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Re: "Oxygen Helped Mammals Grow, Study Finds"
If you want to consider the impact on archosaurs of the supposed oxygen
levels, you really need to look at the plot in Figure 2 of the Science
Oh, it's in Science... oopsie...
Paul G. Falkowski, Miriam E. Katz, Allen J. Milligan, Katja Fennel, Benjamin
S. Cramer, Marie Pierre Aubry, Robert A. Berner, Michael J. Novacek, Warren
M. Zapol: The Rise of Oxygen over the Past 205 Million Years and the
Evolution of Large Placental Mammals, Science 309, 2202 -- 2204 (30
"On the basis of a carbon isotopic record of both marine carbonates and
organic matter from the Triassic-Jurassic boundary to the present, we
modeled oxygen concentrations over the past 205 million years. Our analysis
indicates that atmospheric oxygen approximately doubled over this period,
with relatively rapid increases in the early [sic] Jurassic and the Eocene.
We suggest that the overall increase in oxygen, mediated by the formation of
passive continental margins along the Atlantic Ocean during the opening
phase of the current Wilson cycle, was a critical factor in the evolution,
radiation, and subsequent increase in average size of placental mammals."
The paper claims that oxygen concentration was at 10% only at the start of
the Jurassic, and by implication in the Triassic, although the paper does
not plot oxygen levels before the Jurassic. (The curve is based on Bob
Berner's work, but I think it's calculated freshly in this paper using
some new data.) They show oxygen level rising to about 17% in the early
Jurassic, then dropping down to 12% and rising to 19% at the end of the
middle Jurassic. From the start of the cretaceous through until the early
Eocene, oxygen levels range from 15% to 18% or so.
As a calibration point, air pressure at an altitude of 5 kilometers is
about half that at sea level, so air at that altitude has about as much
oxygen as sea-level air would have at 10% oxygen concentration.
This obviously trounces the 13-% figure!
One more quote (refs removed):
"Whereas the relatively rapid decline in oxygen at the end-Permian and early
Triassic is suggested to have been a major factor contributing to the
extinction of terrestrial animals (mostly reptiles) at this time, the rise
of oxygen over the ensuing 150 My almost certainly contributed to evolution
of large animals. Animals with relatively high oxygen demands, including
theropod dinosaurs (the group that includes living birds) and small mammals,
evolved by the Late Triassic. Avian and mammalian metabolic demands are
three to six times as high per unit biomass as those of reptiles. Although
the reproductive strategies of the earliest mammals are not known with
certainty, both the fossil record and molecular divergence indicate that
superordinal diversification of placental mammals occurred between 65 and
100 Ma. This radiation corresponds to a period of relatively high and stable
oxygen levels in the atmosphere (Fig. 2). Although placental evolution is
not unique to mammals, this reproductive strategy, which can facilitate
geographic expansion of a species, requires relatively high ambient oxygen
concentrations. In the placenta, maternal arterial blood, with oxygen levels
near ambient alveolar pressure, mixes with placental venous blood in a
sinuslike vascular structure. Fetal umbilical arterial (really venous) blood
arrives in a capillary network in the maternal sinus where oxygen diffuses
into the fetal blood. The nature of this exchanger requires the mammalian
fetus to live at a very low arterial oxygen pressure. Although at low
oxygen, placental hemoglobin binding affinity for O2 is modified by pH
(i.e., the Bohr effect), with exceptions, few extant mammals reproduce above
elevations of ~4500 m, corresponding to atmospheric oxygen levels in the
Apart from the Atlantic, blame it on the diatoms and coccolithophores. They
bury all they carbon.
Interesting how the authors use "secular" -- apparently for "long-term".