Field of Science

A New Exhaustive Phylogenetic Analysis of Archosauria

I've been hinting at this for a bit but it is finally out. Congratulations to Sterling Nesbitt on an amazingly detailed and robust study of archosaurian relationships and their early evolution and distributions. This will be the new standard for archosaurian phylogeny and biogeography.

Nesbitt, S. J. 2011. The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades. Bulletin of the American Museum of Natural History 352:1-292. [Free Download]

Abstract - Archosaurs have a nearly 250 million year record that originated shortly after the Permian-Triassic extinction event and is continued today by two extant clades, the crocodylians and the avians. The two extant lineages exemplify two bauplan extremes among a diverse and complex evolutionary history, but little is known about the common ancestor of these lineages. Renewed interest in early archosaurs has led to nearly a doubling of the known taxa in the last 20 years.

This study presents a thorough phylogenetic analysis of 80 species-level taxa ranging from the latest Permian to the early part of the Jurassic using a dataset of 412 characters. Each terminal taxon is explicitly described and all specimens used in the analysis are clearly stated. Additionally, each character is discussed in detail and nearly all of the character states are illustrated in either a drawing or highlighted on a specimen photograph. A combination of novel characters and comprehensive character sampling has bridged previously published analyses that focus on particular archosauriform subclades.

A well-resolved, robustly supported consensus tree (MPTs  =  360) found a monophyletic Archosauria consisting of two major branches, the crocodylian-line and avian-line lineages. The monophyly of clades such as Ornithosuchidae, Phytosauria, Aetosauria, Crocodylomorpha, and Dinosauria is supported in this analysis. However, phytosaurs are recovered as the closest sister taxon to Archosauria, rather than basal crocodylian-line archosaurs, for the first time. Among taxa classically termed as “rauisuchians,” a monophyletic poposauroid clade was found as the sister taxon to a group of paraphyletic “rauisuchians” and monophyletic crocodylomorphs. Hence, crocodylomorphs are well nested within a clade of “rauisuchians,” and are not more closely related to aetosaurs than to taxa such as Postosuchus. Basal crocodylomorphs such as Hesperosuchus and similar forms (“Sphenosuchia”) were found as a paraphyletic grade leading to the clade Crocodyliformes. Among avian-line archosaurs, Dinosauria is well supported. A monophyletic clade containing Silesaurus and similar forms is well supported as the sister taxon to Dinosauria. Pterosaurs are robustly supported at the base of the avian line.

A time-calibrated phylogeny of Archosauriformes indicates that the origin and initial diversification of Archosauria occurred during the Early Triassic following the Permian-Triassic extinction. Furthermore, all major basal archosaur lineages except Crocodylomorpha were established by the end of the Anisian. Early archosaur evolution is characterized by high rates of homoplasy, long ghost lineages, and high rates of character evolution. These data imply that much of the early history of Archosauria has not been recovered from the fossil record. Not only were archosaurs diverse by the Middle Triassic, but they had nearly a cosmopolitan biogeographic distribution by the end of the Anisian.

11 comments:

  1. Congratulations to Dr Nesbitt!

    Just a quick question. In the paper it's stated (on p. 216) that Asilisaurus "has a clear hemicylinderical condyle on the calcaneum for the fibula (Nesbitt et al., 2010)". If that's the case, then shouldn't this character (378 - "Calcaneum, articular surface for the
    fibula") be coded "1" ("convex and hemicylindrical shaped") rather than "0" ("convex") for Asilisaurus?

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  2. The homology is still up in the air. The state in Marasuchus and Asilisaurus is similar to that of pseudosuchians but not exactly - the fibulae fossa is a bit wider in the avian-line taxa. I experimented by scoring the two taxa as 1, and it made no difference. I am currently looking into the morphology of the ankle of Asilisaurus.

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  3. I wonder who that mysterious Anonymous commenter could have been.

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  4. Lots of good things here, but also many red flags. Once again phytosaurs/Euparkeria/Proterochampsia come out basal to pteros (!) and dinos. This just doesn't make sense. Here was a great opportunity to TEST the addition of Huehuecuetzpalli, Sharovipteryx, Cosesaurus and Longisquama to a large cladogram but they were, once again, ignored. So frustrating.

    Likewise, Vancleavea may not be an archosaur. It doesn't look like any other archosaur. Why not throw in Helveticosaurus and Miodentosaurus to see which way the parsimony lies?

    When alternate candidates are not employed you're left with proving tradition. Science is all about TESTING. Why is everyone tiptoeing around these other possible candidates??

    See the alternate tree, the one that is MORE inclusive at: www.reptileevolution.com/reptile-tree.htm and links therein. Comments welcome!

