A Guide to the Dinosaurs of Colorado

Dinosaur Extinction
The Author's Own Theory

If you are of a sufficiently devious mind, you might also suspect that I deliberately omitted from my discussion other serious theories of mass extinction, simply to cast my own in a more favorable light. That is most emphatically not true. I have presented, and expounded on every major extinction theory that I am aware of, and some that aren't so major.

My personal extinction theory is complex enough do warrant its own web site, which you are invited to visit. A brief summary of its salient features will be listed here.

"Changing Paleoclimates and Mass Extinctions"
by Donald L. Blanchard

My thesis derives from three independent lines of evidence. The first is the dramatic change that occurred at the end of the Cretaceous Period. During the Cretaceous, and most of the Jurassic as well, it appears that freezing conditions were unknown on the planet. Evidence of tropical or subtropical flora, characterized by an intolerance of frost, and such animals as crocodilians at polar paleolatitudes has been mentioned previously. Global warming cannot account for the warmness evidenced at high latitudes, where average temperatures today can run below 0°F, or -18°C. A global warming of even 50°F or 28°C, would barely raise the polar regions to subtropical conditions, yet would render equatorial regions of the globe utterly uninhabitable, at around 135°F or 40.5°C on average. This is about the highest temperature ever recorded on Earth today, at Death Valley, California and in the deserts of Libya.

However, the average temperature of the Earth today is around 50 to 55°F, or 10 to 13°C. This is the temperature we would have if all the Earth's atmosphere were thoroughly mixed, and is quite warm enough to support a subtropical ecosystem, provided the atmosphere remains mixed, so that this is the minimum as well as the maximum temperature that exists. What could cause such thorough mixing of the atmosphere? Read on....

The second line of evidence is the fact that the three greatest extinction events known occurred at intervals of 190 to 200 million years: the K/T at 65 Mya, the Late Permian at 248 to 258 Mya (an interval of 183 to 193 million years), and the Late Ordovician at 438 to 448 Mya (an interval of around 190 million years. An even earlier extinction event has been reported in the Vendian Period (Late Precambrian) at around 650 Mya (an interval of 202 to 212 million years). Only one lineage of calcareous algae suffered extinction in the Vendian event, but then there was little else alive at that time to become extinct. Using dates of 256½ Mya for the Permian, 448 Mya for the Ordovician, and 639½ Mya for the Vendian, I come up with an average interval between major extinction events of 191½ million years. (Geological dating is still an imprecise science, and correlating protracted extinctions with precise dates is probably a hopeless task, but these figures are definitely within the ball park.)

The third line of evidence comes from a curious cosmic coincidence; the fact that the Plane of the Ecliptic (the plane in which the Earth orbits the Sun) comes within a few degrees of intercepting the center of mass of the Milky Way galaxy. All other planets orbit in planes that are within a few degrees of (but not exactly the same as) the Earth's orbital plane. Now, the Solar System is itself in orbit around the center of mass of the galaxy; the size and duration of this orbit are currently unknown, but estimated at around 200 to 400 million years. In the absence of outside perturbing forces, the Plane of the Ecliptic should be stable, meaning that in one quarter of a galactic orbit, the Ecliptic should be perpendicular to the center of mass of the galaxy. Thus, the current near alignment would be a once in 100 million year coincidence.

My thesis is that such an outside perturbing force does exist, and that the Plane of the Ecliptic actually rotates as the Solar System orbits the galaxy. Thus, the current (near) alignment should be a constant, not a coincidence. The significance of this is that if the Plane of the Ecliptic rotates, the Earth's obliquity changes. Obliquity is measured as the angle between a line perpendicular to the orbital plane (the Ecliptic) and the Earth's spin axis. The Earth's present obliquity is 23½°, which means that the Sun is directly overhead at 23½ degrees north latitude at noon on the first day of summer in the Northern Hemisphere, and at 23½ degrees south latitude at the Southern Hemisphere summer solstice. The obliquity thus controls how much sunshine, and thus how much solar heating, various latitudes on the Earth receive. It also has a profound affect on atmospheric circulation.

