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.
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.
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