The end of the Cretaceous period 65 million years ago represents one of the greatest natural catastrophes in recent earth history. Nearly 70% of dinosaurs and other animal life, and 60% of plant life, disappeared essentially overnight.
In 1980, father and son scientists Walter and Luis Alvarez proposed the idea that an asteroid impact might be linked to the dinosaur extinction. This assertion was based upon the discovery of a high abundance of iridium in rock layers from this period of earth history. Iridium is rare in earth rocks, and any abundance of the element is likely due to an extraterrestrial source. This controversial idea was later supported by the discovery of a ringed structure in the Gulf of Mexico north of the Yucatan peninsula. At 124 miles in diameter, the crater, named Chicxulub for a nearby town, is of the right size and age to be linked to the dinosaur extinction.
In 2016, scientists sponsored by the International Ocean Discovery Program (IODP) obtained rock samples from the buried crater. This was done by drilling into the sediments and bedrock under the ocean waters. The scientists retrieved samples of overlying limestone sediments, breccia associated with a massive tsunami, and shocked, friable granite bedrock. The scientists had identified a portion of a “peak ring”, an internal ring of upraised bedrock associated with immense impact craters.
Despite being a large impact, its effects would likely have remained local but for some mitigating factors. The high volume of impact debris flung above the earth likely turned the atmosphere into a giant, radiating oven. Fires appeared all over the earth, and this also contributed to the opacity of the atmosphere. Photosynthesis was reduced, and subsequently animals and plant-based life became extinct. Due to striking an area covered by sedimentary rocks, the asteroid impact vaporized rocks containing the sulfur-bearing mineral gypsum. The sulfur likely resulted in a corrosive mix of acid rain, which likely devastated coastal waters and rivers. This acid precipitation in combination with reduced sunlight may have led to the decimation of much ocean life. Along with the dinosaurs, large aquatic reptiles such as mosasaurus became extinct, as well as the coiled cephalopods named ammonites.
A few small mammals were able to survive the extinction, and they quickly filled the niches vacated by the extinct dinosaurs. Within 10 million years of the KT extinction, mammals as large as some dinosaurs came into being. The KT extinction was also a pivotal event in the emergence of Homo sapiens as a species. Without the extinction, dinosaurs would likely have remained the dominant animal life on earth.
Did all dinosaurs become extinct? Yes and no. Rock layers younger than 65 million years display zero dinosaur fossils. Yet similarity between the anatomy of birds and dinosaurs led to the idea that modern birds represent the descendants of past dinosaurs. The discovery of feathered dinosaur fossils has leant further support to this hypothesis.
The PDF contains a two-sided video worksheet consisting of 49 multiple choice and true-or-false questions, along with an answer key, and an MS Word download link. You will need to obtain a DVD of the video, use the PBS Internet site, or a streaming site such as dailymotion