I do so dislike the term ‘transitional fossil’, like ‘missing link’ and ‘ancestor’ these words and phrases have been warped by the media (I think) to the point where they seem to be accepted as technical scientific terms for what were largely informal general concepts and the whole things has become a bit of a minefield. When you add to that the willful misuse and manipulation of the terms by some to try to challenge evolution then it gets even worse. However there are times when something is so obvious and clear and simple that it is hard to use any other term. No, I don’t like ‘transitional fossil’, and this is not a ‘transition’ in the sense of an ancestor because, well that’s not how palaeontology works, BUT, if you want an example of ingtwo separate body plans and shunt them together into some kind of 50-50 version, this is it. Yes, today saw the publication of the new pterosaur Darwinopterus and as the title of this post and introductory paragraph gives away, it is a real intermediate between two groups of pterosaurs.
Anyone who knows even the bare minimum about pterosaurs will know that they are split into a clade of derived pterodactyloids and a group of basal rhamphorhynchoids (and if you don’t, try this) with a number of major differences in the anatomy between the two notably the big head of the former and long tail of the latter. While both are obviously flying reptiels sharing a great many unique characteristics, equally there was a fairly big gap between the two. They were obviously related, but the pterodactyloids appeared in the fossil record pretty much complete and fully formed and trying to work out at which point which characters changed from the rhamphorhynchoid condition to the pterodactyloid one was pretty much an exercise in futility. This, it should be noted, is no great surprise given the rarity of pterosaurs in general and especially of any putitive ancestor or transitional animal that represented both clades.
However, Darwinopterus sheds an enormous and brilliant spotlight onto this dark area of pterosaur history and reveals some very, very surprising things. Firstly this is an amazing find and while in one sense you always assume that sooner or later you are going to fill a big gap in the fossil record, this is a very big gap and one that apparently occured very quickly and as noted in a group not known for often preserving huge numbers of specimens so this is an extraordinary find. About the only bigger gap in pterosaurs that could be filled would be to find what links them to the other archosaurs. Secondly, we are exceptionally lucky that there are multiple specimens, some with soft tissues preserved, and so we have a ton of information here and not just one scrappy fossil. Equally, a number of these specimens were prepared and worked on from scratch by a professional academic team so anyone wanting to suggest these might be fakes coming from China is barking up the wrong tree. What is most striking however is that the fossils show a great example of modular evolution – these animals do not just show a combination of characters seen in rhamphorhynchoids and pterodactyloids, but these are distributed in a disjunct manner.
To look at Darwinopterus with a pterosaur researcher’s eye, the head and neck are absolutely classically pterodactyloid – there is a big fenestra at the front, the skull is big, the neck long and there are no cervical ribs (among other features), but once you get past the neck, it’s all rhamphorhynchoid – a long bony tail, elongate 5th toe, basal style humerus and more. It really looks as if the front of one animal has been bolted onto the rear of another. What is clear is that almost all of the cranial characteristics of pterodactyloid pterosaurs evolved first and possibly in unison, before the rest of the body got going and started changing towards the pterodactyloid condition. This is really impressive and to see it demonstrated so clearly and definitively is fantastic.
This is an amazing animal and great example of an intermediate. Hopefully this will become obsolete quickly, by which I mean that if we find all manner of close relatives of Darwinopterus, then that huge gap between pterodactyloids and rhamphorhynchoids which has already been cut in half with this discovery, can be split into finer and finer gaps until it becomes meaningless and disappears. Then we really will have a complete record of the transition of every character from one group to the other and while it has taken us over 200 years to get this far, there is surely more to come. I’ll leave you with the accompanying picture of Darwinopterus hunting a small feathered dinosaur as just to ice the cake a little further, this animal is from the Middle Jurassic and thus appears in the fossil record immediately before the first pterodactyloids – no temporal paradoxes here!
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