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For instruction on how to read this Causal Loop Diagram, see Holistic Earth Causal Loop Diagram.
Climate triggers extinctions via its effects upon females. In response to environmental heat, a female (mammal, reptile, or bird) shunts part of its blood supply to the skin to transport body heat to the environment. This reduces the supply of blood to the uterine tract. Uterine blood flow is a developing embryo's source of oxygen, water, nutrients, and hormones, and also removes damaging metabolic heat from the embryo. Heat-induced reduction of uterine blood flow can kill developing embryos outright, or damage them, producing dwarfs and skeletal abnormalities. Large animals are most affected. I first developed this model for the Pleistocene-Holocene mammalian extinctions, and have now extended it to the dinosaurs (McLean, 1995).
For the dinosaurs, during Late Cretaceous, they were adapting to climatic cooling. The abrupt K-T climatic reversal to greenhouse warming triggered chaos among dinosaurian reproductive systems. In effect, the dinosaurs went into a thermal net that caught relatively large animals. Whereas the thermal inertia inherent of large body size would have been advantageous to large dinosaurs facing climate cooling, their small surface-to-volume ratio ratios would have been disadvantageous during warming, and they would have retained excessive body heat. Small size is advantageous in a rapidly warming world.
Female dinosaurs, attempting to thermoregulate in a greenhouse world, would have shunted blood supply to the skin, reducing the flow of blood to the uterine tract where fertilization takes place, causing it to heat above optimum for fertilized eggs and embryos. Maternal hyperthermy transmitted to fertilized egg during the first cleavage triggered massive embryo death likely triggered the K-T transition final phase of the dinosaurian extinctions.
Today, via this mechanism, summer heat kills mammalian embryos on a vast global scale. Any modern greenhouse can only increase embryo death.
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Copyright © 1995 Dewey M. McLean