Field of Science

Dinosauria vs. Pseudosuchia - New paper in Science

Very recently there has been a resurgence of interest in the early appearance and diversification of the Dinosauria mainly due to the recognition that there exists strong convergence between early dinosaurs and pseudosuchian archosaurs such as Revueltosaurus and Shuvosaurus, and that dinosaur precursors such as Dromomeron and Silesaurus not only survived into the Late Triassic but also coexisted with the dinosaurs for millions of years (Dzik, 2003; Ezcurra, 2006; Irmis et al. 2007b; Nesbitt et al., 2007; Parker et al., 2005; Nesbitt and Norell, 2006). This has been accompanied by studies demonstrating that in some faunas (especially those of North America) dinosaurs were neither dominant or diverse, and that in fact there is no unambiguous evidence of Triassic ornithischians or sauropodomorphs in North America, and that the global record of Triassic ornithischians is extremely poor (Irmis et al., 2007a; Nesbitt et al., 2007). These and other studies have also demonstrated that Late Triassic pseudosuchians were extremely diverse and that their occurrence together with ornithodirans in most Late Triassic assemblages demonstrates that they were filling similar ecological roles. Thus, one of the biggest mysteries is why the majority of pseudosuchian lineages die out at the end of the Triassic, while the more conservative dinosaurs go on to have great success for the next 140 million years.

Today in the new issue of Science, Brusatte et al. provide the results of a multifaceted study addressing this question. They provide a new phylogenetic analysis of the Archosauria (supplementary materials) and compare evolutionary rates and morphological disparity between pseudosuchians and ornithodirans. Interestingly they found that the dinosaurs had lower disparity and represented a lesser amount of morphospace occupation compared to the pseudosuchians. Furthermore rates of character evolution between the two groups were indistinguishable. Previous hypotheses that the dinosaurs were more successful due to physiological superiority and were “preordained for success” are discounted (as was also argued by Irmis et al., 2007b). Instead Brusatte et al., suggest that the “dinosaurs were the beneficiaries of two mass extinction events – and some good luck”.

I admit that I am not surprised at all by their findings, but am probably biased because this trend is readily apparent in North America (where I work) where with the exception of the Hayden and Coelophysis Quarries at Ghost Ranch New Mexico (and trackways in the youngest Triassic units) there is a marked paucity of Triassic dinosaur fossils and an abundance of diverse pseudosuchians. I am a bit flummoxed over the basal positioning of Revueltosaurus in their phylogeny, but this is based on an incomplete coding which I have not thoroughly reviewed.

Overall I find the paper to be a useful contribution in the attempt to discern why such a wonderful diversity of crocodile-line archosaurs lineages was extinguished at the end Triassic. Their data helps quantify some of the trends seen by other workers, especially that the competition model is most likely untenable. However, disproving the competition scenario does not necessarily support the "lucky break" hypothesis. Furthermore, I have not seen strong evidence for a Carnian-Norian terrestrial extinction in the fossil record, a claim that is even more weakened by the recent announcement of a Rhaetian dicynodont, which supports known Norian dicynodonts in Arizona and rhynchosaurs in Brazil and Argentina. Recent published and unpublished studies revising the Late Triassic timescale demonstrate that much of the hypothesized Carnian terrestrial strata worldwide is probably actually Norian, thus at best there are very few Carnian age terrestrial assemblages (e.g., Muttoni et al., 2004; Furin et al., 2004). There is still much work to be done on this mystery and I for one am not quite ready yet to simply attribute it a “lucky break”; however if this is the case then I truly rue what would appear to be a cruel twist of fate, and can only wonder what might have come to pass if the pendulum had swung the other way.

REFERENCES

Brusatte, S.L., Benton, M.J., Ruta, M., and G.T. Lloyd. 2008. Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs. Science 321:1485-1488.

Dzik, J. A beaked herbivorous archosaurs with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23:556-574.

Ezcurra, M.D. 2007. A review of the systematic position of the dinosauriform archosaur Eucoelophysis baldwini Sullivan & Lucas, 1999 from the Upper Triassic of New Mexico, USA. Geodiversitas 28:649-684.

Furin, S., Preto, N., Rigo, M., Roghi, G., Gianolla, P., Crowley, J.L., and S. A. Bowring. 2006. High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs. Geology 34:1009-1012.

Irmis, R.B., Parker, W.G., Nesbitt, S.J., and J. Liu, 2007a. Early ornithischian dinosaurs: the Triassic Record. Historical Biology 19:3-22.

Irmis, R.B., Nesbitt, S.J., Padian, K., Smith, N.D., Turner, A.H., Woody, D., and A. Downs. 2007b. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317:358-361.

Muttoni, G., Kent, D. V., Olsen, P. E., DiStefano, P., Lowrie, W., Bernasconi, S. M., and F. M. Hernández. 2004. Tethyan magnetostratigraphy from Pizzo Mondello (Sicily) and correlation to the Late Triassic Newark astrochronological polarity timescale. Geological Society of America Bulletin 116:1043-1058.

Nesbitt, S.J, and M.A. Norell. 2006. Extreme convergence in the body plans of an
early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society of London Series B 273: 1045–1048.

Nesbitt, S.J., Irmis, R.B., and W.G. Parker, 2007. A critical reevaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5:209-243.

