Chapter 3 - 6

06 Biology

Diversity in the Cambrian Seas

The seas during the Cambrian period of the Earth’s history were teeming with new forms of life.

Scientists once thought these early multi-celled creatures could not possibly be as varied as today’s ocean inhabitants.

However, this theory has been disputed by the discovery of two major fossil sites.

These sites provide evidence that organisms during the Cambrian Period were actually much more diverse than sea life today.

Around 545 million years ago, a large number of complex multi-celled organisms appeared over a five-to-ten-million-year period.

This phenomenon occurring in such an evolutionarily brief amount of time is called the Cambrian Explosion.

This sudden burst of life is responsible for most of the major animal groups that are recognized today.

The majority of fossils found from the Cambrian period have been classified as arthropods.

They are related to today’s insects, spiders, and crustaceans.

During the Cambrian Period, the first shelled animals with exoskeletons appeared, called trilobites.

Natural selection is thought to have promoted larger sizes in organisms, and there was a need for hard skeletons to provide structural support.

The exoskeleton helped preserve organisms in fossil form.

By studying examples found at two major fossil sites, scientists now know much more about these Cambrian species than before.

The Chengjiang Biota in China preserved many diverse organisms from the Early Cambrian Period in a section of mudstone fifty meters thick known as the Maotionshan Shale.

The fossils are embedded one to two centimeters deep in the stone, providing the oldest Cambrian examples of diverse and welk-preserved soft-bodied organisms.

Though their presence has been known since 1900, it was the discovery of a trilobite fossil in 1984 that revealed the information Chengjiang could offer.

The unique preservation of the soft body parts and internal soft tissue of organisms, as opposed to just the hard skeletal elements usually found in fossils, presented a wealth of knowledge about the organisms in the Cambrian period.

More than 120 species have been classified at Chengjiang Biota, and more than ten animal phyla recognized.

However, there were also a number of fossils that could not be categorized.

These mysterious fossils exhibited features that have not been seen in modem organisms.

It is believed these organisms may have failed at evolution and died out as quickly as they appeared.

No doubt the Chengjiang Biota demonstrates a wide diversity of Cambrian life.

However, the most significant discovery that suggests animals in the Cambrian Period were more diverse than modern animals is the Burgess Shale in the Canadian Rockies of British Columbia.

It was first discovered in 1909 by Charles Doolittle Walcott, secretary of the Smithsonian Institute in Washington, DC.

He spent eight years collecting more than 65,000 specimens.

He took them back to the Smithsonian, where they stayed undisturbed until 1971.

That year, a professor and two graduate students re-examined the fossils using modern technology to project large images of them on the wall.

This magnified details that could not be seen earlier.

The fossils from the Burgess Shale indicated that the multi-celled organisms inhabiting the Cambrian Period had a much broader range of body styles than previously thought.

Like the Chengjiang Biota, many of the fossils at the Burgess Shale contain preserved soft parts of organisms, and there is an extensive amount of overlap in the large number of species found at the two sites.

Two examples of the diverse organisms found during the Cambrian period that have no modern counterparts in today’s seas are the Marella and the Anomalocaris.

The Marella is the most common arthropod from the Cambrian Period.

It had two pairs of curving spines that may have functioned as armor, two pairs of antennae, and twenty-six body segments.

Each body segment had a pair of branched appendages to move it across the sea floor.

The greatest enemy of the Marella was the Anomalocaris, a tubular organism with flexible lobes on the sides of its body to move it through the water.

It measured from sixty centimeters to two meters in length.

It snatched its prey using two spiked feeding arms, and then ate them with its circular mouth.

Many fossils from the Cambrian period show several organisms like Marella and Anomalocaris that do not fit into any of the modern biological classification systems and have no recognizable descendants in today’s oceans.

The study of fossils from the Chengjiang Biota and the Burgess Shale give insight to the kinds of creatures that inhabited the seas during the Cambrian Period.

Though it is still uncertain how these multi-celled organisms developed so quickly, it is certain that they were more diverse in form than animals today.