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  5. "Once again phytosaurs/Euparkeria/Proterochampsia come out basal to pteros (!) and dinos. This just doesn't make sense."

    Why would a Wookiee, an 8-foot-tall Wookiee, want to live on Endor, with a bunch of 2-foot-tall Ewoks? That does not make sense!


    ;) All mockery aside, I reviewed Nesbitt's paper briefly on my blog and found it to be excellent.

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  6. Mickey,
    I hope this will be the last time you resort to mockery in a serious scientific discussion. Really. It's not helpful.

    I also hope that you (or anyone) can provide a list of traits that pterosaurs share with phytosaurs and other basal archosaurs not found in larger numbers (= more parsimoniously) in any other known taxa. That will go a long way towards settling the debate.

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  7. "I also hope that you (or anyone) can provide a list of traits that pterosaurs share with phytosaurs and other basal archosaurs not found in larger numbers (= more parsimoniously) in any other known taxa. "


    I heard--- Nesbitt, S. J. 2011. The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades. Bulletin of the American Museum of Natural History 352:1-292. --- might be a good place to start!

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  8. "I heard--- Nesbitt, S. J. 2011 might be a good place to start! "

    That was the hope! Unfortunately, as mentioned before, the fenestrasaur candidates were not listed on his cladogram. That's the key that seems to be missing. There's an assumption in place that pterosaurs were archosaurs and ornithodires. The test would be to see if they are indeed by permitting them to nest elsewhere if they do. Unfortunately Nesbitt 2011 decided NOT to test these other taxa, the ones that actual provide a series of ancestral sisters with a growing list of pterosaur synapomorphies. See www.reptileevolution.com/cosesaurus.htm to see a prepubis and a pteroid on a pre-pterosaur.

    Nesbitt also considered the "hole" in the mandible of Dimorphodon a mandibular fenestra, and archosaur character, but that is a mistake. Details and images at: reptileevolution.com/dimorphodon.htm

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  9. I couldn't resist the similarity with South Park's Chewbacca Defense. ;)

    In all seriousness, Nesbitt's analysis won't let us test the position of Huehuecuetzpalli or Miodentosaurus. You could plug them in, but without the characters for Archosauromorpha, Lepidosauromorpha, Thalattosauria, etc., their position wouldn't mean much. Might as well plug them into the TWG matrix. The converse of this is that it also doesn't let us test if pterosaurs, Vancleavea or any other included taxon are archosauromorphs. To test phylogeny at the level you disagree with (i.e. the highest one) would necessitate a HUGE analysis of well over a thousand characters.

    Incidentally, on my blog I've started to add some of the archosauromorphs you've considered close to pterosaurs to Nesbitt's matrix. Longisquama, Cosesaurus and Langobardisaurus so far, with Vallesaurus and Megalancosaurus in progress. So far, pterosaurs are still avemetatarsalians, but the addition of these taxa does make it easier for pterosaurs to be prolacertiforms.

    I don't expect anyone's analysis to convince you though, since you see things in photos no one else does. If I coded things with your interpretations, I very well might get your topology, but I don't trust most of what you draw.

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  10. Mickey,

    Thousands of characters would be interesting, but unnecessary. More taxa helps. As far as seeing things no one else has seen, I urge you to look at the various levels of detail that others have offered and I'm sure you'll agree they are at the level of cartoons. Senter declared there was no antorbital fenestra in Cosesaurus, yet his vague illustration oulined one. Sharov reported the back half of Longisquama was missing, but the missplaced plume he illustrated was actually an aligned femur + tibia/fibula. Jones et al. found subdivided plume shafts on Longsiquama, not realizing those were pedal phalanges aligned with the plumes. You can see photos and interpretations of the fenestrasaurs at reptileevolution.com. And finally, I'm sure that everyone has noticed that pterosaurs have one helluva big fourth finger. I've seen it. You've seen it. Where is that homologous structure within the Archosauria? Same for the large fifth toe. Same for the attenuated tail with chevrons parallel to the centra. Same for the antorbital fenestra without a fossa. Same for the multicusped teeth in basal taxa and the ossified sternum with wrap-around clavicles + coosified interclavicle. The list goes on and on.

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  11. PS, Mickey --

    If you can't or won't test the basal lizard, Huehuecuetzpalli, because Nesbitt's analysis "won't let you," then how did the those other lizards, the fenestrasaurs including pterosaurs, get nested? Just try with whatever characters Nesbitt's analysis offers and report the results. If pterosaurs are indeed archosaurs close to phytosaurs, then they will stay there. If they nest closer to Huehuecuetzpalli, that basal lizard with a an enlarged and retracted naris, a large first phalanx on pedal digit V, and an unfused mesotarsal ankle, then let us know. Testing is good. I applaud your efforts.

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