and its overall appearance was that of an armoured mouse. The Burgess Shale also contains the fossilised remains of a Wiwaxia , although a different species to the Mount Cap type. This illustrates the importance of the Mount Cap fossils. Although they donât represent a diverse community, they contain very close relatives of the Burgess Shale fossils but are some ten million years older. This type of evidence can be used to set the date of the Cambrian explosion more precisely. Wiwaxia is also known from a similar period in the Spence Shale of Utah, USA. In fact the Mount Cap and Burgess Shale fossils now appear to belong to a broadly continuous belt of comparable early and middle Cambrian fossil assemblages extending from southern California through to northern Greenland and Pennsylvania.
Another extensive collection of Cambrian fossils is known from Chengjiang in Yunnan province, south China. The Chinese palaeontologist Hou Xianguang found the first Chengjiang specimen, an unusual trilobite, as a student in 1984. Hou, and his senior at that time, Chen Junguan, dedicated their subsequent years to the Chengjiang site. Extremely well-preserved specimens from several animal phyla were unearthed, and efforts to find further phyla continue at considerable pace today. A study of the ecology of the Chengjiang fauna is receiving the kind of attention previously reserved only for the Burgess Shale fossils. The Chengjiang palaeontologistsâ ammunition has been the age of these fossils - 525 million years old - in addition to the wide diversity of animals represented. So the Chengjiang fauna predate the Burgess fauna by ten million years and reveal that a community structure similar to that we know of 515 million years ago was already in place 525 million years ago.
Cambrian fossil assemblages that can match the Burgess Shale in terms of preservation have been, and no doubt will continue to be, discovered. But the fossils of the Burgess Shale will remain a landmark in the study of evolution, not least because of their services to the Cambrian in the macrocosm of popular science. This may seem a trivial point compared with the wealth of pure science derived from these fossils, but in the modern world of science politics are as important as knowledge, and, without the Burgess Shaleâs rise to fame, the expeditions which led to findings of further Cambrian fossil sites might never have been funded or been made enticing.
The $64 million question
We now know of wonderfully preserved communities of animals where a diversity of animal phyla are represented from the Cambrian, but not before the Cambrian. As stated previously, the internal body plans of animal phyla evolved some 120 to more than 500 million years earlier (depending on who you believe). Hence, the variety of internal body plans found in animals today really was once hidden within the bodies of worms, for tens of millions of years. Now we can really understand
what the Cambrian explosion is. It is the sudden acquisition, 543-538 million years ago, of hard external parts by all the animal phyla found today (except the sponges, comb jellies and cnidarians). It is the simultaneous transition from the prototype worm-shaped or soft-bodied form to complex, characteristic shapes (also known as âphenotypesâ) within each phylum, and it happened in a blink of an eye on the geological timescale. The what of the Cambrian explosion is now understood.
For some reason the early members of each animal phylum did not acquire their hard parts, and hence their characteristic external parts, until the Cambrian. This poses a different question - the why of the Cambrian explosion. Why did it take place? The evolution of hard, external parts was not a chance occurrence. It took place simultaneously in all phyla, after a considerable period during which nothing happened. This extensive correlation must have been forced by an external factor. But what factor? What caused the
Another extensive collection of Cambrian fossils is known from Chengjiang in Yunnan province, south China. The Chinese palaeontologist Hou Xianguang found the first Chengjiang specimen, an unusual trilobite, as a student in 1984. Hou, and his senior at that time, Chen Junguan, dedicated their subsequent years to the Chengjiang site. Extremely well-preserved specimens from several animal phyla were unearthed, and efforts to find further phyla continue at considerable pace today. A study of the ecology of the Chengjiang fauna is receiving the kind of attention previously reserved only for the Burgess Shale fossils. The Chengjiang palaeontologistsâ ammunition has been the age of these fossils - 525 million years old - in addition to the wide diversity of animals represented. So the Chengjiang fauna predate the Burgess fauna by ten million years and reveal that a community structure similar to that we know of 515 million years ago was already in place 525 million years ago.
Cambrian fossil assemblages that can match the Burgess Shale in terms of preservation have been, and no doubt will continue to be, discovered. But the fossils of the Burgess Shale will remain a landmark in the study of evolution, not least because of their services to the Cambrian in the macrocosm of popular science. This may seem a trivial point compared with the wealth of pure science derived from these fossils, but in the modern world of science politics are as important as knowledge, and, without the Burgess Shaleâs rise to fame, the expeditions which led to findings of further Cambrian fossil sites might never have been funded or been made enticing.
The $64 million question
We now know of wonderfully preserved communities of animals where a diversity of animal phyla are represented from the Cambrian, but not before the Cambrian. As stated previously, the internal body plans of animal phyla evolved some 120 to more than 500 million years earlier (depending on who you believe). Hence, the variety of internal body plans found in animals today really was once hidden within the bodies of worms, for tens of millions of years. Now we can really understand
what the Cambrian explosion is. It is the sudden acquisition, 543-538 million years ago, of hard external parts by all the animal phyla found today (except the sponges, comb jellies and cnidarians). It is the simultaneous transition from the prototype worm-shaped or soft-bodied form to complex, characteristic shapes (also known as âphenotypesâ) within each phylum, and it happened in a blink of an eye on the geological timescale. The what of the Cambrian explosion is now understood.
For some reason the early members of each animal phylum did not acquire their hard parts, and hence their characteristic external parts, until the Cambrian. This poses a different question - the why of the Cambrian explosion. Why did it take place? The evolution of hard, external parts was not a chance occurrence. It took place simultaneously in all phyla, after a considerable period during which nothing happened. This extensive correlation must have been forced by an external factor. But what factor? What caused the
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