Precambrian-Cambrian trace fossils from Eastern Yunnan, China: implications for Cambrian explosion. Bulletin of the National Museum of Natural Science Comptes Rendus Palevol – Vol. 8 – N° – p. – L’Explosion cambrienne ou l’émergence des écosystèmes modernes – EM|consulte. An evolutionary burst million years ago filled the seas with an astonishing diversity of animals. The trigger behind that revolution is finally.
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The “Cambrian Explosion” refers to the sudden appearance in the fossil record of complex animals with mineralized skeletal remains.
It may represent the most important evolutionary event in the history of life on Earth. Expolsion beginning of the explosion is generally placed about million years ago, during the Cambrian Period at the start of the Palaeozoic Era the same time the Ediacarans disappear from the fossil record. While the explosion was rapid in geological terms, it took place over millions of years – the Burgess Shale, at million years old, expkosion the tail end of the event.
The explosion is particularly remarkable because all major animal body plans each more or less corresponding to a distinctive Phylum – Mollusca and Chordata, for example appeared during this time, changing the biosphere forever. The origin and diversification of animals during the Cambrian Explosion. Dotted lines represent the probable range of particular groups of animals.
Solid lines represent fossil evidence. Extinct groups are represented with a circled-cross. Cones represent the approximate origin and diversification of the modern phyla the crown groups. The basic body plan of major vambrienne of animals expkosion phyla had already evolved by the time of the Burgess Shale.
The rapid appearance of a wide variety of animals – particularly bilaterians – led to the development of radical new ecological interactions such as predation. Consequently, ecosystems became much more complex than those of the Ediacaran. As the number and variety of organisms increased, they occupied a variety of new marine environments and habitats.
Cambrian seas teemed with animals of various sizes, shapes, and ecologies; some lived on or in the sea floor a benthic lifestylewhile others actively swam in the water column nektonic. Cwmbrienne fundamental ecological structure of modern marine communities was firmly established during the Cambrian.
By the end of the Period, some animals had also made the first temporary forays onto land, soon to be followed by plants. The early record of the Cambrian Explosion is based on fossils – principally camnrienne appearance of mineralized skeletons and complex trace fossils. The typically tiny skeletal elements from this time are called “small shelly fossils.
Unfortunately, many of the fossils remain poorly understood and are difficult to classify within known taxonomic groups. Early Cambrian sclerite-bearing animals. Early Cambrian shell-bearing animals. About million years ago, trilobites made their first appearance in the Cambrian fossil record.
These armoured animals had of three dorsal shelly parts – a explpsion heada segmented thorax bodyand a pygidium tail section. Trilobites eventually became one of the most ubiquitous groups of invertebrate organisms in the Palaeozoic seas.
They survived for almost million years, and their fossils can be found from the Cambrian to the Permian periods. Trace fossils also become considerably more complex and diverse in Early Cambrian rocks. During the late Ediacaran, metazoans produced only simple horizontal traces on the surface of the sea floor. Starting in czmbrienne Cambrian, animals began to tunnel vertically through the sediments and exhibit more varied behaviours, providing indirect evidence that mobile bilaterians with differentiated tissues and organs had already evolved.
Early Cambrian trace fossils. The rise of these bilaterians permanently altered the nature of the sea floor, an event commonly referred to as the Cambrian Substrate Revolution. During the Precambrian, the upper layers of mud, sand or silt on the sea floor ecplosion relatively firm, thanks to bacterial mats that covered and stabilized the surface. These mats also served as cambruenne primary food source for Ediacaran organisms capable ca,brienne grazing along the sea floor see Kimberella.
The burrowing animals of the Cambrian were able to tunnel down through the microbial mats, churning the sediment beneath and making it soupier. The burrowers may have started tunneling to access new sources of food such as the sunken carcasses of planktonic organisms buried on the sea floor or to escape predation by digging deep into the substrate.
Changes in substrate types during the Cambrian substrate revolution. Left, Precambrian Period; right, Cambrian Period. The burrows opened up new ecological niches beneath the sea floor as water and oxygen could now get into the sediment layers. At the same time, bacterial mats were progressively destroyed and forced into more restricted habitats i.
This change in the substrate is thought to be partly responsible for the expposion of the Ediacaran biota.
Cambrian explosion – Wikipedia
Other factors such as a change explowion water chemistry or an increase of predators may also have played important roles in their extinction see above. The revolution turned the once-uniform sea floor into a sxplosion patchwork, opening up a variety of new niches for animals – including those of the Burgess Shale – to exploit. Exceptionally well-preserved soft-bodied fossils of Cambrian age were first described from the Burgess Shale over years ago. Today, dozens of Burgess Shale-type deposits with comparable assemblages of fossils have been found around the world.
These deposits are usually found in Lower and Middle Cambrian rock layers, but may extend as far as the early Ordovician. These deposits are characterized by a similar mode of preservation called “Burgess Shale-type preservation”. Acinocricus cricuspart of a lobopod cambrienne the Spence Shale.
Images of landscapes and fossils from different Burgess Shale-type deposits in Utah. Robert Gaines landscapes and Royal Ontario Wxplosion. Jean-Bernard Caron fossil specimens. Maotianshan Hill, China where the first Lower Cambrian Chengjiang fossils were discovered, including Naraoia spinosa see below.
