Article 20: The Cambrian is the springtime for life on our planet. It is the time when most of the major groups of animals first appear in the fossil record.
The Cambrian Period of the Earth’s history spans about 57 million years, from 542 to 485 million years ago.
Burgess Shale Fauna: Credit to Carel Brest van Kempen (1989)
The Cambrian marks an important point in the history of life on Earth. Prior to the Cambrian, the majority of living organisms on the whole is small, unicellular and simple, the Precambrian Charnia being exceptional. Complex, multicellular organisms gradually become more common in the millions of years immediately preceding the Cambrian, but it is not until this period that mineralized, hence readily fossilized organisms become common. It is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the Cambrian Explosion, because of the relatively short time over which this diversity of forms appears. A sufficient amount of oxygen (and/or the development of eyesight, says Brian Cox) may have triggered the Cambrian Explosion.
A few localities around the world, that preserve soft-bodied fossils of the Cambrian, show that the Cambrian radiation generated many unusual forms, not easily comparable with anything today.
The best-known of these sites is the legendary Burgess Shale in the British Columbian Rocky Mountains. There are at least 12 species of trilobite in the Burgess Shale. The largest predator was Anomalocaris, a free-swimming animal that undulated through the water by flexing its lobed body. It had true compound eyes and two claw-tipped appendages in front of its mouth. It was the largest, most fearsome predator of the Cambrian Period, but did not survive into the Ordovician.
Living among the variety of ancestral forms that make up the Burgess Shale community was the earliest known representative of the phylum to which we ourselves belong. Averaging about one and one half inches in length, Pikaia swam above the seafloor, using its body and an expanded tail fin. Unfortunately, these features are not properly depicted in this preliminary sketch, the only one available to us.
Pikaia, this earliest known chordate animal, was about 1,5 inches (4 centimeters) long. Pikaia had a nerve cord that was visible as a ridge, starting behind its head and extending almost to the tip of the body.
The fine detail preserved in the Burgess Shale clearly shows that Pikaia had the segmented muscle structure of later chordates and vertebrates.
Pikai gracilens after Mount Pika + gracilens (L.) = slender, thin
Please note: Pikaia is not a vertebrate – no one can say if this particular creature is our direct predecessor. Nevertheless, Pikaia is a representative member of the chordate group from which we undoubtedly arose. It resembles a living chordate, commonly known as the lancelet.
Haikouichthys, thought by some to be the earliest jawless fish, was also found in the Burgess Shale.
More than half a billion years old, the fossils of the Burgess Shale preserve an intriguing glimpse of early life on Earth. They were first discovered in 1909 by Charles D. Walcott, then Secretary of the Smithsonian Institution. This group of fossils takes its name from the Burgess Shale rock formation, named by Walcott after nearby Mount Burgess in the Canadian Rockies. The Smithsonian’s National Museum of Natural History currently houses over 65.000 specimens. The museum also has a permanent exhibit of the Burgess Shale fauna near the Dinosaur Hall.
Since Walcott’s original discovery, fossil deposits like these have been found in such widely dispersed areas as China, Greenland, Siberia, Australia, Europe and the USA.
These fossils merit special interest for several reasons:
▪ They were buried in an underwater avalanche of fine mud, that preserved exceptionally fine details of the structure of their soft parts. Only hard parts are preserved in most other Cambrian deposits, obviously limiting information within the geologic record.
▪ They represent an early snapshot of the complexity of evolving life systems. The Burgess Shale fossils as a group have already developed into a variety of sizes and shapes from the much simpler, pre-Cambrian life forms.
▪ Many of them appear to be early ancestors of higher forms, from algae to the chordates, a major group of animals that includes human primates. Others appear unrelated to any living forms and their later disappearance presents an intriguing mystery.
Hallucigenia; illustration by Mary Parrish
The Cambrian follows the Ediacaran Period, during which time the continents have joined in a single supercontinent, called Rodinia (from the Russian word for homeland, Rodina).
As the Cambrian begins, Rodinia (later Pannotia) starts to fragment into smaller continents, which do not always correspond to the ones we see today. By the early to mid-Cambrian there are two continents. Gondwana, near the South Pole, is a supercontinent that later forms much of the land area of modern Africa, Australia, South America, Antarctica and parts of Asia. Laurentia, nearer the equator, is composed of landmasses that currently make up much of North America and part of Europe.
The Cambrian world is bracketed between two ice ages, one during the late Proterozoic, called Iceball Earth, and the other during the Ordovician, the next geological period. During these ice ages the decrease in global temperature leads to mass extinctions. Cooler conditions eliminate many warm water species and glaciation lowers the global sea level. However, during the Cambrian there is no significant ice formation. None of the continents are located at the poles, so land temperatures remain mild. In fact, the global climate is probably warmer and more uniform than it is today.
With the retreat of the Proterozoic ice, the sea level rises significantly. Lowland areas such as Baltica are flooded and much of the world is covered by seas. This event opens up new habitats, where marine invertebrates such as trilobites radiate and flourish. Most animal life during the Cambrian is aquatic, with trilobites assumed to be the dominant life form, which has since proven to be incorrect.
Trilobite: Credit Bill Frische
Arthropods in general are by far the most dominating animals in the ocean, but at that time trilobites were only a minor part of the total arthropod diversity. What makes them different from their relatives is their heavy armor, which fossilized far more easily than the fragile exoskeleton of other arthropods, leaving behind numerous preserved remains, which gave the false impression that they were the most abundant part of the fauna.
By nearly any measure, the most successful animals on the planet are the arthropods. They have conquered land, sea and air, and make up over three-fourths of all currently known living and fossil organisms, or over one million species in all. Since many arthropod species remain undocumented or undiscovered, especially in tropical rain forests, the true number of living arthropod species is probably in the tens of millions. Once recent conservative estimate puts the number of arthropod species in tropical forests at 6 to 9 million species. They are the real rulers of the Earth.
Arthropods range in distribution from the deep sea to mountain peaks, in size from the king crab, with its 12-foot arm-span, to microscopic insects and crustaceans, and in taste from chocolate covered ants to crawfish jambalaya and lobster Newburg. Despite this unbelievable diversity, the basic body plan of arthropods is fairly constant. Arthropods have a stiff cuticle made largely of chitin and proteins, forming an exoskeleton that may or may not be further stiffened with calcium carbonate. They have segmented bodies and show various patterns of segment fusion to form integrated units (heads, abdomens and so on). The phylum takes its name from its distinctive jointed appendages, which may be modified in a number of ways to form antennae, mouthparts and reproductive organs.
Plants have not yet evolved and the terrestrial world is devoid of vegetation and inhospitable to life as we know it. Photosynthesis and primary production are the monopoly of bacteria and algal protists that populate the world’s shallow seas.
Thanks to the University of California, Berkeley, Museum of Paleontology; the Smithsonian National Museum of Natural History; Wikipedia-Cambrian; and to LiveScience. Special thanks to Stephen Jay Gould for his superb book ‘Wonderful Life’