Why Dimorphodon macronyx is one of the coolest pterosaurs

How to make Dimorphodon macronyx fly: chase it down with a Sarcosaurus-like dinosaur. The most recent illustration of the 'reluctant flier Dimorphodon' hypothesis, based on predicted wing parameters of this heavyset pterosaur. Prints of this image are available.

With Jurassic World about to start assaulting the box office and intelligence of palaeontologists around the globe, it seems appropriate to take a look at some of the science behind the animals featured in the film. Being just about to move house (copious books and fossils = Worst. Moving. Experience. Ever.) means I can't write about them all, but we have time to look at one of the pterosaurs they're featuring, and coincidentally also one of my favourite fossil species: Dimorphodon macronyx. I was quite chuffed to hear Dimorphodon was going to make it to the big screen, but... oh dear. Poor Dimorphodon has been really mangled by the infamous reconstruction approach of the Jurassic World film makers, and the information on their website is really awful - powerful talons for snatching fish? Seriously?. From what we've seen so far at least, I wonder if it's one of the worst reconstructions in the film.

"Now that is one big pile of..." From the Jurassic World wikia.

Clearly, the Dimorphodon of Jurassic World is going to be nothing like the Dimorphodon known to researchers. OK, that's hardly a shock, but it's a shame nonetheless. Dimorphodon is not a theropod-headed scaly dragon, but an especially interesting and significant animal to pterosaur researchers. I'm involved in several Dimorphodon related projects at the moment - one should see fruition next week - and thought I'd share some of the basis for my fascination here.

OK, smart guy, what was Dimorphodon really like?

Dimorphodon is one of the best known early pterosaurs. Seemingly unique to Lower Jurassic rocks of Dorset, UK (Mexican material previously referred to Dimorphodon likely represents a different taxon), it is perhaps the oldest pterosaur known from anything like three-dimensional remains. This doesn't include skull material, which is always preserved with the topography of a pancake, but much of our Dimorphodon limb and body fossils have some, if not entire, three-dimensionality to them. Although a complete skeleton has never been found, several half- or near-complete specimens are known along with a lot of associated material. The upshot of this is that a fairly decent understanding of Dimorphodon osteology has been held for almost 150 years (so, yeah, the Jurassic World animal is less accurate than renditions put together by Victorian palaeontologists. It's not the only Jurassic World species to suffer this sort of problem). With most older pterosaur fossils being either mere fragments or entirely squashed skeletons, Dimorphodon represents an important insight into early pterosaur anatomy. This is especially so because some aspects of its skeleton - particularly jaw shape, dental anatomy and wing proportions -indicate it is a rather 'plesiomorphic' species, closely related to some of the oldest known pterosaurs, such as the Triassic taxa Peteinosaurus and Preondactylus. This might make it particularly informative as goes the anatomy of the first pterosaurs, with all sorts of potential for investigating their locomotion and ancestry. It must be said that this is only one interpretation of Dimorphodon phylogenetics however: the interrelationships of early pterosaurs are particularly contentious, and other workers suggest it plots much further away from the base of the pterosaur tree.

Restored Dimorphodon macronyx skeleton. From Witton (2013).

Dimorphodon is widely known for its dentition, its 'two form teeth' providing a generic namesake. The larger teeth of Dimorphodon are sometimes incorrectly portrayed as splaying from its jaws, somewhat like those of rhamphorhynchine or ornithocheird pterosaurs. So far as we can tell, though, they were more-or-less vertically orientated. These bigger teeth possess carinae - cutting surfaces running along the anterior and posterior dental margins. Only the posterior region of the lower jaw has the second type of tooth - very small, sharp cusps which are positioned at regular intervals to make the jaw resemble a hacksaw blade. These were clearly the subject of heavy use in life: some specimens possess broken tips.

D. macronyx tooth morphologies. Note the broken tooth exploded from the main image. From Ősi (2011).

Its not only teeth which make Dimorphodon characteristic, however. The size of the skull is quite remarkable compared to other early flying reptiles, and a forerunner to the trend of large skull sizes that would develop later in monofenestratan pterosaurs. The fact all Dimorphodon skulls end up being flattened indicates that the skull bones were not robust. That said, although likely full of air in life, the skull of Dimorphodon is still large enough to occupy a proportionally large amount of body mass, as were the hindlimbs (Henderson 2010). We tend to think of early pterosaurs as scrawny-legged animals which couldn't walk if their lives depended on it, but Dimorphodon limbs are pretty well built. Indeed, the hindlimbs are so strongly put together that Dimorphodon was the pterosaur behind the controversial 'dinosaur-like bipdeal pterosaurs' concept discussed through the 1980s and 1990s. I've been wondering of late whether we can consider that idea fully refuted now: at least one individual still champions the idea, but they have not really countered the wealth of evidence set against pterosaurian bipedality. For those not keeping score, that evidence includes the anterior centre of gravity occurring in all pterosaurs; issues with hindlimb musculature efficiency at poses imposed by bipdal gaits; problems with neatly folding the wing; the wealth of quadrupedal pterosaur trackways; trackway and osteological characteristics indicating plantigrade feet; and scaling regimes of pterosaur limbs matching those of quadrupedal volant animals, but not bipedal ones. Although a minority of these points have been partially refuted (sometimes controversially), evidence supporting the bipedal pterosaur hypothesis is thin on the ground compared to that for quadrupedality, and this applies to Dimorphodon as much as anything. I'm coming to the opinion that the use of bipedal or quadrupedal gaits is not really a debated topic for pterosaurs now.

