Why There Were Dinosaurs; Why There Are Birds

Presenter: Peter Ward, University of Washington
When: November 18, 2003 12AM PST

New information on the levels of atmospheric oxygen
through the Phanerozoic indicates that the end of the Permian and end of
the Triassic were times of greatly lowered oxygen compared to the present
day, equivalent to the oxygen content at current altitudes in excess of
12,000 feet. Here I propose that the combination of low oxygen and
repeated short spikes in global temperature caused by methane induced
atmospheric greenhouse conditions that were the primary causes of the P/T
and T/J mass extinctions. These lowered oxygen levels, which according to
the models persisted through the Triassic and into the early Jurassic
(with minima at 250 and 200 Ma) may well have lead to the evolution of
bone pneumatization found in modern birds and most lineages of saurischian
dinosaurs examined to date. New physiological studies of this system in
extant birds shows it to be far superior to the respiratory systems of
lizards, amphibians, and mammals in surviving at high altitude (and thus
lowered oxygen). It also appears that vertebrate lineages with this
newly-evolved respiratory system had higher survival rates across the T/J
mass extinction interval than did lineages with the air-sac system.

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