The Earth's atmosphere circulates in six toroidal cells called Hadley Cells; two in the tropics (north and south of the Equator), two in the temperate zones, and two at the poles. Little air mixture occurs between adjacent Hadley Cells. The area covered by the tropical Hadley cells also traps most of the heat from the Sun; its surface temperatures hover around 80°F, or 27°C, all year around. The temperate zones capture considerably less heat from the Sun (because the Earth's obliquity keeps the Sun lower in the sky); temperatures fluctuate between near 0°F (-18°C) in the winter to around 100°F (38°C) in the summer, averaging around 50°F (10°C). The polar regions capture very little heat from the Sun; their temperatures can dip below -40° (C or F) in winter, but rarely rise above 40°F (4.5°C) in summer, averaging around 0°F (-18°C).

My thesis rests on the premise that the Earth's obliquity is constantly changing, at a rate of 180 degrees every 191½ million years (it is currently increasing if I am correct), and that the obliquity was 90° during the Early Cretaceous. At 90° obliquity, the Sun would be directly over the North Pole in Northern Hemisphere summer, and over the South Pole in winter. I predict that during periods of high obliquity, the present six Hadley Cell atmospheric circulation no longer exists, and that air heated directly by the Sun (as the tropics are today) will mix freely with colder air, causing a nearly uniform temperature globally, day and night, winter and summer, of around 50 to 55°F, or 10 to 13°C. The maximum variation I expect would be around 2 or 3 degrees.

Any organisms that evolved during a period of high obliquity would never have to adapt to daily or seasonal changes in temperature. The three (or four) major extinction events occurred at the onset of six Hadley Cell circulation, with its hot tropics and cold poles, its hot summers and cold winters, and its hot days and cool nights. This would have occurred, according to the numbers given above, when the Earth's obliquity dropped to 37.6°. Using the same numbers, obliquity reached 0° 25 million years ago, and is currently increasing.

When it again reaches 37.6° in another 15 million years, six Hadley Cell circulation will once more be halted, and the Earth will again enjoy an isothermal global climate like the dinosaurs experienced. Some extinction can reasonably be expected at that time, as the tropical climate zones will vanish, taking with them all species not adapted to survive cooler temperatures. (It was at such a transition that the Frasnian/Famennian extinction event took place; the fourth greatest extinction event on record.)

So why did the dinosaurs become extinct, according to my theory? Large warm-blooded animals (and the general consensus today is that dinosaurs were warm-blooded) generate body heat in proportion to their body mass, which is proportional to the cube of their length. They radiate excess heat in proportion to their surface area, which is proportional to the square of length. Thus an animal three times as long will generate 27 times as much heat, but only loose it 9 times as fast. Large body size is therefore a logical adaption to cooler temperatures. And dinosaurs tended to have very large bodies. (This tends to support the proposition that Cretaceous times were cool, with an ambient temperature around 50°F, or 10°C.) Dinosaurs had never needed and so never evolved adequate cooling systems to dispell excess body heat. When hot summers and cold winters set in, dinosaurs had no place to escape the hot temperatures of summer. The only areas cool enough for dinosaurs to survive would have been the polar regions, and there isn't enough vegetation there for them to live on.

Another prediction of my extinction theory is that more widely distributed atmospheric circulation caused increased humidity, leading to a continuous cloud cover. It is my contention that no dinosaur living from the mid-Jurassic on ever saw sunshine! When the Sun finally came out at the end of the Cretaceous, small animals could hide from the Sun (even alligators can seek shelter in burrows or underwater), but dinosaurs had no place to hide, and baked.

In the ocean, a more complex explanation is required. Today, with polar ice caps and/or glaciers at both poles, cold sea water is constantly descending to the ocean's depths. Ocean bottom water is currently at around 39°F, or 4°C. Surface water temperatures range from near freezing in polar regions to near 80°F or 27°C in the tropics. But even in the tropics, you don't have to go very far below the surface before the water starts to get cold. Scuba divers in the Caribbean wear wet suits for insulation when working in water as shallow as 100 feet (30 m).

Marine planktonic microorganisms, including calcareous algae and foraminiferans, migrate vertically from the surface to a depth of several hundred feet on a daily cycle. In doing so, they experience a considerable change in temperature. During periods of high obliquity, there is no source for cold bottom water, and the ocean temperatures tend to be more uniform; vertical thermal gradients disappear. Microorganisms evolved under such conditions that migrate vertically don't have to be adapted to changes in temperature. When vertical thermal gradients reappeared, they couldn't survive the daily transitions. With the loss of the plankton, the whole ocean ecosystem collapsed, taking with it ammonites, plesiosaurs, mosasaurs, and all the other animals that depended on a lush crop of plankton for food.

"Changing Paleoclimates and Mass Extinctions"
at
http://webspinners.com/dlblanc/climate/