Thursday, 19 February 2015

People-snatching pterosaurs

I'm sure by now everyone has seen the recent Jurassic World trailer and palaeontologists and dinosaur fans alike have been salivating over it. The paleontological community is mostly in uproar over the scientific inaccuracies, mainly related to the lack of feathers on the theropod dinosaurs (for a few examples see Brian Switek here, Mark Witton, etc), but there have also been a few other comments about some problems with the creatures seen in the movies.

Of course for me, I notice how poorly done the pterosaurs are with respect to their contemporaneous dinosaurian relatives (remember, pterosaurs aren't dinosaurs!). I think Mark Witton put it the best:
I'm not going to go into the general inaccuracies of the pterosaurs (e.g. they should be covered in fibres, more meaty, etc), but I will talk about this problem of people-snatching pterosaurs. This is something that goes back quite far in Hollywood and dinosaur-related movies. There is always an image of a large pterosaur (typically Pteranodon) swooping down and picking up a person and flying away.
Nice grainy image of the Pteranodon flying away with a poor, unsuspecting woman in Jurassic World
Painting of a Peregrine falcon by John Gerrard
Keulemans. Notice the foot on the front bird and
how it is grasping the branch. 

Drawing of a Golden Eagle foot by Lydekker (1895)
showing the 4-digit structure of the foot.
There are several reasons for this, and I'm going to demonstrate this by comparing them with birds. In order to pick something up like that, you need some kind of grasping foot. Anyone who has had any type of raptor (as in the bird raptors - hawks, eagles, etc.) sit on their arm knows what this feels like. Their feet have 3 forward-facing clawed toes, and one reversed digit, known as the reversed hallux that faces backwards, allowing for a 4 digit grasping claw. This is what allows birds to perch on a branch, as they are able to grasp the branch to prevent themselves from falling off. This is also what allows these kinds of birds to pick up their prey as they swoop down. An important thing to note about that is that their prey is typically quite a bit smaller than they are (e.g. mice, rabbits, fish, etc.), although some of the larger birds have been known to kill bigger animals such as deer, antelope, etc. However, if a bird does this, it doesn't fly off with the prey, but rather will kill it and eat it in place. If it's going to fly off with it, it'll go for something much smaller. Another key bit of information with birds that do this - they have strongly muscled legs. Birds take off with their legs, and in comparison have very muscly feet and legs.

With that in mind, let's think about pterosaur feet. Pterosaurs have slender, weakly muscled feet. While the earlier non-pterodactyloids had 4 long, slender clawed digits that would have been flat on the ground during walking (in a plantigrade posture), the 5th digit was still elongated but did not touch the ground when walking (but was also not reversed as seen in birds). In more derived pterodactyloids, the 5th digit is almost entirely lost. None of these digits are reversed like in birds, and do not show the grasping structure as is typically shown in movies. Furthermore, pterosaur legs are weakly muscled, with most of their musculature occurring in the wings. They simply would not have had the musculature present to grasp prey in the same way that birds do.
Drawing of pterosaur hindlimbs from Witton (2013). A represents a pterodactyloid hindlimb (Anhanguera) with the nearly missing digit V, while B shows a non-pterodacctyloid (Rhamphorhynchus) with an elongated (but not clawed) digit V. 
The final "nail in the coffin" so-to-speak about people-snatching pterosaurs is the problem of weight. As I mentioned above, birds pick up small prey, typically much smaller than their body mass. Pterosaurs, however, are depicted picking up children or full grown humans. As it's typically Pteranodon being represented this way, we'll look at them. A large Pteranodon had a wingspan of about 6 m, and weighed probably somewhere around 35 kg if you go with the heavier estimates that I tend to favour from Witton (2008). The woman shown above that was carried off quite easily by a Pteranodon was probably somewhere around 60 kg at least. That means that it would have had to have been capable of carrying something and flying off with more than double it's initial mass. Considering there are already debates about if large pterosaurs were capable of flight (ok this isn't normally debated in Pteranodon, but I'm trying to prove a point!) there is no way it could fly if it was suddenly responsible for flying off with an additional 60 kg. Flight is hard enough as it is, and that additional mass would make it impossible. Then when you consider that 35 kg is a heavy estimate, moving do the lighter estimates and more "shrink-wrapped" pterosaurs to quote Witton, there is just no way.

So next time you see a pterosaur flying off with a person in tow in any kind of movie/tv show/etc., remember that it just couldn't happen. If pterosaurs were alive today, that would not be a concern we would have to deal with!

Special thanks to Tony Martin for giving me the idea for this post!

Witton, MP (2008) A new approach to determining pterosaur body mass and its implications for pterosaur flight. Zitteliana B28: 143-158.
Witton, MP (2013) Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press, Princeton, USA. 304 pages.