Cambrian life
From Palaeos
| Cambrian period 542-488 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Early Cambrian 542-521 | Middle Cambrian 521-501 | Furongian 501-488 | |||||||
| Series 2 521-510 | Series 3 510-501 | ||||||||
| Fortunian 542-535 | Stage 2 535-521 | Stage 3 521-517 | Stage 4 517-510 | Stage 5 510-506 | Drumian 506-503 | Guzhangian 503-501 | Paibian 501-496 | Stage 9 496-492 | Stage 10 492-488 |
| Cambrian topics: Stratigraphy | Geography | Climate | Life | Important Fossil Sites | References | Links | |||||||||
At the beginning of the Cambrian period, about 540 million years ago, life was entirely confined to the oceans. During the 53 million years that the Cambrian period lasted there was the sudden appearance and diversification of almost every major group (phylum) of animal life, as well as many types that later died out. Animals with shells and exoskeletons appeared for the first time, including trilobites, brachiopods, molluscs, and many other groups. This sudden evolutionary burst was so spectacular that it has been termed the "Cambrian explosion". There hasn't been anything like it on Earth before or since.
The most characteristic animals of the Cambrian period were the trilobites, a primitive form of arthropod remarkable for it's highly developed eyes (unusual in such an early organism).
The trilobites appear suddenly during the Atdabanian epoch (several groups are known, including large spiny types and small planktonic forms), reached their fullest development in the middle Cambrian and the following (Ordovician) period, and gradually declined after that and became extinct by the end of the Paleozoic era.
Other very important groups of Cambrian animals were the sponges, echinoderms (represented by a large number of different classes), and most interesting of all the soft-bodied priapulids, which were burrowing worm-like creatures, which seemed to have been the most important and predominant carnivores of the time.
The earliest gastropods (marine snails) also appeared in this period, as did the cephalopods (during the late Cambrian) and other now extinct lineages of Molluscs. Molluscs however were still relatively rare, they did not become an important element of the marine fauna until the Early Ordovician period.
The first chordates (vertebrate ancestors) occurred as did the first foraminifers (shelled amoebas).
Many Cambrian creatures however did not fit into modern categories of organisms; they seemed to have been representatives of unknown or experimental phyla.
Greatest of all the Cambrian beasties were as the Anomalocarids. Averaging 45 to 60 cm, with exceptional specimens reaching 1 or even 2 metres, these animals dwarfed even the largest trilobites. They were armed with twin grasping organs and a wicked mouth with a ring of teeth in shape rather like a pineapple slice. Many trilobite exoskeletons have been found with large bites taken out of them, the result of an encounter with Anomalocaris. Like the shark of today, Anomalocaris . was perfectly adapted to its environment, and a single species existed without change for some 30 million years or more. Scientists do not agree what group of modern creatures Anomalocaris was most closely related too. Suggestions most often include arthropods, but the aschelminthe group of worm-like creatures is another possibility. I am wary of pigeonholing extinct organisms with living types on the basis of superficial similarity. Like many Cambrian creatures, Anomalocaris is best put in a phylum of its own.
Halkeria is one of a number of the strange armoured "coat of mail" creatures that inhabited the early Cambrian oceans. In addition to the calcareous scales it possessed two mollusc-like limpet-shaped shells. Later forms such as the Middle Cambrian Wiwaxia lost the shells and developed long spines instead. It is not certain that these strange creatures even are molluscs, although a radula-like structure has been found in Wiwaxia. Jan Bergström suggests that they represent a group of late surviving "Procoelomates", the ancestors of all higher (Coelomate body plan) animals.
The enigmatic sponge-like cup-shaped Archaeocyathids were very common during the early Cambrian, forming great reefs, but died out almost completely by the middle of the period, leaving no descendents.
Plants of the Cambrian period included only algae (seaweeds). There were no known land plants, the land was still bare of any life other than microorganisms.
