Now this, this is a tricky one. Like some of the other areas I have (and indeed will) write about, this is one of the ones where many people seem to have dipped their oar in over the years and everyone seems to think they know what’s going on, when the truth is in fact quite a bit more complex when you start digging around under the surface.
Well, I have done a lot of digging and in some rather unusual ways as this subject made up the bulk of my PhD at Bristol and of course as a result I was pretty much immersed in this problem for the best part of three years. I also still have a few things ongoing and have been able to discuss this problem with just about everyone involved at the Wellnhofer pterosaur meeting in Munich. So hopefully this will be as definitive as possible, but prepare for some disappointment – it does not end well.
To cut a long story short, pterosaurs are damned difficult to place in the reptile tree. Skipping over a few historical anomalies (pterosaurs as flying marsupials, pterosaurs as bird ancestors and far worse) we are left with actually quite a limited, but complex, rage of possibilities. Thanks to the two posterior holes in the skull (the upper and lower temporal fenestrae for those in the know) we know pterosaurs are diapsid reptiles, but after this it gets really tricky, and the diapsids are a big group (it includes snakes, crocodile, dinosaurs, lizards, various marine reptiles, and all kinds of odds and sods like rhynchosaurs).
Pterosaurs, rather obviously, are highly adapted for their flying lifestyle and as a result their anatomy has changed a huge amount. Not just the obviously elongate fourth finger, but the skull bones have kind of merged, the humerus has changed dramatically to accommodate the flight muscles and the vertebrae have reshaped themselves to provide better support and plenty more besides. What this means in practice is that all those little anatomical details we rely on to link clades to each other have either disappeared, or changed so profoundly we are not sure what they have evolved from.
Normally we use a method called cladistics (it sort of tots up the similarities and differences of these anatomical changes to give you a family tree) to link together taxa but when the differences are few and far between things get tricky. Basically we don’t have enough information to link the pterosaurs to any of the diapsids with great confidence. Cutting out the details, we essentially have two options – pterosaurs either evolved from the dinosauromorphs (basically dinosaur-like reptiles) or from the base of the prolacertiforms (a group of early diapsids that includes all kinds of odd-balls). However, there is only a very limited amount of evidence to tie them to either group, so the whole thing hangs in the balance.
There are problems with both possible solutions – the prolacertiforms (and their relatives) are all pretty chunky, long-necked, sprawling lizard-like beasts which are hardly the best stock to produce small, short-necked, arboreal flying basal pterosaurs. With the dinosauromorphs they tend to have long legs and short arms (not very pterosaurian) and are bipedal (very un-pterosaurian – despite what you might have read). Hmm, not much to go on there then. When pterosaurs fall out with the dinosauromorphs, they almost inevitably end up in close association with Scleromochlus. Sadly this reptile is exceptionally poorly known and there is a great limit to what we can accurately determine about its skeleton which does not help matters.
How can we solve this problem? Well, we can add more characters to the analysis in the hope to accumulate more evidence (which is not really possible as we have pretty much exhausted them already), we can try new methods of cladistics to look for new solutions (the basis of my PhD and it didn’t work – ***Important point alert*** – Science does not always work!!!) or we can try and find new fossil material. Oddly enough the last of these actually looks the most reasonable right now, which should tell you a lot about the others! There is no guarantee that any possible ‘transitional’ pterosaur even fossilised, let alone that we will ever find one. Right now we are stuck without a pterosaurian Archaeopteryx (something that would unambiguously link pterosaurs to another diapsid group with unique characters) and given the size and age of such a creature it will probably never be found.
So there you go, placing pterosaurs is really tricky. But what is the answer? I have avoided giving you any hints until now, so here it is: drum-roll please……
Look, I told you it didn’t end well. The truth of the matter is that currently the best supported hypothesis is that pterosaurs derived from the dinosauromorphs and thus are very close relatives of the dinosaurs. However, it would not take much to swing the balance the other way and move pterosaurs to a more basal position in the reptile tree to link them with the prolacertiforms. As I said above, this one hangs in the balance, and at the moment, just tips towards dinosauromorphs.
However, at the risk of being really irritating, I do know of a couple of analyses that are slowly making their way towards publication that might actually reinforce the ‘dinosauromorph’ argument and make things a bit more clear cut. There are also a few anecdotal links between pterosaurs and dinosaurs that suggest a link and which would firm up this relationship still further. Nothing concrete for now, sadly, but the scales are tipping and right now the money is on dinosauromorphs.
PS Anyone in the know should feel free to not mention Sharovipteryx – it is simply not worth the effort. It has nothing to do with pterosaurs. Really.
This is a revised Mk.1 post, to see the original with comments etc., go here.