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From Wikipedia, the free encyclopedia

Cambrian Period
541–485.4 million years ago
Mean atmospheric O
2
content over period duration
c. 12.5 vol %[1][2]
(63 % of modern level)
Mean atmospheric CO
2
content over period duration
c. 4500 ppm[3]
(16 times pre-industrial level)
Mean surface temperature over period duration c. 21 °C[4]
(7 °C above modern level)
Sea level (above present day) Rising steadily from 30m to 90m[5]
Key events in the Cambrian
view • <span style="color:#002bb8;" title="Discussion about this template">discuss</span> • edit
-550 —
-540 —
-530 —
-520 —
-510 —
-500 —
-490 —
Orsten Fauna
Archaeocyatha extinction
SSF diversification, first brachiopods & archaeocyatha
Treptichnus pedum trace
Large negative peak δ 13Ccarb excursion
First Cloudina & Namacalathus mineral tubular fossils
Stratigraphic scale of the ICS subdivisions and Precambrian/Cambrian boundary.

The Cambrian Period ( /ˈkæmbriən/ or /ˈkmbriən/) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon.[6] The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago (mya) to the beginning of the Ordovician Period 485.4 mya.[7] Its subdivisions, and its base, are somewhat in flux. The period was established (as “Cambrian series”) by Adam Sedgwick,[6] who named it after Cambria, the Latinised form of Cymru, the Welsh name for Wales, where Britain's Cambrian rocks are best exposed.[8][9][10] The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells. As a result, our understanding of the Cambrian biology surpasses that of some later periods.[11]

The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian Charnia being exceptional. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized—hence readily fossilized—organisms became common.[12] The rapid diversification of lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla. Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa (animals) evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.

Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust[13] and a few molluscs that emerged to browse on the microbial biofilm known to have been present.[14] Most of the continents were probably dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia. The seas were relatively warm, and polar ice was absent for much of the period.

