What exactly is going on with the dinosaurs in the Early Jurassic?

It is considered by some to be, and should have been, a classic example of adaptive radiation. At the end of the Triassic the majority of pseudosuchians go extinct, removing the biggest competitors of the dinosaurs and leaving the door open for an evolutionary explosion of the dinosaurs. Interestingly, however, this is not what happened according to a new study by Steve Brusatte and colleagues (Brusatte et al., 2008b) who found, in the continuation of their research comparing morphospace disparity between ornithidirans and pseudosuchians (Brusatte et al., 2008a), that dinosaur disparity remained relatively unchanged through the Triassic/Jurassic boundary. It would be expected that once the extinction of the pseudosuchians freed up a large amount of morphospace, the dinosaur record (with whom the pseudosuchians occupied a lot of the same niches and had similar body plans) would show a strong response, yet the dinosaurs show only a "slight non-significant increase" (Brusatte et al., 2008b). Thus, these authors argue, "different aspects of dinosaur radiation (diversity, disparity, and abundance) were decoupled, and the overall macroevolutionary pattern of the first 50 Myr of dinosaur evolution is more complex than often considered (Brusatte et al., 2008b).

Adam Yates had discussed this (and his hypothesis) a few weeks ago at Dracovenator and I had provided some follow-up discussion here. Nonetheless, despite the timing of the extinction it is apparent that not to much is going on for the dinosaurs immediately after the TR/J extinction (which by the way took out the non-dinosaurian dinosauromorphs). Sure to the record of coelophysoids and sauropodomorphs you add a few large theropods such as Dilophosaurus and you see the first good records of heterodontosaurids and the earliest thyreophorans (including the first ornithischians and sauropodomorphs in N. America), but you do not see a true explosion of dinosaur diversity until you get to the Late Jurassic. How much of this is a sampling and/or preservation problem is unclear, but simply look at the Weishampel et al. (2004) chapter on dinosaur distribution in the 2nd edition of The Dinosauria and compare the faunal lists for these epochs. You really have to clean up the Late Triassic portion removing many of the Ornithischia references, indeterminate theropods (could be shuvosaurids), and all of the footprint evidence (no ornithischian or sauropodomorph tracks in N. America; the "theropod" tracks worldwide could be made by convergent dinosauriforms, and pseudosuchians), not to mention the really messed up stratigraphy for the Chinle and Dockum which caused some duplicate entries. Now compare the Late Triassic, Early Jurassic, and Middle Jurassic lists to the rest of the chapter. Surprised? I commend Brusatte et al. (2008a, 2008b) for setting the stage and providing a baseline framework for some much needed future research to address this enigma.

REFERENCES

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

Brusatte, S.L., Benton, M.J., Ruta, M., and G.T. Lloyd. 2008b. The first 50 Myr of dinosaur evolution: macroevolutionary pattern and morphological disparity. Biology Letters, doi:10.1098/rsbl.2008.0441, published online.

Weishampel, D. B., Barrett, P. M., Coria, R. E., Le Loeuff, J., Gomani, E. S., Zhao Z., Xu X., Sahni, A., and C. Noto. 2004. Dinosaur distribution. In: Weishampel, D. B., Dodson, P., and Osmólska, H. eds. The Dinosauria. 2nd edition. Univ. California Press, Berkeley. pp. 517-606.