Parker, W.G., Irmis, R.B., Nesbitt, S.N., Martz, J. W., and L. S. Browne, 2005. The pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. Proceedings of the Royal Society London B 272:963-969.

11 comments:

  1. I'm not so surprised that dinosaurs "lucked out." It happens a lot in mass extinctions. Dicynodonts died out in the Permio-Triassic extinction except for Lystrosaurus, who shows no obvious adaptations to make it through unharmed. Neornithine birds survived the KT extinction while Enantiornithines were wiped out.

    And this is the part where I beg for a copy of the paper in electronic form. ;-) I won't be able to get my hands of a physical copy of the new Science for at least a week (our university library has a terrible lag time).

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  2. I know I am going to state this wrong, so forgive me as I do it, but isn't it Benton's pet idea that faunas don't out compete each other, but rather they have been the beneficiaries of mass extinction events for some time?

    It gives the impression that he doesn't "like" competition much as reasons for faunal replacement at all.

    Also wasn't there a Cretaceous dicynodont found in Oz none-too-long ago?

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  3. Even at the Hayden Quarry, dinosaurs are still not as abundant as pseudosuchians. We've found alot of dinosaur specimens, but they represent a fraction of the total number of specimens excavated.

    Zach - I agree with Bill in cautioning people about the influence of a Carnian-Norian extinction event on the rise of dinosaurs. With all the new constraints on the Late Triassic timescale, it seems like the evidence for such an extinction is very weak. That of course doesn't rule out the influence of an extinction at the Triassic-Jurassic boundary.

    Will - You are correct that Benton's idea was a lack of competition. So, I'm not sure why you're confused. The new paper supports this idea, as does Irmis et al. 2007 (in Science last year). However, as Bill points out, competition vs. lucky break is not a simple dichotomy - there are many other options that have yet to be ruled out.

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  4. ok, time for less delicacy. I was trying to be polite about this, but...

    does this constitute 'proof' or merely flogging a hobby horse?

    sorry, too tired to phrase it nicely.

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  5. Will,

    The article is simply providing more evidence against the idea of competition, but unfortunately not really offering a new hypothesis as the conclusions again were that the dinosaurs were "lucky beneficiaries" of another group's misfortune. Still I would not quite put the paper in the cateory of "flogging the hobby horse" because it offers a new approach and some new data. I'll admit that I've been going back and forth with this myself.

    There is a purported Cretaceous dicynodont from Australia, but this is based on scrappy material and to be cliche "extraordinary claims require extraordinary evidence" and it is just not there IMHO (see my earlier Polish Dragon post for the citation).

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  6. Zach - I wouldn't be so hasty to attribute Lystro's survival to random dumb luck. Recent FEA work comparing Lystro to a more standard Permian dicynodont (can't remember which) shows that it had a much stronger, stiffer skull which was thought to be an adaptation to eating much tougher vegetation, probably horsetails. One thing about the exceedingly poor plant fossil record from the Early Triassic is that horsetails are about all you can find. So speculative as it might be, it seems to me to be a plausible idea that reason for Lystrosaurus' success is that its food formed the dominant vegetation in the wake of the P-Tr event.
    Also Lystosaurus was not the sole dicynodont survivor. There were at least two emydopoid survivors represented by Myosaurus and Kombuisia. The late Permian Vivaxosaurus is also more closely related to Triassic Kannemeyeriformes than to Lystrosaurus indicating that this clade also had to have had at least one surviving lineage crossing the P-Tr boundary.

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  7. There is a purported Cretaceous dicynodont from Australia, but this is based on scrappy material and to be cliche "extraordinary claims require extraordinary evidence" and it is just not there IMHO (see my earlier Polish Dragon post for the citation).

    IIRC, there was a suspected locale that the material came from. Has anyone followed up with that? Or tried to? Ir realize funding is short, but that would be quite a feather in someone's cap if they confirmed the find with more material.

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  8. Everyone I've talked to is happy with the specimen being a dicynodont, but they're all very skeptical of its provenance.

    There is locality info for the specimen, but the issue is that it was found as float, not in situ. The authors argued that because that area of Australia has Cretaceous bedrock for miles around, the specimen should be Cretaceous. The problem is that Australia is very flat and deeply weathered. Some land surfaces have been exposed since the Miocene! So, the specimen could have eroded out of Triassic rocks quite a long time ago, and then just sat around on or near the land surface for literally millions of years.

    The key would be to compare Rare Earth Element (REE) signatures of the matrix around the fossil to the nearby Cretaceous rocks, as well as potential Triassic source rocks. As far as I'm aware, no one has done this.

    For those wondering, the reference is:

    Thulborn, T., and S. Turner. 2003. The last dicynodont: an Australian Cretaceous relict. Proceedings of the Royal Society of London, Biological Sciences 270:985-993. DOI: 10.1098/rspb.2002.2296

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  9. >The key would be to compare Rare >Earth Element (REE) signatures of >the matrix around the fossil to >the nearby Cretaceous rocks, as >well as potential Triassic source >rocks. As far as I'm aware, no >one has done this.

    This should work. An excellent idea.

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  10. There is more discussion of the Cretaceous dicynodont over at Dracovenator, Adam Yates' blog.

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