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Transcription

Welcome back to our mini-series on the history of life on Earth! So far, we’ve covered the Archean and Proterozoic eons -- the first 3 billion years of life, which was mostly single-celled that whole time. Next comes the Phanerozoic eon, and here, we’re going to zoom in a bit: to the first chunk of the Phanerozoic eon, the Paleozoic era, which lasted from 542 to 252 million years ago. Right at the beginning of the Paleozoic, there was a huge explosion of more complex life. And that’s when things started to get really interesting. The Paleozoic era is divided up into 6 periods. And by the first one, the Cambrian, multicellular life -- including animals -- already existed. But it hadn’t … done much. Animals were simple things like sponges. They didn’t have complex organs, and really, they weren’t much more than lumps, eating bacteria they strained out of the water. All that changed about 542 million years ago, with the Cambrian Explosion. There was an increase in oxygen levels just before the beginning of the Cambrian, caused by a boom in life that produced oxygen using photosynthesis. Scientists still debate how big this oxygen event was and when exactly it happened, but it might have made predation -- the predator-prey relationship -- possible for the first time. And the filter-feeding lumps got gobbled up. Predation involves chasing after the things you want to eat, and that takes more energy than sitting on the ocean floor waiting for food to come to you. To maintain that high-energy lifestyle, predators needed a whole lot of oxygen. Once predators evolved, prey started to evolve better defenses and ways to run away -- which led to predators getting faster and better at capturing their prey. It was basically an evolutionary explosion. Hard body tissues like shells and skeletons begin to show up in the fossil record just before the Cambrian. It took a lot of energy to produce that tissue, but it was worth it since these animals were less likely to be eaten. Officially, the beginning of the Cambrian was when animals started to burrow under the thick mat of bacteria on the ocean floor to escape predators. These predator-prey relationships combined with other factors, like changes in the minerals in the oceans and flooding that opened up shallow habitats. This led to such an enormous boom in diversity that almost every major animal group that exists today evolved during the Cambrian -- including arthropods, molluscs, and the chordates that eventually gave rise to vertebrates. The second period of the Paleozoic era was the Ordovician, which started 485 million years ago. The name comes from a Celtic tribe, since many of the best-studied rocks from the Paleozoic come from Britain. The Ordovician was when vertebrates first appeared. They were fish without jaws. Then came the Silurian period, named for another Celtic tribe, which started 443 million years ago. Sometime during the Ordovician and into the Silurian, life made the jump to land. Moving to land wasn’t as easy as washing up onshore and going about your business. Life began in water, and living in water has some advantages over living in air. Water holds your body up. It helps you with gas exchange. And if you release your sperm or eggs into the Big Blue, it’s much more likely that they’ll meet up with other gametes to reproduce. None of that is true for air. So the earliest land organisms had to evolve support structures, new respiratory systems, ways to avoid drying out, and methods of reproduction that were a little more controlled. Extremely simple plants might go back as far as the early Ordovician or even the Cambrian. They were spore-forming and had very little in the way of internal support. The oldest fossil we have that isn’t a plant spore or a single-celled organism is a fungus called Tortotubus. It’s about 440 million years old, from around the Ordovician-Silurian cutoff. This fungus was a total game-changer. Tortotubus probably helped pave the way for more complex plants, and for animals too. It lived as an underground network of filaments, much like modern fungi. It might have formed mushrooms to disperse its spores, but we don’t know for sure. It fed by rotting the few other organisms on land, like early plants and microbes. By breaking down nutrients, it helped develop the soil on Earth’s surface. That helped complex plants grow and develop soil even further. Now, there are Dr. Who creatures called Silurians. But they’re pretty badly named, because there were no land vertebrates during the Silurian period, let alone intelligent humanoid reptiles. But! There were insects and other arthropods, earthworms, and other terrestrial invertebrates colonizing the land. They formed the first simple land ecosystems, along with plants and fungi. The fourth period of the Paleozoic, called the Devonian, started 419 million years ago and ended 359 million years ago. It’s sometimes called the Age of Fishes, because it’s when the first fish with jaws appeared. And it was followed by a LOT of fish with jaws. These fish, called placoderms, had tough, bony armor surrounding their skulls. They were the earliest vertebrates with jaws, and jaws were pretty useful for eating stuff, so they were a big success, in an evolutionary sense. Placoderms showed up in the middle Devonian, before the Devonian was over, the first tetrapods, or four-footed creatures with backbones, had already evolved. In fact, there’s evidence that tetrapods may go back 395 million years or more -- smack in the middle of the Devonian. Which means that those fish -- the ancestors of birds, mammals, reptiles, and amphibians -- got around to having legs and crawling out of the ocean almost instantly, on an evolutionary timescale. At the beginning of the Devonian, jawless vertebrates were the most complex life around. By the end of the Devonian: there were early, amphibian-like land dwellers walking around. That is a gigantic leap. Arthropods