The name Chengjiang comes from a nearby village in Yunnan Province. Compared to conventional fossil deposits, in which only the remains of more durable body parts are typically preserved, Burgess Shale-type deposits provide a much more complete picture of a normal Cambrian marine community.
In modern marine settings, animals with mineralized body parts shells, carapaces, etc. This is also the case in most Burgess Shale-type deposits where the shelly assemblage usually represents a small percentage of specimens collected. Thus, without the fossilized remains of soft-bodied organisms, especially from the Burgess Shale, our knowledge of Cambrian ecosystems would be extremely limited.
Similarities among various Burgess Shale-type deposits around the world suggest the deep marine ecosystem was geographically uniform and evolutionarily conservative from the Lower to at least the Middle Cambrian i. The characteristic assemblage of organisms is often referred as the Burgess Shale-type biota.
Why did the Cambrian explosion happen when it did, and why was it such a unique event? While there is no current consensus among scientists, most researchers agree the explosion cannot be ascribed to a single, simple causal mechanism.
The potential triggers can be classified in three main categories: Deciphering the impact of each of these factors remains one of the most important challenges faced by palaeontologists today.
Very few organisms ever enter the fossil record; after death, their remains are usually completely destroyed and recycled. Animals with hard body coverings, such as trilobites, are much more likely to be preserved than those with only soft body parts. So the evolutionary development of mineralized shelly parts by different groups would be marked in the rock record by a sudden jump in fossil numbers.
Thus, preservation bias alone could create the appearance of an “explosion” of new life forms at the beginning of the Cambrian. When he published On the Origin of Species inCharles Darwin puzzled over the apparently sudden appearance of complicated organisms in the fossil record at the beginning of the Cambrian Period.
He noted this could be used as an argument against his controversial new theory, which predicted a more gradual appearance of simpler organisms. At the time, Darwin pointed to the imperfection of the fossil record as his only defence, arguing complex animal life must have lived long before the Cambrian, but traces of that life had not yet been found. The presence of large, soft-bodied, putative animals problematic as they may be in Ediacaran seas does indeed make the “explosion” appear less abrupt.
But the fact remains that the Early Cambrian was a time of major change in marine animal communities and environments, with the rapid and unprecedented advent of disparate new body plans and novel ecological niches.
Explosion Cambrienne – Bibliographie
By the end explosoin the period, every major animal phylum was firmly established, and expoosion after the Cambrian was radically different from cambriennf had gone before. So it is safe to call this event an “explosion” – it was crucial to the evolution of life on Earth as we know it. Before complex animals could evolve on Earth, there had to be an environment favourable for their survival.
Researchers have examined a number of environmental factors that might have been instrumental in the evolution of new body plans, but the two strongest contenders are a cabmrienne in oxygen levels and the end of extreme glacial conditions. Multicellular animals use oxygen to fuel their metabolism. At low oxygen levels, they don’t function well … without it, they cannot survive. Photosynthesis could have caused a rise in the amount of oxygen in the seas and atmosphere near the beginning of the Cambrian, allowing the evolution of larger, more complex animals with respiratory and circulatory systems.
However, there does not seem to be much variation in oxygen levels across the Ediacaran-Cambrian boundary. Earlier increases might have triggered the evolution of large Ediacaran metazoans prior to the explosion, and a subsequent post-explosion rise in oxygen levels may have allowed animals to adopt more active, energy-intensive lifestyles such as swimming and hunting.
Another possible environmental explanation for why the explosion occurred when it did involves glaciers. Some researchers have suggested the entire Earth was cambrienns with ice before the Cambrian explosion. This is known as the “Snowball Earth” hypothesis. The ice would have limited the number of evolutionary niches for life in the sea, and blocked most of the sunlight on which cyanobacterial mats and algae depend.
But once the glaciers receded, huge expanses would suddenly be opened for life: Unfortunately for the hypothesis, the last worldwide glaciation seems to have ended around million years ago – nearly 90 million years before the first signs of the Cambrian explosion in the fossil record which was followed by another major regional glaciation around million years exploaion.
Even if there is no direct triggering link between Precambrian glaciations and the Cambrian explosion, the post-glacial period was a crucial time in evolution. The appearance of the first large and complex multicellular organisms shortly after the return to a warmer global climate suggests that environmental cambgienne had become ripe for them to evolve.
Some scientists have argued there was nothing in the environment during the Precambrian-Cambrian transition that was particularly unique.
For these researchers, the answer to the question cambrirnne did the Cambrian explosion take place when it did? According to this approach, life first had to evolve the ability to develop new and diverse body plans. Developmental genes in animals regulate cambfienne and when other genes operate to “build” the organism through its earliest life stages.
Many important developmental genes are shared between widely-divergent animal groups. They are so closely shared that control genes from a lab mouse work perfectly well in a fruit cxmbrienne. This conservation means those control genes must also have been present in the last common ancestor to both the fruit fly and the mouse.
Very small changes in developmental genes can have a surprisingly large impact on the resulting organism.
For instance, changes to the so-called hox genes in fruit flies can cause a fly to sprout an extra set of wings, or to grow legs where the antennae should be. From this, it could be argued that the fuse setting off the Cambrian explosion may have been ignited when the genome cambriennee the ancestor of all modern animals reached a level of complexity including the evolution of hox -like developmental genes sufficient to create radically new body plans.
This would have provided more raw material for natural selection to act upon.