Cambrian

From Palaeos.org

two Cambrian lobopods: To the left is the lobopod Luolishania longicruris, from Chengjiang, and the right is the lobopod Aysheaia pendunculata, from the Burgess Shale. © Stanton Fink

[edit] Introduction

The first eon of Earth’s history, from the first coalesence of the planet, about 4500 Mya, to about 542 Mya, is referred to as the Precambrian. From this hint, one might well suppose that the Cambrian comes next -- which it does, in a way. Actually, this is the biggest break point in all of geology. It marks the beginning of the Phanerozoic Eon, the Paleozoic Era, the Cambrian Period, the Terreneuvian Epoch, and the Fortunian Age (alternatively and previously known as the Manykaian Age or Nemakit-Daldynian Age). The Cambrian Period was named in 1835 by the geologist Adam Sedgwick, after the region of Cambria in North Wales, where rocks of this age were first found. The name "Cambria" is a version of Cumbria, a latinisation the Welsh Cymry (= countryman, compatriot against the invading Anglo-Saxons).

Long before it was a formal stratigraphic unit, the Cambrian was a concept about Earth history. It was understood to be the earliest period in which one could find the fossils of multi-celled animals (Metazoa). Since then, metazoans and their fossilized traces have been found well before 542 Mya. In particular, ediacarans, a group of very strange and poorly understood creatures - but obviously metazoans - have been found in many parts of the world with ages pushing the 600 Mya mark.

Consequently, paleontologists now view the Cambrian as the period when the Bilateria first appeared and, almost at the same time, the first metazoans with shells. The Bilateria include all living metazoans except cnidarians and sponges. Bilaterians have a head at one end of an elongate body which is bilaterally symmetrical (hence the name). Their embryos all develop a separate layer of embryonic cells, called mesoderm, between the gut or coelom and the outer wall of the animal. If this whole description suggests a worm, you’ve got the right idea. A flatworm is the most basal living form of bilaterian.

Unlike many other, somewhat arbitrary, geological markers, the base of the Cambrian Period is defined with reference to the underlying paleontological concept. Small worms rarely leave body fossils, but their burrows are frequently preserved. The burrows of bilaterian worms are fairly distinctive. Trace fossils are often given names as if they were organisms, and the earliest well-known bilaterian trace fossil is a type of fossilized burrow referred to as Treptichnus pedum. The base of the Cambrian is currently defined as the first occurrence of T. pedum at Fortune Head, near the town of Fortune, on the north coast of western Newfoundland, Canada.

[edit] Stratigraphy

Main page: Cambrian stratigraphy

The International Commission on Stratigraphy has not yet finalized an international stratigraphical scheme for the Cambrian. In the meantime we use a mixture of the bits they have decided on and various informal names, mostly from the Siberian platform:

[edit] Geography

The Cambrian saw most continents located in the southern hemisphere at low paleolatitudes (near the equator). The Ediacaran supercontinent of Pannotia continued to assemble in some regions but fragmented into Gondwana, Laurentia, Baltica and various mostly submerged Asian blocks.

Laurentia stretched across the Cambrian equator, partly submerged by the Iapetus ocean, with a mostly submerged Baltica and Siberia approaching from the south-east.

Gondwana remained the largest supercontinent. Other continents included Kazakhstania and China (actually China, Thailand, Malaysia, and Indochina). Seas were for the most part shallow, especially along the edges of the continents.

Global (eustatic) transgressions occured in the Middle and Late Cambrian, as shallow seas repeatedly invaded the land, providing a perfect habitat for many types of marine invertebrates. These shallow epeiric seas covered much of the continents except for Gondwana, where there were highlands. Other highlands could be found in Eastern Siberia and Central Kazakhstan.

[edit] Climate

The Cambrian climate was generally warm, wet and mild. As there were no continental landmasses located at the poles, ocean currents were able to circulate freely, hence there was no significant ice formation. As a result temperatures worldwide were mild. The Cambrian constituted a benign spell between two great ice ages - the late Proterozoic Snowball Earth and the Late Ordovician Ice Age.

[edit] Life

Main page: Cambrian life

At the beginning of the Cambrian, 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 arthropods remarkable for their 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 seem 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 amoebae).

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 aschelminth group of worm-like creatures is another possibility. One should be 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.

Halkieria is one of a number of the strange armoured "coat of mail" animals 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.

Many Cambrian creatures however did not fit into modern categories of organisms; they seemed to have been representatives of unknown phyla or phyla which weren't successful in the long run.

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 descendants.

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.

The first possible tracks on land, presumably made by arthropods date back to the Cambrian period (c 530 MYA).[1]

[edit] Important Fossil Sites

Main page: Cambrian sites

Cambrian life is known from several spectacular Lagerstätten, most famously the Burgess Shale in British Columbia, Canada, but also including Chengjiang in China and Sirius Passet in Greenland.

[edit] Links

• The Cambrian Period, at the University of California Museum of Paleontology. best on the web

• International Subcommision on Cambrian Stratigraphy The Cambrian World - Stratigraphy - Localities - Life in the Cambrian

• The Cambrian Period - good essay, from from Peripatus

• The Cambrian Period - Dr Pamela Gore - some useful study notes, complete with links and a few photos of several types of trilobite. An excellent introduction, if you don't mind the Geology 102 format. Includes material on the famous Burgess Shale site.

• American Scientist: January-February 1997 - Origin of Animal Body Plans - explains how multicelluar animals suddenly appeared during the Cambrian, after billions of years of nothing but micro-organisms

• MSN Encarta - Cambrian Period - Short entry, summarizes the life forms and geologic activity that mark this time period.

Palaeos com - Cambrian

[edit] Credits

page uploaded on Kheper Site on 28 May 1998, page uploaded on Palaeos Site 11 April 2002, last modified 26 October 2002
checked ATW040707
wikified various people 2007-2009
unless otherwise indicated, content © M.Alan Kazlev 1998-2002