and land plants had a huge boom too, meaning those simple land-based ecosystems from the end of the Silurian were a lot more complex by the end of the Devonian. Then, 359 million years ago, the 5th period of the Paleozoic began:, the Carboniferous period. You might’ve heard that fossil fuels are made of dinosaurs. But they’re actually much, much older than that. The Carboniferous was when land plants really started to establish themselves. The climate was mild enough for plants to grow year-round, and huge forests grew. The word “carboniferous” means coal-bearing, and for good reason: hundreds of millions of years later, we’re digging up the remains of those forests as coal. The forests pumped oxygen into the atmosphere like crazy -- much more oxygen than there is today -- which led to the development of the first big land animals: arthropods. Bugs, basically. They grew huge in the oxygen-rich atmosphere. That’s right. 350 million years ago, Earth was full of giant bugs. Land vertebrates were still fairly small in the Carboniferous, but they did develop one major evolutionary innovation: the amniotic egg, which is the reason you can store a chicken egg without it drying out. Amniotic eggs don’t need to be kept in water, because they have a tough shell and membranes to manage gas exchange without letting the embryo dry out. The reptiles that laid these eggs were less dependent on water than the first tetrapods, who still had to return to the water to lay their eggs. But the amniotes could spend their entire life cycle on land, and they got better and better at it. And they got bigger. The Permian, the last period of the Paleozoic, began 299 million years ago, arthropods, It was the first age that was dominated by land vertebrates -- including the first big vertebrate land predators, like the fin-backed Dimetrodon. If you had a dinosaur-themed coloring book or toy set that featured Dimetrodon as a kid, you should know two things: First, dinosaurs didn’t evolve until after the Paleozoic era, during the Mesozoic era. Dimetrodon is way older than those guys! Second, Dimetrodon was on the same evolutionary branch as today’s mammals, not today’s reptiles and birds -- so it’s more closely related to you than to any dinosaur. It was a member of the group of so-called “mammal-like reptiles” that came before the dinosaurs. Even though they weren’t technically reptiles, it can be a helpful way to think of them. Not mammals yet, but getting there. Dimetrodon was a carnivore, but there were synapsids that ate plants, as well. Like the similar-looking Edaphosaurus, which Dimetrodon probably ate. Plant-eating was its own kind of evolutionary innovation, because herbivores couldn’t really survive until there were enough plants to sustain the animals that ate them. Plus, herbivorous animals had to evolve digestive systems that could extract nutrients from leaves, which is much harder and less energy-efficient than getting all your calories from meat. So plant-eating was another major evolutionary development that happened during the Permian. At the end of the Permian, 251 million years ago, the Paleozoic era ended. And everything else nearly ended along with it. There was a mass extinction event so unimaginably widespread that it’s sometimes called the Great Dying. Something like 90% -- or more -- of Earth’s marine species went extinct. Most of those big synapsids died out. Marine species were hit even harder. Something so awful happened that life nearly met its match. There were ice ages and smaller extinctions throughout the Paleozoic. But this one was the big one. So what was it? What caused the Great Dying? We don’t know for sure. The prime suspect is a plume of lava in present-day Siberia that was deposited 250 million years ago, just when the Permian extinction took place. This was a volcanic eruption of sorts, but if you’re imagining a Vesuvius or Krakatoa, think bigger. A huge plume of heat welled up under Earth’s crust and melted it for hundreds of square kilometers. The region was flooded by enough lava to cover two thirds of the United States. The reason this volcanic plume is such a likely suspect is because it could have done all sorts of life-ending things. It could have caused rapid cooling by blocking out the sun. It could have also set fire to buried coal, releasing carbon dioxide and causing runaway global warming -- there’s evidence for both kinds of temperature extremes. It could have released chemicals into the atmosphere that led to large-scale acid rain, or changed the chemistry of the oceans. We don’t know exactly what those eruptions did, but we know they did something and it probably wasn’t pretty. Other suspects include methane-producing bacteria warming the planet, a catastrophe that somehow got rid of all the oxygen in the oceans, an asteroid impact. The Great Dying could also have been caused by the formation of the supercontinent Pangea -- continents crashing into each other would have destroyed a lot of continental shelf habitat, killing some of the richest parts of the oceans. Having one big continent in one place would also have rearranged ocean currents and altered the climate. But Pangea formed a little too early to account for such a widespread die-out. And all the other hypotheses have their strengths and weaknesses, too -- none of them can explain everything. So, some scientists have suggested what’s known as the Murder on the Orient Express Hypothesis: #spolier like in Agatha Christie’s classic novel, there are multiple culprits. It’s like an exam question where the answer might be “some of the above” or “all of the above.” Whatever the cause, nearly everything died. But a few lucky life forms hung on, clearing the way for the archosaurs, the group that includes the dinosaurs. They were the dominant vertebrates during the Mesozoic era, which we’ll talk about next time. Thank you for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, just go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!

