Dinosaur Extinction Theories

Calvin W. Lew

20 February 1996

Dinosaur extinction theories have been widely speculated over the years and have evolved as new evidence is discovered. Theories differ from some that explain the cause of extinction for all dinosaurs; while others appear to only account for some. These theories include drastic changes in the earth’s climate due to several causes; a world-wide physiological epidemic, some incredible cataclysm or catastrophe, changes in vegetation, an asteroid from outer space striking the earth, and even the Biblical flood (with little standing room in Noah’s Ark for dinosaurs!) among others. The ultimate cause may have been one of these theories or a combination of several.

One widely accepted theory of dinosaur extinction is that of great climatic changes which the dinosaurs were not able to adapt to. There are many possible causes to these changes. And there are more disputes about these causes than the eventual result (the earth’s environmental change and the dinosaurs’ incompatibility to it). Some of these changes may have been a result of the drifting of the continents. In this time of mountain building, the earth’s crust would have undergone violent upheavals.

With the shifting of the continents came great climatic changes in the environment. There became increases in the difference of latitudinal temperature. During the time of the dinosaurs, the temperatures between the equator and the poles were relatively similar. When these differences occurred, also came different seasonal temperatures. With these changes in temperature “(it became too hot or too cold, for at least part of the year) and in rainfall (it became too dry, causing the disappearance of the swamps and lakes in which many of the dinosaurs are supposed to have lived, or too wet, causing floods). Movement of the earth’s crust, up or down, could have produced the same effects. The Walt Disney film Fantasia chose to show the Cretaceous dinosaurs (including a Jurassic Stegosaurus !) dragging themselves hopelessly to their deaths over a hot waterless desert” (Charig 1983).

But perhaps these changes did not take place so suddenly. “From the fossil evidence it is difficult to see much climatic difference between the late Cretaceous and early Cenozoic time” (Colbert 1983). Another argument against the climatic change theory is would this have affected all the dinosaurs? For example, why has the crocodile survived? All the dinosaurs, small and large, became extinct. If the crocodile survived, why didn’t other small dinosaurs survive? Perhaps the earth’s waters experienced less drastic changes? Or perhaps the water already had varying degrees of temperature and the animals within it were accustomed to these temperature variations. But, many aquatic creatures have also become extinct.

Another cause in the dinosaurs’ demise may have been a change in the vegetation that the dinosaurs fed upon. Changes which caused the vegetation to be inedible would have led to obvious results. Perhaps more plants evolved to be poisonous to the dinosaurs. If dinosaurs were similar to present day reptiles, they would have had a poor sense of taste; and thus would not have been able to taste the bitter and poisonous alkaloids contained in some of the flowering plants that evolved in Late Cretaceous times.

Or, there may have been just a sudden lack of vegetation due to several reasons. The climatic changes mentioned before certainly would have had this outcome. With the evolution of plants and the arrival of flowers, also came along butterflies and moths. Butterflies and moths fed almost entirely on plants; thus decreasing the supply. Today, their numbers are kept down by natural enemies such as birds. But these natural enemies to moths and such were absent at the time of the dinosaurs (Charig 1983).

The dinosaurs’ enemy may have been smaller and more deadly than plant-eating moths. An unknown, wide spread epidemic or disease may have plagued them. With the environment evolving, new diseases, bacteria, viruses and such could arrive. And again, the question arises; would this have affected all the dinosaurs? “Diseases and epidemics are generally rather specific in their effects; closely related animals may suffer from the same diseases, but more distantly related forms are usually immune. There were many genera and species of dinosaurs, belonging to numerous families and higher categories” (Colbert 1983).

Rather than these gradual and natural developments, several theories argue that a tremendous world-wide cataclysmic event caused the sudden extinction of the dinosaurs. Many of these theories include some event that caused great changes to the atmosphere and environment that was impossible to adopt to. One such theory is that in this time of continental upheaval and drastic change to the earth’s crust, great volcanic eruptions emitted so much dust and ash as to block the sun and change the atmosphere. Resulting effects would have been a drastic reduction in photosynthesis of plants; the base of the food chain on earth. These same effects may have also been caused by a great comet, meteor, or asteroid from outer space. These catastrophic theories may have explained the wide, global nature of this event; but why were not all the animals affected? Would not the mammals be affected by these same drastic changes in the earth’s atmosphere?

Some discredit the cataclysm theory by again indicating the global and far-reaching nature that such an event would had to have had. “The cataclysmic hypothesis can receive no support. We have shown that dinosaurs were world-wide in distribution, and, of course, they did not live alone. Many other kinds of creatures lived on land, in the air, and in the sea whose history would have been profoundly affected by any great catastrophic change in environmental or other conditions” (Swinton 1970).

Other arguments not mentioned before include the over-abundance of meat-eaters. These meat-eaters may have ate so many plant-eaters that they themselves starved. Another theory is that the little mammals of the Cretaceous period liked to eat dinosaur eggs; so much so that the dinosaurs died out.

These varied arguments are near impossible to verify and may not answer the question, “This caused the extinction of all the dinosaurs everywhere ?” The dinosaurs of the late Cretaceous were so varied and populated such a large area of the continents that a cause of extinction would have had to been gradual, but global enough to effect all the dinosaurs; or if the cause was sudden, it would have had to be so catastrophic and immense in nature as to effect every corner of the globe.

Evidence that dinosaurs died out in a short time can be seen at the North Horn Formation in Utah. At a certain point in ancient sediments dinosaur bones can clearly be seen. “Then at a certain point in the sequence there are no more dinosaur fossils, while within a few feet above that level are the remains of early mammals. There are no changes in the sediment to indicate the break (no disconformity, in geological parlance), yet the fossil evidence is clear and unequivocal. The dinosaurs died out and a short time later...primitive mammals appeared in force” (Colbert 1983).

Whether or not the extinction was sudden or gradual, the question still remains as to why it was not complete. Many crocodiles, turtles, lizards, snakes, fish, amphibians, and other invertebrates survive to the present day; while many “ichthyosaurs, plesiosaurs, mosasaurs (...giant sea-going lizards), flying reptiles, and certain invertebrates...were going the way of the dinosaur” (Ostrom 1984). Those searching for an answer to dinosaur extinction look for a common lethal factor. But, perhaps it was not just one cause, but several of these hypotheses working in concert. For now, this question remains one of the great mysteries of science.

References

Charig, Alan. 1983. A New Look at the Dinosaurs. Facts on File, Inc., New York.

Colbert, Edwin H. 1983. Dinosaurs: An Illustrated History. Hammond Inc., New Jersey.

Ostrom, John H. 1984. Dinosaurs. Carolina Biological Supply Company, North Carolina.

Swinton, W. E. 1970. The Dinosaurs. George Allen & Unwin Ltd., London.