Posts Tagged 'archosaurs'

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.

Guest Post: Rauisuchians – the neglected archosaurs

Today I’m delighted to bring you a guest post by Stephan Lautenschlager. Pterosaur researchers might remember Stephan from the Munich Flugsaurier meeting where he was generous enough to help me arrange the meeting during his last year of study there. Since then he’s moved on to start a PhD in the UK on therizinosaurs (cool!) and has just returned from a trip to Larry Witmer’s lab. Here he introduces the rauisuchians (a group of archosaurs I really should have written more about before – though the one behind him might be familiar…) and discusses his recent paper on them.

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Phytosaur skull

I don’t tend to cover things like phytosaurs simply because I don’t know much about them, rather than that I don’t find them interesting. Writing posts at the rate I do (generally close to 6 a week) means I try to stick to things I know well to save me doing much (if any) background reading on the issue at hand. Still, I do get pangs of guilt about not delving into other archosaurian lineages and especially the wealth of interesting things from the Triassic like phyosaurs, aetosaurs and rauisuchians, though this is somewhat alleviated by the presence of blogs like Chinleana which tend to avoid saurischians and pterosaurs and favour these guys, so the information is out there.

Despite the name, phytosaurs were not herbivores or indeed anything like really, but crocodile, even gaharial, analogues. Long snapping jaws with numerous sub-cylindrical teeth made many species ideal piscivores and the rest of the body was very like that of modern crocodylians. Here is a classic phytosaur skull (though I’m sorry I don’t know which taxon it belongs to) on display in Stuttgart that was too nice to ignore, even if I can’t say much about it, or the group as a whole.

Limusaurus confusion

ResearchBlogging.orgThe advent of the description of Limusaurus and the associated hypothesis of digit homologies in the paper is likely to generate quite a lot of interest in dinosaurian and avian circles. However, it will, I suspect, also generate a fair bit of confusion in the short and perhaps even long term, even if the hypothesis is rapidly refuted or is not adopted by the palaeontological community. It’s a problem that comes up occasionally in palaeontology (and I imagine other fields too) and is worth commenting on at least a little.
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Just some nice (living) archosaurs

imgp1446Blah, blah, short post, no time, pretty pictures. These ones come from the Ueno zoo in Tokyo, a truly super inner-city zoo. There were some wonderful rarities and personal favourites among the collections and while I have lots of photos of mammals, amphibians and fish too, given the tile of this blog, I thought I should stick to the archosaurs. For those who have not yet read the ‘what is an archosaur‘ post, among living animals it the birds and crocodilians. And here they are:

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So what exactly is an archosaur anyway?

This whole ‘Science Basics’ thing is not as easy as you might think. Even simple concepts take in several different branches of biology that some readers probably don’t know at all so while I can try and build up ‘A’ from scratch, it’s still quick tricky without a bit of background in ‘B’, which requires some knowledge of ‘C’, which is reliant on at least a partial understanding of ‘A’. And on and on and on. That is not to denigrate my audience, but I have to assume that at least some people reading this really don’t know much or any biology or palaeo at all, and since I’m not writing a straight through ‘what is biology’ series, but cherry-picking ideas I want to expand on, or I think are especially important or interesting, I can’t just do ‘A’ now, ‘B’ next week and ‘C’ the week after and hope the audience follows the thread. So, with that in mind, I’ll try to keep everyone on board and not bore the “experts” as I delve into the world of archosaurs.

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Fodonyx – a new genus of rhynchosaur

Yes my new paper is out (co-authored with my PhD supervisor, colleague and all-round-rhynchosaur expert Mike Benton) and as a result I want to talk about rhynchosaurs. OK, so they are not really dinosaurs, in fact they are not actually even archosaurs, but they are reptilian, lived alongside the dinos and are dead so they more or less count. And given what has passed for ‘dinosaur’ posts by me of late something on some other archosaurs seems appropriate. Besides, it’s not like you lot are going to stop me is it?

So rhynchosaurs then – the chisel lizards – so called because of their bizarre ‘tusks’ at the front of the jaws. These look rather like giant incisors and as a result give them a bit of a rabbit-like appearance, but these things aren’t teeth. In fact they are just modified jaw bones (the premaxilla above and dentary below) that in life would have stuck out through the flesh of the animal to form these pseudo teeth. In fact you can see marks on the bones where the skin would have stopped and the bony protrusion started.
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Pterosaur origins – where did they come from?

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.
Continue reading ‘Pterosaur origins – where did they come from?’

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