A Brief Overview of the Cambrian Period for Middle School Science Classes

Dec 14, 2024

3 min read

Earth history is a fascinating topic! It is also a massive topic! The reading passage below will teach you all about the Cambrian Period. If you want to learn more (and why wouldn't you?!), you can check out my Earth History page. I also have all of my passages available at Teachers Pay Teachers. They come with so many extras to get your students thinking about the content! I also recommend scrolling to the bottom of the page to check out my digital picture book on the Cambrian Period!

Cambrian Period Reading Passage for Middle School Science

The Cambrian period began with the Cambrian explosion. For millions of years, life on Earth was simple creatures floating through the oceans. Suddenly, everything changed. Of course, in geological terms, suddenly means over fifty million years.

Naming the Cambrian Period

Geologist Adam Sedgwick named the Cambrian period. He first discovered fossils from this period in Wales, so he named it after the Roman name for Wales, which was Cambria.

Geology

As the Cambrian period began, the supercontinent Rodinia split into two pieces. The new major continents were Gondwana and Laurentia.

Climate

The ice ages of the Proterozoic Eon ended. The temperatures on Earth were warmer than they are today. During the Cambrian period, the average temperature on Earth was 72 degrees Fahrenheit.

Cambrian Explosion

Scientists aren’t sure what caused the explosion of life during the Cambrian period, but they have some guesses. The most popular theory is that as the Earth warmed, glaciers melted. The melting glaciers released rocks and dirt, adding sediments to the oceans. These sediments fed the oxygen-producing bacteria in the oceans. The ocean waters filled with oxygen. With oxygen, organisms can produce fifteen times more energy than without oxygen. This additional energy can be used to grow, develop, and move.

No matter the cause, the Cambrian explosion was the largest appearance of new species in Earth’s history. Every animal phyla that exists today first appeared during the Cambrian period. One exception is the Bryozoa phylum. Many scientists believe bryozoans did live during the Cambrian period, but fossils haven’t been found yet.

Predators and Prey

Perhaps as a result of the increased oxygen levels, animals first appeared during the Cambrian period. Animals are multicelled organisms that eat food. The organisms from the Proterozoic Eon didn’t eat food; they absorbed nutrients from the water around them. Animals eating food meant predators developed. These were animals that ate other animals. The relationship between predators and prey may have added to the diversity of life during the Cambrian period. Predators had to be great hunters to survive. Prey had to be great hiders or fighters to survive. This relationship is what drives evolution today.

Anomalocaris

The largest predator of the Cambrian period was Anomalocaris. It swam through the water by flexing its muscles the way eels swim today. It was three feet long and had large claws extending from its mouth. It would use these claws to push smaller animals into its mouth. Unlike older species, Anomalocaris had eyes. Before the Cambrian period, organisms had no nervous systems. No nervous system meant they didn't have eyes, and they couldn’t see. Eyes were just one part of the predator/prey arms race of the Cambrian period.

Exoskeletons and Shells

Animals of the Cambrian period had the first exoskeletons and shells. One animal with an exoskeleton was the trilobite. Trilobites were the most abundant animals of the period. Fossils of trilobites exist all over the world, and scientists have discovered over ten thousand different species of trilobites. The largest trilobites were twenty-eight inches long. The smallest trilobites were less than a millimeter long. While species were slightly different, all trilobites had a hard exoskeleton covering their soft bodies. They had a head, a segmented thorax (body), and a tail. Trilobites also had legs, so they are a part of the Arthropoda phylum. Insects and spiders are animals in the Arthropoda phylum today.

End of the Cambrian Period

Around 485 million years ago, Earth’s temperature dropped, and another ice age began. The ice age brought lower sea levels, colder temperatures, and a drop in oxygen levels. During the Cambrian period, all life existed in the oceans. As the ocean changed, many animals went extinct. This extinction marks the end of the Cambrian period and the beginning of the Ordovician period.

Cambrian Period Picture Book

Looking for another way to learn about the Cambrian Period? Check out this picture book version. The pages are a part of my Earth History bundle on Teachers Pay Teachers.