1. Dinosaur Extinction Theories

2. The K/T Extinction is one of the greatest mysteries of paleontology. What caused all non- avian dinosaurs to go extinct at the end of the Cretaceous Period?

3. Falsifiability There are many hypotheses about what caused the extinction, but they are not scientifically valid if there is no evidence to either support or refute them.

4. Hay fever killed the dinosaurs. Flowering plants evolved in the late Cretaceous period. Dinosaurs could have died from allergic reactions to these new plants. Is this a valid hypothesis?

5. Hay Fever Theory Invalid Flowering plants were around for millions of years before the dinosaurs disappeared. We have no evidence whatsoever that their pollen or other pathogenic products killed the dinosaurs, and it is unlikely that we would find evidence if it existed. It still wouldn't explain the massive marine extinction; there have never been marine angiosperms. Indeed, around the K-T boundary there is the phenomenon known as the "fern spike": all land plants except ferns show a dramatic decline in diversity. How could the dinosaurs be wiped out by pollen if a lot of the pollen producers were gone too?

6. Sniffles killed the dinosaurs? Near the end of the Cretaceous period, the continents were shifting, opening new routes for dinosaurs to cross into other areas previously inaccessible to them. So the mingling dinosaurs spread diseases and wiped each other out. Is this a valid hypothesis?

7. Dino Sniffle Theory Invalid We have no evidence of widespread disease among the last dinosaurs; they seem quite normal (we could prove disease in a dinosaur by the bone pathology — diseased animals often have deformed, weakened bones). This doesn’t explain what happened to the oceanic and non-dinosaur organisms that also went extinct.

8. Dinosaurs got too big.

9. Dino Crush Theory Invalid There is no general trend of increasing size among all dinosaurs. None became so large that they couldn't move; that is an evolutionary impossibility. Most dinosaurs were of medium or small size, even at the end of the Cretaceous.

10. Mammals are to blame. Mammals were outcompeting the dinosaurs for food, space, or other resources.

11. Mammal Culprits Invalid Dinosaurs and mammals evolved together for most of the Mesozoic era; mammals remained quite small, and only slowly increased in diversity. If they were outcompeting the dinosaurs, we would see a trend of decreasing dinosaur diversity and increasing mammalian diversity. We don't. Mammals and dinosaurs probably did not occupy similar ecological niches; the small mammals could exploit rare food resources, while the larger dinosaurs could not survive eating the same things that the mammals did. If they did not occupy similar niches, they probably did not compete. Mammals were not found in the oceans at that time, so the marine extinction is still not explained.

12. Mammals ate all of the dinosaurs eggs.

13. Egg Eaters Invalid No evidence of mammals eating them. But no egg-eaters could eat all of the dinosaurs' eggs; they would eat themselves into extinction if they did. Mammals couldn't have eaten all of the marine animals' eggs since there were no marine mammals at that time.

14. Cosmic rays killed the dinosaurs.

15. Cosmic Ray Theory Invalid. There is no evidence of extraterrestrial events that were occurring at the K-T boundary that could have emitted sufficient dangerous radiation to affect life on Earth significantly. There is no evidence for irradiated dinosaurs, either; this would have to show up in the bones to be noticed.

16. With the dismissal of other hypotheses, we are left with two different schools of thought: the gradualists and the catastrophists.

17. Gradualists These scientists believe that the fossil record indicates a gradual decline over 5-10 million years. These scientists believe that plate tectonic forces caused extensive volcanic activity in India and perhaps elsewhere that resulted in dense clouds of soot being released into the air.

19. Catastrophists These scientists believe that the fossil record indicates a sudden decline that is more consistent with a catastrophic event such as a massive asteroid impact. This theory was first proposed in 1980 by Walter Alvarez.

20. You will now examine pieces of evidence from the K/T extinction and sort them into support for the gradualist theory or the catastrophist theory. Some may fit in both categories. When you have finished, decide which hypothesis you believe is correct and identify the best supporting evidence for it. Identify any evidence that refutes your hypothesis.

21. Extinction Theories Gradualist Catastrophist