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Competition
Toward the end of the dinosaurs' reign, mammals began to populate the land. These mammals -- with their more active lifestyles, maneuverability and omnivorous diets -- could have competed with the cumbersome dinosaurs. Meng Jin, a paleontologist with the American Museum of Natural History, and colleagues detailed the lifestyles of early mammals belonging to the group Repenomamus. The dog-sized, badger-like Repenomamus giganticus, possibly the largest mammal of that time period, competed with the dinosaurs for food. Its cousin, Repenomamus robustus, preyed on young dinosaurs, as evidenced by the skeletal remains of a juvenile Psittacosaur found in its belly.
Evolution of Angiosperms
With the decline of the gymnosperms, which comprise the pines and palms, plant-eating dinosaurs lost their main sources of food. Gymnosperms dwindled because of a global decline in temperature while angiosperms, or flowering plants, because of their successful adaptations, gradually replaced the gymnosperms. Herbivorous, or plant-eating, dinosaurs, probably could not adapt in such a short time period to the new angiosperm food. Looking through the stomach contents of fossilized dinosaurs, scientists found evidence that the dinosaurs chose to eat gymnosperms, even in the midst of angiosperm abundance.
Supernova
Supernovae occur with some regularity near earth about once every 50 million years. Dinosaur fossils in Kenya show signs of calcium buildup near the joints, possibly caused by the development of cancerous tumors. Raynelle Deans and Jason Lee, researchers at the Carnegie Lab, simulated the radiation effects of a supernova on animals in the laboratory. Animals that suffered the most incidences of cancer when exposed to low levels of radiation for two months coincided with the same types of animals that perished during the K-T extinction event. The radiation emitted by the supernova would have soaked into the atmosphere and rained down on earth and the life on its surface.
Rise of Insects
Insects became vectors, or carriers, of disease; the dinosaurs, unfamiliar with these new diseases, fell ill and succumbed in great numbers. George Poinar, Jr., a researcher with Oregon State University, presented the insect hypothesis as a mechanism of dinosaur extinction. Poinar and his wife examined the bodies of insects that were trapped in amber, a sticky, viscous substance formed from the sap of trees that hardened over years. These entrapped insects, which became preserved in a lifelike fashion, housed disease-causing, or pathogenic, organisms within their bodies, including one responsible for a type of malaria found in lizards and birds. Poinar offered this answer to intimate that the dinosaurs became extinct over an extended period of time.
Tectonic Activity
During the age of the dinosaurs, masses of land began drifting apart. Land previously at the interior of the continents became coasts and cooled rapidly. The spreading of continents triggered volcanic activity all over the world, throwing ash into the atmosphere. Earth grew dark and cold, and plants died because they lacked the sunlight to power the photosynthetic reactions they needed to make food. As plants declined, organisms that fed on them also decreased, along with other animals higher up on the food chain. Scientists cite as evidence lava flows in India; the presence of iridium, an element usually found deep within the planet; and the occurrence of shocked quartz, a type of cracked crystal caused by an explosion or contact with a massive body.
Asteroid Impact
The theory of dinosaur extinction via collision of a massive asteroid with earth has garnered the most support from the scientific community. This theory also tells of a climate change on earth; the impact would have generated acid rain, widespread fires, massive tsunamis and global cooling and subsequent warming because of the greenhouse effect. Discovered in the 1980s, the 150-mile wide Chicxulub Crater near the Yucatan Peninsula, which dates back 65 million years, seems evidence of the likely culprit. Supporting evidence includes the presence of iridium and shocked quartz at the impact site and impact ejecta, melted rock caused by a colossal collision.