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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 paper.

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 September 2005)

Abstract:
"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 Early Jurassic."


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".