Dinosaur Survivorship Across The Triassicjurassic Boundary

The end Triassic mass extinction was one of the so called "Big Five": one of the five most catastrophic of all mass extinctions. It took a toll on all kinds of life, and vertebrates were not spared. As with any mass extinction, clues to cause can be found by comparing "winners" (survivors) and losers (those clades undergoing complete extinction). In this case the biggest winners, in that they were the only clade to actually increase in numbers across the Triassic-Jurassic (T-J) boundary, were the saurischian dinosaurs. This can be tracked with synoptic data, but this finding is supported by a local finding. Olson et al. (2002) noted that the size, abundance, and diversity of dinosaur footprints increased across the T-J boundary in the Newark Basin, the site where footprint records are perhaps best preserved for this time interval. These authors, who (then) favored large body impact as the largest single cause of the T-J mass extinction, have more recently abandoned the impact hypothesis for this event. Whatever the cause, it must have somehow favored survivability of those clades that were pneumatized.

The synoptic data at hand for this event are shown in Figure 2.4, which looks at families of vertebrates and classifies them by hypothesized lung type. The two aspects of this graph that are most notable are that the saurischian dinosaur body plan somehow conferred higher survivability

Triassic Jurassic Extinction
Fig. 2.4 Survivorship across the Triassic-Jurassic boundary mass extinction. The groups are put into larger clades, but the data making up the spindles are number of genera taken at the stage level. (Diagram from Ward (2006), with permission.)

than the others, and that even after the extinction was long over, in fact over most of the Jurassic Period, vertebrate diversity including dinosaurs remained low. This is contrary to our usual view of the Jurassic as a time of highly diverse dinosaur genera. This problem can be further assessed by looking at the number of dinosaur genera with respect to atmospheric oxygen in the Jurassic.

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