Field of Science

Biogeography of Triassic tetrapods

Interesting approach and personally timely as I just happen to really be into Triassic biogeography at the moment; however, I think this study suffers somewhat from the usual culprits, lack of taxon sampling and unresolved taxonomic issues (weak primary data).  For example, basal phytosaurs are completely left out which have an early and widespread appearance in the Late Triassic.  These are extremely important as they are found in Europe, North America, North Africa, and interestingly in India.  Including these would have changed several of the paper's conclusions.  Likewise, the aetosaur Calyptosuchus (Stagonolepis) wellesi is purposefully left out as is Coahomasuchus, which are two important datapoints for aetosaurs in the Ischigualastian of North America.  I wish I had been contacted regarding this study as I would have gladly suggested some less "key" aetosaur taxa to leave out if better resolution was needed for the basal aetosaurines.  It is also not entirely clear if the Ischigulastian and Coloradian are applicable outside of South America.  It appears that they are presented as equivalent to the Otischalkian + Adamanian and Revueltian + Apachean of North America.  This was first put forth by Max Langer in 2005 and has some support, but needs further testing, especially regarding the relationships of all of the Stagonolepis-like aetosaurs, the lack of good phytosaur material in South America, and lack of good (or any) comparable rhynchosaur, cynodont, and sauropodomorph material in North America.

On a side note, it was nice to see Heliocanthus used in there.

Ezcurra, M. D. 2010. Biogeography of Triassic tetrapods: evidence for provincialism and driven sympatric cladogenesis in the early evolution of modern tetrapod lineages. Proceedings of the Royal Society B, doi:10.1098/rspb.2010.0508 Published online.

Abstract - Triassic tetrapods are of key importance in understanding their evolutionary history, because several tetrapod clades, including most of their modern lineages, first appeared or experienced their initial evolutionary radiation during this Period. In order to test previous palaeobiogeographical hypotheses of Triassic tetrapod faunas, tree reconciliation analyses (TRA) were performed with the aim of recovering biogeographical patterns based on phylogenetic signals provided by a composite tree of Middle and Late Triassic tetrapods. The TRA found significant evidence for the presence of different palaeobiogeographical patterns during the analysed time spans. First, a Pangaean distribution is observed during the Middle Triassic, in which several cosmopolitan tetrapod groups are found. During the early Late Triassic a strongly palaeolatitudinally influenced pattern is recovered, with some tetrapod lineages restricted to palaeolatitudinal belts. During the latest Triassic, Gondwanan territories were more closely related to each other than to Laurasian ones, with a distinct tetrapod fauna at low palaeolatitudes. Finally, more than 75 per cent of the cladogenetic events recorded in the tetrapod phylogeny occurred as sympatric splits or within-area vicariance, indicating that evolutionary processes at the regional level were the main drivers in the radiation of Middle and Late Triassic tetrapods and the early evolution of several modern tetrapod lineages.

5 comments:

  1. Hi Bill!
    Your are right regarding the taxon problem, but ambiguous phylogenetic relationships are a large problem at the time of performing this kind of analyses. At first hand, I included Calyptosuchus wellesi and Coahomasuchus in the analyses, but I have to pruned them because the program TreeMap doesn't allow politomies at the phylogeny. The same problem is with for example Parasuchus, since no recent quantitative phylogenetic analyses found non-ambiguous relationships for this taxon.
    But I'll try to find the way to include these taxa in the new and larger biogeographic analysis that I'm working right now. Bill, I will be happy to receive any suggestions to my e-mail.

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  2. Hi Martin,

    Taxon sampling will always be a problem when dealing with fossils and biogeography, however, I just didn't agree with some of the taxa you were forced to exclude as I believe their non-inclusion distorted your results.

    There is a phytosaur phylogeny by Michelle Stocker that is in press and includes many basal North American taxa, and I am currently building a larger phylogentic dataset for the aetosaur with the goal of elucidating the relationships of the Aetosaurus-like and Stagonolepis-like taxa. My preliminary results are rather interesting.

    Let's talk about this some more for your next project.

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  3. Bill,
    Thanks for the info. Please, let me know when these analyses will be available. I'll be happy to have your opinion about my next Triassic biogeographic analysis in the future.

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  4. Well, the best way to deal with polytomies for biogeographic analysis is to re-run the analysis with each of the different resolutions of the MPTs. In alot of cases, they'll have a very consistent signal. The great part about newer likelihood-based methods is that you can actually use a likelihood ratio test to help decide which different MPTs give you the result that best fits the data (or conversely, tells you that you cannot reject any of the possibilities).

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  5. Its a very good experience to have thanks for posting your experience of Triassic tetrapods.

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