Earth's Cambrian Period

The Oceanic Evolutionary Explosion

Agaric, Yahoo! Contributor Network
Mar 7, 2007 "Share your voice on Yahoo! websites. Start Here."

The Cambrian Period is one of the most important in Earth's history. It marks the point at which major groups of animals begin to appear on the fossil record. This was a time of great evolutionary progress in which there was an explosion of diversity throughout the life on planet Earth. The Cambrian Period lasted between 542 million and 488 million years ago.

It is believed that due to continental breakup during the Cambrian Period, the Earth was covered in a number of shallow seas. During the Proterozoic period there was one supercontinent on Earth known as Pannotia. The breakup of this gigantic land mass allowed shallow seas and waterways to form over and between landmasses. Cambrian climates were significantly warmer than at other points in Earth's history and the poles were not frozen. This lack of polar glaciation can help to account for the preponderance of the shallow Cambrian seas across the planet. At this point, no land plants existed and in general the Cambrian continents were barren places of bare rock and sand.

Although the Cambrian Period saw an evolutionary explosion, that drama was being played out predominantly in the oceans. Despite the fact that the continents were uncolonized, all major phyla made their first firm appearance during this Eon. Oxygen that was being pumped out into the atmosphere since the Proterozoic by marine algae had begun to dissolve into the oceans. Anaerobic organisms generally have a greater chance at diversifying with their efficient use of oxygen. Anaerobic organisms tend to be much smaller, softer, and less diverse that aerobic organisms due to the relative inefficiency of the fermentation processes they use in order to produce energy.

Life on Earth during the Cambrian Period generally was restricted to invertebrates, or animals that lack a backbone. Echinoderms, which include the modern starfish, sea cucumbers, and sand dollars began to appear in the Cambrian seas. However, the modern examples given had not yet evolved and many Cambrian Echinoderms did not become prolific until later periods in Earth's history. Cephalopods which include modern-day squid and octopi also began to colonize the seas, as did gastropods (snails and slugs). Although mollusks made their first appearance during the Cambrian Period, they were still relatively rare compared to other marine organisms.

Arthropods, the family that includes present-day insects and spiders began to make a foothold in the Cambrian seas. These organisms are known for their hard outer shells called exoskeletons that provide natural armor but also restrict growth. One of the most famous Cambrian animals, the trilobite, made its appearance in the shallow seas covering Earth. This peculiar arthropod resembles a modern-day pill bug with its segmented back and oval shape and accounted for nearly 50% of all hard-bodied marine species. However, the trilobites were dwarfed in size by arthropod predators known as anomalocarids. These predators could grow up to a meter in length and boasted a pair of grasping organs at their front in order to bring prey into their teeth-lined mouths.

The first vertebrates appeared in the Cambrian, although they were tiny, jawless creatures that do not appear very often in the fossil record. A tiny fishlike creature called haikouichthys would form the evolutionary basis for bony and cartilaginous fish like sharks later on down the road. It wouldn't be until the Ordovician Period that vertebrates began their long rise to the top of the evolutionary ladder.

Published by Agaric

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www.ucmp.berkeley.edu/cambrian/camb.html

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Earth's Cambrian Period

The Oceanic Evolutionary Explosion

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