Cambrian Explosion

Cross references:   In the Beginning   The Origin of Life  
Timeline of Evolution
     Neoproterozoic      Ediacaran Period  

Cambrian (Wiki) 
542 to 488 million years ago. 
The Cambrian period marked a profound change in life on Earth. Before the Cambrian, life was on the whole small and simple. Complex organisms became gradually more common in the millions of years immediately preceding the Cambrian, but it wasn't until this period that mineralised - hence readily fossilised - organisms became common.[8] This diversification of lifeforms was relatively rapid, and is termed the Cambrian explosion. This explosion produced the first representatives of most modern phyla

The Cambrian marked a steep change in the diversity and composition of Earth's biosphere. The incumbent Ediacaran biota suffered a mass extinction at the base of the period, which corresponds to an increase in the abundance and complexity of burrowing behaviour. This behaviour had a profound and irreversible effect on the substrate"   

Cambrian Substrate Revolution (Wiki)   

Before and after the Cambrian substrate revolution

"The "Cambrian substrate revolution"[1] or "Agronomic revolution"[2], evidenced in trace fossils, is the diversification of animal burrowing during the early Cambrian period." 

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Before this "widening of the behavioural repertoire",[3] bottom-dwelling animals mainly grazed on the microbial mats that lined the surface, crawling above or burrowing just below them. These microbial mats created a barrier between the water and the sediment underneath, which was less water-logged than modern sea-floors, and almost completely anoxic (lacking in oxygen). As a result the substrate was inhabited by sulfate-reducing bacteria, whose emissions of hydrogen sulfide (H2S) made the substrate toxic to most other organisms.[4]

Around the start of the Cambrian, organisms began to burrow vertically, forming a great diversity of different fossilisable burrow forms as they penetrated the sediment for protection or to feed.  These burrowing animals broke down the microbial mats, and thus allowed water and oxygen to penetrate a considerable distance below the surface. This restricted the sulfate-reducing bacteria and their hydrogen sulfide emissions to the deeper layers, making the upper layers of the sea-floor habitable for a much wider range of organisms. The upper level of the sea-floor became wetter and softer as it was constantly churned up by burrowers

Ironically, the first burrowers probably fed on the microbial mats, while burrowing underneath them for protection; this burrowing led to the downfall of the mats they were feeding on.

The rise of burrowing represents such a fundamental change to the ecosystem, that the appearance of the complex burrow Trichophycus pedum is used to mark the base of the Cambrian period.

Cambrian Explosion (Wiki)
"Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years the rate of evolution accelerated by an order of magnitude (as defined in terms of the extinction and origination rate of species[4]) and the diversity of life began to resemble today’s."  "Rocks dating from 565 to 543 million years ago contain fossils of the Ediacara biota, organisms so large that they must have been multi-celled, but very unlike any modern organism.

The earliest Cambrian trilobite fossils are about 530 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.
Trilobites first appear in the fossil record during the Early Cambrian period (540 million years ago) and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders, with the sole exception of Proetida, died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago."

File:SamGonIII 3lobes.png

"When trilobites first appear in the fossil record they were already highly diverse and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left, with some 17,000 known species spanning Paleozoic time."  "Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in trilobites, except for parasitism. Some trilobites (particularly the family Olenida) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.

A Guide to the Orders of Trilobites (Wiki)
Good reference with many pictures. 

Darwin's dilemma: the realities of the Cambrian ‘explosion’
Full length HTML and PDF available online for free. 

Solution to Darwin's dilemma: Discovery of the missing Precambrian record of life
Full length HTML and PDF available online for free.   

The evolution and distribution of life in the Precambrian eon-global perspective and the Indian record.
Only abstract available online. 

The Cambrian "explosion" of metazoans and molecular biology: would Darwin be satisfied?
Full length PDF available online for free. 

What sparked the Cambrian explosion? : Nature

Hyperactive magnetic field may have led to one of Earth’s major extinctions   
Rapid reversals of Earth’s magnetic field 550 million years ago destroyed a large part of the ozone layer and let in a flood of ultraviolet radiation, devastating the unusual creatures of the so-called Ediacaran Period and triggering an evolutionary flight from light that led to the Cambrian explosion of animal groups. That’s the conclusion of a new study, which proposes a connection between hyperactive field reversals and this crucial moment in the evolution of life.