Contents

Stratigraphy

Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified. The base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage.[15] The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult as the occurrence of very similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia, Spain and Newfoundland, and possibly, in the western USA. The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, and probably in Spain.[16][17]

Subdivisions

The Cambrian Period followed the Ediacaran Period and was followed by the Ordovician Period. The Cambrian is divided into four epochs (series) and ten ages (stages). Currently only three series and six stages are named and have a GSSP (an internationally agreed-upon stratigraphic reference point).

Because the international stratigraphic subdivision is not yet complete, many local subdivisions are still widely used. In some of these subdivisions the Cambrian is divided into three series (epochs) with locally differing names – the Early Cambrian (Caerfai or Waucoban, 541 ± 1.0 to 509 ± 1.7 mya), Middle Cambrian (St Davids or Albertan, 509 ± 1.0 to 497 ± 1.7 mya) and Furongian (497 ± 1.0 to 485.4 ± 1.7 mya; also known as Late Cambrian, Merioneth or Croixan). Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.

Trilobite zones allow biostratigraphic correlation in the Cambrian.

Each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance:

Chinese North American Russian-Kazakhian Australian Regional
C
a
m
b
r
i
a
n
Furongian Ibexian (part) Ayusokkanian Datsonian Dolgellian (Trempealeauan, Fengshanian)
Payntonian
Sunwaptan Sakian Iverian Ffestiniogian (Franconian, Changshanian)
Steptoan Aksayan Idamean Maentwrogian (Dresbachian)
Marjuman Batyrbayan Mindyallan
Miaolingian Maozhangian Mayan Boomerangian
Zuzhuangian Delamaran Amgan Undillian
Zhungxian Florian
Templetonian
  Dyeran Ordian
Cambrian Series 2 Longwangmioan Toyonian Lenian
Changlangpuan Montezuman Botomian
Qungzusian Atdabanian
Terreneuvian
Meishuchuan
Jinningian
Placentian Tommotian
Nemakit-Daldynian*
Cordubian
Precambrian Sinian Hadrynian Nemakit-Daldynian*
Sakharan
Adeladean

*In Russian scientific thought the lower boundary of the Cambrian is suggested to be defined at the base of the Tommotian Stage which is characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.[18][19][20]

Dating the Cambrian

The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago.

The lower boundary of the Cambrian was originally held to represent the first appearance of complex life, represented by trilobites. The recognition of small shelly fossils before the first trilobites, and Ediacara biota substantially earlier, led to calls for a more precisely defined base to the Cambrian period.[21]

After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, which was to be correlated worldwide by the earliest appearance of Treptichnus pedum.[21] Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, and it is the T. pedum ichnofossil assemblage that is now formally used to correlate the base of the Cambrian.[21][22]

This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago quickly gained favour,[21] though the methods used to obtain this number are now considered to be unsuitable and inaccurate. A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago.[23] The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, and to the disappearance of distinctive Ediacaran fossils (Namacalathus, Cloudina). Nevertheless, there are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which would mean that dates from other, more suitable sections, ranging from 544 or 542 Ma, are more suitable.[21]

Paleogeography

Plate reconstructions suggest a global supercontinent, Pannotia, was in the process of breaking up early in the period,[24][25] with Laurentia (North America), Baltica, and Siberia having separated from the main supercontinent of Gondwana to form isolated land masses.[26] Most continental land was clustered in the Southern Hemisphere at this time, but was drifting north.[26] Large, high-velocity rotational movement of Gondwana appears to have occurred in the Early Cambrian.[27]

With a lack of sea ice – the great glaciers of the Marinoan Snowball Earth were long melted[28] – the sea level was high, which led to large areas of the continents being flooded in warm, shallow seas ideal for sea life. The sea levels fluctuated somewhat, suggesting there were 'ice ages', associated with pulses of expansion and contraction of a south polar ice cap.[29]

In Baltoscandia a Lower Cambrian transgression transformed large swathes of the Sub-Cambrian peneplain into an epicontinental sea.[30]

Climate

The Earth was generally cold during the early Cambrian, probably due to the ancient continent of Gondwana covering the South Pole and cutting off polar ocean currents. However, average temperatures were 7 degrees Celsius higher than today. There were likely polar ice caps and a series of glaciations, as the planet was still recovering from an earlier Snowball Earth. It became warmer towards the end of the period; the glaciers receded and eventually disappeared, and sea levels rose dramatically. This trend would continue into the Ordovician period.

Flora

Although there were a variety of macroscopic marine plants[which?][citation needed] no land plant (embryophyte) fossils are known from the Cambrian. However, biofilms and microbial mats were well developed on Cambrian tidal flats and beaches 500 mya.,[31] and microbes forming microbial Earth ecosystems, comparable with modern soil crust of desert regions, contributing to soil formation.[32][33]

Oceanic life

Most animal life during the Cambrian was aquatic. Trilobites were once assumed to be the dominant life form at that time,[34] but this has proven to be incorrect. Arthropods were by far the most dominant animals in the ocean, but trilobites were only a minor part of the total arthropod diversity. What made them so apparently abundant was their heavy armor reinforced by calcium carbonate (CaCO3), which fossilized far more easily than the fragile chitinous exoskeletons of other arthropods, leaving numerous preserved remains.[35]

