Disappearing Act

There have been at least six well-documented mass extinctions on Earth over the past 500 million years, a phenomenon that has both puzzled and intrigued scientists for centuries. Various types of organisms on Earth become extinct fairly frequently, but mass extinctions are distinguished by the large numbers of species that become extinct over a relatively short period of time. The Cretaceous-Tertiary extinction, responsible for the demise of the dinosaurs some 65 million years ago, is perhaps the most famous mass extinction. It’s forever branded into our brains from being the topic of feature films, artists’ paintings, and numerous books.

Yet mass extinctions are not the dramatic fare served up by Hollywood renderings. Many students tend to think of mass extinctions in technicolor, special effects-laden terms, with enormous quantities of dinosaurs dropping in their tracks in one fell, dramatic swoop. In reality, mass extinctions tend to happen much more slowly. The scientific rule of thumb is that an event qualifies as a mass extinction if 20-50% of many diverse organisms on Earth become extinct over a period of not more than one million years. The majority of scientists today believe that we are currently in the midst of another mass extinction, caused by humans and our inhabiting virtually every part of the globe. Named the “Sixth Extinction” or the “Holocene extinction,” this event is characterized by decreasing biodiversity as a result of human activity. Many scientists believe that, collectively, the human species has the ability to halt the current extinctions; this viewpoint could make for interesting classroom discussion and debate.

There are numerous theories about what has caused mass extinctions in the past. The Ordovician extinction that occurred 444 million years ago is thought to have been triggered by gamma rays, while the Cretaceous-Tertiary extinction is believed to have been prompted by a large asteroid crashing into the Earth near Chicxulub, Mexico, and sending up vast quantities of dust and debris into the atmosphere. Recently, some scientists have posited that a dark satellite orbits the Sun once every 27 million years, each time smacking a flood of comets out of a celestial cloud at the edges of our solar system and sending them crashing to Earth. Other scientists chalk mass extinctions up to natural evolution, or Nature’s version of hitting a reset button. Students must realize that at this point, all of the theories surrounding mass extinctions are just that – theories. For the time being, no one really knows what ultimately caused mass extinctions in the past. While some theories are more plausible and have more adherents than others, the ability to discuss the various theories is a valuable exercise in critical thinking skills at all grade levels.

My picks this week all focus on the concept of mass extinctions, with age appropriate resources for various grade levels. I’ll also be featuring many more lessons, articles, and other resources on mass extinctions throughout the week on our Facebook and Twitter pages, so please be sure to take a look.


Dinosaurs 1: Where Are the Dinosaurs?
Subjects: Paleontology, Natural History
Grade: K-2
The extinction of a species can be a difficult concept for younger kids to grasp. In this lesson, students explore the concept of extinction by studying dinosaurs. This lesson was created by Science NetLinks, which offers standards-based lesson plans and resources that are reviewed by scientists and educators.

Big Burp: A Bad Day in the Paleocene
Subjects: Ecology, Biology
Grade: 5-6
The focus of this lesson is global warming and the Paleocene extinction. In this activity, students will be able to describe the overall events that occurred during the Paleocene extinction event, describe the processes that are believed to result in global warming, and infer how a global warming event could have contributed to the Paleocene extinction event. This lesson was prepared by the National Oceanic and Atmospheric Administration’s (NOAA) Ocean Explorer program, which provides public access to current information on NOAA scientific and educational explorations and activities. While most resources here focus on the marine environment, there are also resources related to other scientific areas as well.

Extinction in the Classroom
Subjects: Natural History, Paleontology
Grade: 10-12
Using images of evidence from the fossil record, students are asked in this lesson to consider whether dinosaur biodiversity was stable, growing or diminishing at or near the end of the Cretaceous Period, and to identify those species that have successfully survived this 65 million-year-old mass extinction event. Students will also evaluate different theories explaining this last great mass extinction event, and have a chance to share and debate their insights with their peers. A French version of this resource is also available here. This resource was prepared by the Canadian Museum of Nature, which is Canada's national natural history museum located in Ottawa, Ontario. In addition to lesson plans, the Museum also offers educational workshops, activities, and games.

~Joann's Picks - 12/3/2010~