The Kotlinian Crisis, as it is known, saw widespread extinction and put an end to the Ediacaran Period. During this time, large (up to meter-sized) soft-bodied organisms, often shaped like discs or fronds, had lived on or in shallow horizontal burrows beneath thick mats of bacteria which, unlike today, coated the sea floor. The slimy mats acted as a barrier between the water above and the sediments below, preventing oxygen from reaching under the sea floor and making it largely uninhabitable.

The Ediacaran gave way to the Cambrian explosion, 542 million years ago: the rapid emergence of new species with complex body plans, hard parts for defense, and sophisticated eyes. Burrowing also became more common and varied, which broke down the once-widespread bacterial mats, allowing oxygen into the sea floor to form a newly hospitable space for living.

Scientists have long argued over what caused the Cambrian explosion in the first place. Potential explanations have included rising levels of atmospheric oxygen because of photosynthesis, allowing for the development of more complex animals; the rise in carnivorous species and new predatory tactics, such as the flat and segmented, armor-crushing creatures known as anomalocaridids; and the breakup of the supercontinent Rodinia, which may have created new ecological niches and isolated populations as the continents drifted apart.

In their new study, however, geologist Joseph Meert of the University of Florida in Gainesville and his colleagues propose a different hypothesis: that these evolutionary changes might have been connected to rapid reversals in the direction of Earth’s magnetic field. During a reversal, magnetic north and south trade places—an event which, in geologically recent times, occurs about once every million years.

Yet in the Ediacaran, such reversals were a lot more common, the team proposes. Certain minerals in rocks can preserve a record of the direction of Earth’s magnetic field when the rock formed. While studying these magnetic records in 550-million-year-old, Ediacaran-aged sedimentary rocks in the Ural Mountains in western Russia, the team discovered evidence to suggest the reversal rate then was 20 times faster than it is today. “Earth’s magnetic field underwent a period of hyperactive reversals,” Meert says.

Previous research has suggested that Earth’s protective magnetic field would be weaker across such periods of frequent reversal, compromising its ability to shield life from harmful solar radiation and cosmic rays. On top of this, the duration of each individual reversal episode—thought to take an average of 7000–10,000 years—would likely see the field temporarily weakened even more before growing back in the opposite direction.

This weakened shielding would have allowed more energetic particles into the upper atmosphere, which would have begun to break down the ozone layer that protects Earth from harmful UV radiation, Meert says. Twenty to 40% of ozone coverage might have been lost—in turn, doubling the amount of UV radiation that reached Earth’s surface, the team reports in a paper in press in Gondwana Research. “Organisms with the ability to escape UV radiation would be favored in such an environment.”

This flight from dangerous levels of UV light, therefore, might explain many of the evolutionary changes that occurred during the Late Ediacaran and Early Cambrian, Meert says. Creatures with complex eyes to sense the light and the ability to seek shelter from the radiation—for example, by migrating into deeper waters during the daytime—would have been more successful. The growth of hard coatings and shells would afford additional UV protection, as would the capacity to burrow deeper into the sea floor.

In turn, these changes may have opened up new environments. The development of shells, for example, helps creatures colonize intertidal areas, protected not only from UV rays but also stronger waves and the risk of drying out. Similarly, the breakdown of the bacterial mats by early burrowing would have opened up the upper sea floor further for life.

Looking forward, the researchers are now hoping to examine other Ediacaran sediments from around the globe to verify the rapid reversals’ signal, along with hunting for biological or chemical evidence for high doses of UV radiation in the fossil record.

There are many factors that may explain why the Cambrian explosion occurred, but the researchers’ “escape from light” idea adds a novel possibility to the debate, says David Harper, a paleontologist at Durham University in the United Kingdom who was not involved in the study. “The authors have opened up yet another exciting and imaginative area of research within which to frame and test new hypotheses for the origin and early evolution of animal-based communities.”

Geobiologist Joseph Kirschvink of the California Institute of Technology in Pasadena, is skeptical, however. Although the idea that UV radiation increases without Earth’s magnetic field is long-established, its effect on the evolution of life at this time should be limited, he says, as the radiation would not be able to reach and damage the germ line, the cells of the body used in sexual reproduction to pass genetic information to offspring. The radiation “would affect the outer skin … but the germ cells are usually internal and protected.” As such, he argues, the idea that increased levels of UV radiation significantly affected the evolution of life in the Ediacaran is problematic.