarchids. However, this isn’t necessarily such a problem given that – as we’ve seen on Tet Zoo before – teeth can be, and have been, re-evolved in the jaws of clades that had otherwise lost them. Having said that, we might expect the Lank to have more a pterosaury-looking head: as a beaked herbivore it should perhaps look more like a long-billed goose. The Lank’s head actually looked too mammalian. Qilin parungulatus has serrated tomia that function as pseudo-teeth, so teeth just aren’t needed [in image below, giraffes from Longleat's giraffe page].
All in all however, the Lank does not fare that badly when put up against a more contemporary vision of what flightless azhdarchids might look like.
In case you’re wondering, this does not mean that the other Dixonian flightless pterosaurs are ok. The world of The New Dinosaurs was populated by a diverse pterosaur assemblage that, besides lanks, included the savannah-dwelling Flarp, the wading sifts and parasos, the raptor-like Harridan Harpyia latala, shorerunners, the immense, long-winged oceanic Soar Cicollum angustalum, penguin-like plungers, and the flightless, muppet-like wandles and kloons of New Zealand. Paul (1990) noted that, given their low diversity prior to the end-Maastrichtian event, the survival of pterosaurs into the Cenozoic might be doubtful even if a mass-extinction event failed to occur. Unwin (1992) noted that many of the lineages portrayed in The New Dinosaurs seemed unrealistically diverse and speciose in view of the fortunes of their Maastrichtian ancestors. This was particularly true of pterosaurs: so far as we know at the moment, azhdarchids were the only pterosaurs present in the Maastrichtian (thanks to Mark, I’ve had to revise my previously-held idea (Martill & Naish 2006) that tupuxuarids also made it into the Maastrichtian), so they’d be the only ones able to make it into the Cenozoic.
Cursorial Miocene lank-like azhdarchids never did exist, of course. But I now think it’s possible that something like Shemhazai could have existed, in which case its fossils might await discovery in some yet-to-be-described Upper Cretaceous island-endemic assemblage. We can but dream.
Refs – -
Brown, D. M., Brenneman, R. A., Koepfli, K.-P., Pollinger, J. P., Milá, B., Georgiadis, N. J., Louis, E. E., Grether, G. F., Jacobs, D. K. & Wayne, R. K. 2007. Extensive population genetic structure in the giraffe. BMC Biology 2007, 5: 57 doi:10.1186/1741-7007-5-57
Dixon, D. 1988. The New Dinosaurs: An Alternative Evolution. Salem House Publishers, Topsfield, MA.
Martill, D. M. & Naish, D. 2006. Cranial crest development in the azhdarchoid pterosaur Tupuxuara, with a review of the genus and tapejarid monophyly. Palaeontology 49, 925-941.
Paul, G. S. 1990. An improbable view of Tertiary dinosaurs. Evolutionary Theory 9, 309-315.
Unwin, D. M. 1992. The New Dinosaurs: An Alternative Evolution (review). Historical Biology 6, 61-71.
Witton, M. P. & Naish, D. 2008. A reappraisal of azhdarchid pterosaur functional morphology and paleoecology. PLoS ONE 3 (5): e2271. doi:10.1371/journal.pone.0002271
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 22, 2008
September 23, 2008
September 23, 2008
September 23, 2008
September 24, 2008
September 29, 2008
September 29, 2008
October 1, 2008
October 2, 2008