Leigh Van Valen and Bob Sloan offered a more complex theory. During the Late Cretaceous, the continents were moving and major new mountain chains began to rise. Many of the shallow Mesozoic seas dried up. Van Valen and Sloan suggest that this caused the world's climate to change. Evidence from fossil plants at their Montana study site suggests that it got about ten centigrade points colder in the Late Cretaceous. A temperature decline that large would affect the earth. It would have become colder in the mountains, and new plants would have replaced the warm-weather plants.
Van Valen and Sloan have argued that dinosaurs were at a disadvantage in the new forests of coniferous trees. So, they left the mountains and moved toward the tropics, to a better climate. This theory suggests that the dinosaurs survived longer in the tropics than in the mountains. Because plants in the tropics survived, something else must have caused dinosaur extinction in these areas.
One suggestion is that placental mammals (mammals that give live birth and have a placenta) became abundant in the mountains, because they no longer competed with dinosaurs. Later, the mammals escaped these regions and went into the tropics, where they drove dinosaurs to extinction. There are problems with this theory. Some studies of foraminifera (the shells of protozoans talked about earlier) show that there was a short-term warming trend in the Late Cretaceous. But they are right about the long-term cooling trend. There is no evidence that dinosaurs lasted longer in one area than another. Although competition between animals may explain the extinction of many living things at the end of the Cretaceous, other groups, especially the foraminifera, seem to have disappeared without competitors.
Another climate change theory is due to recent research. Some living reptiles, turtles and crocodiles, lay eggs. It was recently found that the sex of their offspring is decided by the temperature of the nest. So if the same were true for dinosaurs, cooling temperatures in the Late Cretaceous may have caused all the young to be of the same sex, so the species would not have been able to continue.