An arrangement of archosaurs

As I’m sure many readers are aware that Sterling Nesbitt has just published a monumental chunk of his PhD thesis as a major monograph on archosaur phylogenetics. While obviously new phylogenetic analyses are constantly being published, these tend to rehash and recycle existing work heavily (and that’s not necessarily a bad thing) and tend to be limited in scope. Very big, detailed, comprehensive analyses are incredibly rare since they require a huge amount of time to be applied to the problem. You have to have a really good understanding of anatomy, the historical aspects of taxonomy and phylogeny of the groups at hand, go over all the existing datasets, see tons of material, and then analyse and reanalyse and write it all up. To do all of that for a group as vast and complex as the archosaurs is a truly major achievement and he’s done it in incredible depth.

Few of the results are perhaps really even remotely surprising, but that’s by the by. That gives us great confidence in our other, smaller, studies and tells us we were on the right track. More importantly, Sterling has thrashed out (as far as I can tell, it’s 300 pages long! and I’ve not read a fraction of it in detail yet) some of the more poorly defined characters and redefiend them and their states and then as a result of his work, redefined every major node on the archosaur tree.

This is the real boon for me, making explicit statements about character evolution across the tree and allowing us to say more about what changed when and how and in conjunction with what else. The amount of characters too (over 400) also means that some important nodes now have a lot more support than they used to.

Archosaur phylogeny. From Nesbitt, 2011.

And the rest of it. From Nesbitt, 2011.

Obviously I’m especially interested in pterosaur origins, not least through having looked at some of the characters that tie them to the archosaurs with Sterling just this year. Some of these are disucssed and assessed here, but overall what we see is a much stronger support for an ornithodiran (that is pterosaurs + the dinosaursomorphs) position of pterosaurs than ever before.

Sterling recovers some 12 characters directly the supporting ornithodia, (with perhaps 13 more depending on issues of transformation that I won’t go into here). That is, in the context of something this big, really quite a lot. In fact it’s as many characters as he finds to support the dinosaurs (also 12) and many more than supports the dinosauromorphs (just 5). In short, what has always been a very poorly supported node is, in this analysis, an incredibly well supported node.

There are of course a few little caveats to this in that this is non a dedicated analysis of the pterosaurs and their relationships, and despite the huge size of this work, there are some more detailed and specialised characters that can be added in and there will doubtless be quibbles about exact codings and taxa. I can see a few things I would explore if I was to go and do this. That’s not to denigrate his work one iota (after all this isn’t what he’s trying to do!), I’m merely saying that IF you were to use this as the basis of pterosaur origins, I think there’s more that should be done. However, this is very robust indeed as nodes go, and I think at the very least it adds a lot of support to this hypothesis where previously it had been rather less clear cut.

The only thing I can really say to conclude is congratulations to Sterling and thank you. This is a piece of work which will aid almost every archosaur worker for, quite probably, decades. That is a real achievement.

4 Responses to “An arrangement of archosaurs”


  1. 1 Joshua Dyal 09/05/2011 at 9:06 pm

    Which also raises (or re-raises) the somewhat speculatory yet fascinating question of if some kind of fibrous coating is primitive to dinosaurs (it’s a tantalizing yet dubious prospect) what are the chances that they’re homologous to pterosaur pycnofibers?

    Interesting times.

  2. 3 Joshua Dyal 10/05/2011 at 12:48 pm

    Oh, I know. I meant merely that the question certainly comes back to mind. It certainly was a bit more on the back-burner while the status of the debate about the placement of pterosaurs was less certain.


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