LECTURE 4: THE CAMBRIAN EXPLOSION
The Cambrian is a geologic period and system that began about 542± 0.3 Ma (million years ago) at the end of the Proterozoic eon and ended about 488.3± 1.7 Ma with the beginning of the Ordovician period (ICS, 2004)[5]. It was the first period of the Paleozoic era of the Phanerozoic eon. The Cambrian takes its name from Cambria, the classical name for Wales, the area where rocks from this time period were first studied.The Cambrian is the earliest period in whose rocks are found numerous large, distinctly fossilizable multicellular organisms. This sudden appearance of hard body fossils is referred to as the Cambrian explosion. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks it was not until 1994 that this time period was internationally ratified. The base of the Cambrian is defined on a complex assemblage of trace fossils known as the Trichophycus pedum assemblage. This assemblage is distinct from anything in the Precambrian as it has ecologically tiered vertical burrows which are absent from the Precambrian.
Cambrian continents are thought to have resulted from the breakup of a Neoproterozoic supercontinent called Pannotia. The waters of the Cambrian period appear to have been widespread and shallow. Gondwana remained the largest supercontinent after the breakup of Pannotia. It is thought that Cambrian climates were significantly warmer than those of preceding times that experienced extensive ice ages discussed as the Varanger glaciation. Also there was no glaciation at the poles. Continental drift rates in the Cambrian may have been anomalously high. Laurentia, Baltica and Siberia remained independent continents since the break-up of the supercontinent of Pannotia. Gondwana started to drift towards the South Pole. Panthalassa covered most of the southern hemisphere, and minor oceans included the Proto-Tethys Ocean, Iapetus Ocean, and Khanty Ocean, all of which expanded by this timeOf those modern animal phyla that fossilize easily, all save the bryozoans have claimed representatives in the Cambrian. Many extinct phyla and odd animals that have unclear relationships to other animals also appear in the Cambrian. The apparent "sudden" appearance of very diverse faunas over a period of no more than a few tens of millions of years is referred to as the "Cambrian Explosion". Also, the first possible tracks on land, such as Protichnites and Climactichnites, dating to about 530 mya and found in Ontario, Canada, and northern United States, appeared at this time. The conodonts, small predatory primitive chordates known from their fossilised teeth, also appeared during the Furongian epoch of the Cambrian period. The conodonts thrived throughout the Paleozoic and the early Mesozoic until they completely disappeared during the Late Triassic period when the first mammals were evolving.
The best studied sites where the soft parts of organisms have fossilized are in the Burgess shale of British Columbia. They represent strata from the Middle Cambrian and provide us with a wealth of information on early animal diversity. Similar faunas have subsequently been found in a number of other places — most importantly in very early Cambrian shales in the People's Republic of China's Yunnan Province (see Maotianshan shales). Fairly extensive Precambrian Ediacaran faunas have been identified in the past 50 years, but their relationships to Cambrian forms are quite obscure.
Generally it is accepted that there were no land plants at this time although molecular dating suggests that land plant ancestors diverged from the algae much earlier, in the Neoproterozoic about 700 ma[citation needed], and that fungi diverged from the animals about 1 billion years ago[citation needed]. The land at this time was barren, mostly desert and badlands.
The Cambrian-Ordovician extinction event occurred approximately 488 million years ago. It was the first major extinction event in the Phanerozoic eon and it eliminated many brachiopods, conodonts, and severely reduced the number of trilobite species. The Cambrian-Ordovician event ended the Cambrian period, and led into the Ordovician period in the Paleozoic era. The Ordovician-Silurian extinction event rendered 49% of genera of fauna on earth extinct.
READING FOR THE NEXT LECTURE
