Fig. 2.5 Atmospheric oxygen percentage plotted against number of dinosaur genera. This figure supports the hypothesis that higher oxygen supported a higher diversity of dinosaurs. Part of the reason for this may be due to the fact that rising oxygen levels opened up more habitable areas at altitude, a prediction from Huey & Ward (2005). (Diagram from Ward (2006), with permission.)
Middle Triassic and Late Triassic through most of the Jurassic. It is not until the latter part of the Late Jurassic that dinosaur numbers started to rise significantly, and this trend then continued inexorably to the end of the Cretaceous, with the only (and slight) pause in this rise coming in the early part of the Late Cretaceous, and this slight drop may be due to the very small number of localities known of this age yielding dinosaurs. By the end of the Cretaceous (in the Campanian Stage) there are hundreds of times more dinosaurs than during the Triassic to Upper Jurassic. So what was the cause of this great increase?
The relationship shown in Figure 2.5 suggests that oxygen levels have either played a major role in dictating dinosaur diversity, directly (by physiological effects on the animals themselves) or indirectly (by somehow affecting success of dinosaurs through food supply or available habitat area). Through the Late Triassic and first half of the Jurassic, dinosaur numbers were both stable and low. While originating in the latter part of the Triassic, they stayed relatively few in number until a moderate rise at the end of the period - a rise that seemed to coincide with the end-Triassic mass extinction itself. Gradually, if the oxygen results from GEOCARBSULF are even approximately correct, oxygen rose in the Jurassic, hitting 15% or more in the latter part of the period. It is then that the numbers of dinosaurs really begin to increase. It is also at this time that the sizes of dinosaurs increased as well, culminating in the largest dinosaurs ever evolved appearing from latest Jurassic into the first half of the Cretaceous. Oxygen levels steadily climbed through the Cretaceous, and so too did dinosaur numbers, with a great rise in dinosaur numbers found in the Late Cretaceous, the true dinosaur heyday. It must be noted that of the two great stocks of dinosaurs, the saurischians and ornithischians, it was the latter that made up the greatest number of new forms in the Cretaceous. Because these species were without pneumatization, as well as being mainly her-vibores (and thus less active than saurischian carnivores, all with pneumatization) it appears that oxygen had to rise to certain thresholds for ornithischians to get first a toe-hold, and then dominance among herbivorous animals of the latter parts of the Mesozoic.
There were surely other reasons for this Cretaceous rise. For instance, in mid-Cretaceous times the appearance of angiosperms caused a floral revolution, and by the end of the Cretaceous Period the flowering plants had largely displaced the conifers that had been the Jurassic dominants. The rise of angiosperms created more plants, and sparked an insect diversification. More resources were available in all ecosystems, and this may have been a trigger for diversity as well. But even with these caveats, the correlation between numbers of dinosaurs and oxygen levels is clear.
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