The hands and feet of Dimorphodon are also robust, and equipped with large, trenchant claw bones (these, of course, provide the specific namesake, 'macronyx'). There are indications that the extensor muscles controlling these might have been powerful, as every claw on both hands and feet is equipped with a neighbouring sesamoid - those intra-tendinous bones serving to enhance muscle output or protect tendons against powerful joint motion. Interestingly, the only other animals with these claw-adjacent sesamoids are lizards and a 'bottom walking' fossil stem-turtle - more on that another time. As with all pterosaurs, there is no indication that their hands or feet were for grasping, and their claws are really nothing like talons (take that, Jurassic World website).

Dimorphodon wings are interesting for their contrasting proportions to the rest of the body, as well as those of most other pterosaurs. Although the wing fingers of Dimorphodon are decently sized - they occupy over half the length of the entire arm - the overall wing length is a bit on the small size, at least compared to predicted Dimorphodon masses. At least 3 studies have independently predicted relatively high wing loading in Dimorphodon, suggesting those relatively big skulls and legs were not accommodated for with increases in wingspan. First-principle interpretations of these results - that Dimorphodon flight may have been a bit more fraught and energy-demanding than similarly-sized pterosaurs - is being borne out in assessments of wing shape (Witton 2008) and flight studies (which I'll be talking about at Flugsaurier 2015). I went so far in 2008 as to suggest that Dimorphodon was a 'reluctant flier', because its predicted wing parameters seemed to closely match those of game birds and other woodland avians - those which take flight when they have no alternative, and keep their flight durations short (see illustration at top of post). Early indications from more detailed assessments are that aspects of flight we normally assume for pterosaurs - soaring and gliding - may well have been challenging, or effectively impossible, for Dimorphodon. These predictions of a heavyset pterosaur by myself and others are something of a first for flying reptile studies: mostly, we've remarked about how lightweight and glide-efficient pterosaurs were, not the opposite.

The Puffinodon: another palaeoart meme? 

What sort of lifestyle did Dimorphodon lead? Considering we're talking about a pterosaur, you can almost guarantee that someone has proposed that Dimorphodon ate fish. Some authors - perhaps Bakker (1986) was the first - have noted similarity between the skull of Dimorphodon and that of puffins, taking this to mean that these animals lived similar lives of diving into the water in pursuit of nektonic prey. Lots of artists have been inspired by this idea.

The palaeoart meme of 'Puffinodon'. Note that related puffin-inspired Dimorphodon art, not shown here, exists where dark body colours contrast with a variably coloured, vertically-striped bill. I'm as guilty of the latter as anyone.

The 'Puffinodon' concept doesn't do very well under testing. For one, the skulls of Dimorphodon and puffins aren't really that alike. Most of what makes up the deep cranial profile of puffin bills is soft-tissue, not bone. Moreover, puffins and other diving birds have wings well-adapted for 'flight' under water in that their wing bones are somewhat flattened, with thick bone walls. Dimorphodon wings, by contrast, are actually broader in some respects than those of other pterosaurs (stay tuned for more on that), and there are no indications that it had thickened bone walls. To the contrary, there are indications that its postcrania was pneumatised, at least in part.

That's the skull of Atlantic puffin (Fratercula arctica) on the left, Dimorphodon on the right. Grey shading indicates soft-tissue. Puffin skull modified from Schufeldt (1889).

So what did Dimorphodon eat? A comprehensive study of early pterosaur skulls and teeth concluded that Dimorphodon jaws were well-suited to a diet of insects, carrion and small vertebrates (Ősi 2011). That actually chimes pretty well other interpretations of Dimorphodon palaeobiology: there are indications that Dimorphodon was an adept climber, a fast runner, and - as discussed above - possibly flight restricted. A diet of insects and small vertebrates fits with these assessments of locomotor habits suited to terrestrial realms pretty well, and we might imagine Dimorphodon as better adapted to chasing down lepidosaurs and large beetles than it was diving for fish. Indeed, there are some pretty cool aspects of Dimorphodon anatomy indicating it may have been really at home on land - as in, as much as pterodactyloids were. This might come as a surprise to some, seeing as non-pterodactyloids have largely been thought of as terrestrially inept. We'll have to wait just a bit longer before I can talk about those, however.

So that's Dimorphodon in a few paragraphs, then: nothing like the animal we'll be seeing this summer at the cinema, and perhaps nothing much like other pterosaurs, either. If early pterosaurs and their lifestyles are your thing, stay tuned for some new ideas on that very soon.

References

• Bakker, R. T. (1986). The Dinosaur Heresies. London: Penguin.
• Henderson, D. M. (2010). Pterosaur body mass estimates from three-dimensional mathematical slicing. Journal of Vertebrate Paleontology, 30(3), 768-785.
• Ősi, A. (2011). Feeding‐related characters in basal pterosaurs: implications for jaw mechanism, dental function and diet. Lethaia, 44(2), 136-152.
• Shufeldt, R. W. (1889). Contributions to the Comparative Osteology of Arctic and Sub-Arctic Water-Birds: Part V. Journal of anatomy and physiology, 24(Pt 1), 89-116.
• Witton, M. P. (2008). A new approach to determining pterosaur body mass and its implications for pterosaur flight. Zitteliana, 143-158.
• Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.