The period marked a steep change in the diversity and composition of Earth's biosphere. The Ediacaran biota suffered a mass extinction at the start of the Cambrian Period, which corresponded to an increase in the abundance and complexity of burrowing behaviour. This behaviour had a profound and irreversible effect on the substrate which transformed the seabed ecosystems. Before the Cambrian, the sea floor was covered by microbial mats. By the end of the Cambrian, burrowing animals had destroyed the mats in many areas through bioturbation, and gradually turned the seabeds into what they are today.[clarification needed] As a consequence, many of those organisms that were dependent on the mats became extinct, while the other species adapted to the changed environment that now offered new ecological niches.[36] Around the same time there was a seemingly rapid appearance of representatives of all the mineralized phyla except the Bryozoa, which appeared in the Lower Ordovician.[37] However, many of those phyla were represented only by stem-group forms; and since mineralized phyla generally have a benthic origin, they may not be a good proxy for (more abundant) non-mineralized phyla.[38]

A reconstruction of Margaretia dorus from the Burgess Shale, which were once believed to be green algae, but are now understood to represent hemichordates.[39]
A reconstruction of Margaretia dorus from the Burgess Shale, which were once believed to be green algae, but are now understood to represent hemichordates.[39]

While the early Cambrian showed such diversification that it has been named the Cambrian Explosion, this changed later in the period, when there occurred a sharp drop in biodiversity. About 515 million years ago, the number of species going extinct exceeded the number of new species appearing. Five million years later, the number of genera had dropped from an earlier peak of about 600 to just 450. Also, the speciation rate in many groups was reduced to between a fifth and a third of previous levels. 500 million years ago, oxygen levels fell dramatically in the oceans, leading to hypoxia, while the level of poisonous hydrogen sulfide simultaneously increased, causing another extinction. The later half of Cambrian was surprisingly barren and show evidence of several rapid extinction events; the stromatolites which had been replaced by reef building sponges known as Archaeocyatha, returned once more as the archaeocyathids became extinct. This declining trend did not change until the Great Ordovician Biodiversification Event.[40][41]

Some Cambrian organisms ventured onto land, producing the trace fossils Protichnites and Climactichnites. Fossil evidence suggests that euthycarcinoids, an extinct group of arthropods, produced at least some of the Protichnites.[42][43] Fossils of the track-maker of Climactichnites have not been found; however, fossil trackways and resting traces suggest a large, slug-like mollusc.[44][45]

In contrast to later periods, the Cambrian fauna was somewhat restricted; free-floating organisms were rare, with the majority living on or close to the sea floor;[46] and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry.[46]

Many modes of preservation are unique to the Cambrian, and some preserve soft body parts, resulting in an abundance of Lagerstätten.

Symbol

The United States Federal Geographic Data Committee uses a "barred capital C" ⟨Ꞓ⟩ character to represent the Cambrian Period.[47] The Unicode character is U+A792 LATIN CAPITAL LETTER C WITH BAR.[48][49]

Gallery

See also

References

  1. ^ Image:Sauerstoffgehalt-1000mj.svg
  2. ^ File:OxygenLevel-1000ma.svg
  3. ^ Image:Phanerozoic Carbon Dioxide.png
  4. ^ Image:All palaeotemps.png
  5. ^ Haq, B. U.; Schutter, SR (2008). "A Chronology of Paleozoic Sea-Level Changes". Science. 322 (5898): 64–8. Bibcode:2008Sci...322...64H. doi:10.1126/science.1161648. PMID 18832639. 
  6. ^ a b Wikisource Chisholm, Hugh, ed. (1911). "Cambrian System". Encyclopædia Britannica (11th ed.). Cambridge University Press. 
  7. ^ "Stratigraphic Chart 2012" (PDF). International Stratigraphic Commission. Archived from the original (PDF) on 20 April 2013. Retrieved 9 November 2012. 
  8. ^ Sedgwick and R. I. Murchison (1835) "On the Silurian and Cambrian systems, exhibiting the order in which the older sedimentary strata succeed each other in England and Wales," Notices and Abstracts of Communications to the British Association for the Advancement of Science at the Dublin meeting, August 1835, pp. 59-61, in: Report of the Fifth Meeting of the British Association for the Advancement of Science; held in Dublin in 1835 (1836). From p. 60: "Professor Sedgwick then described in descending order the groups of slate rocks, as they are seen in Wales and Cumberland. To the highest he gave the name of Upper Cambrian group. ... To the next inferior group he gave the name of Middle Cambrian. ... The Lower Cambrian group occupies the S.W. coast of Cærnarvonshire,"
  9. ^ Sedgwick, A. (1852). "On the classification and nomenclature of the Lower Paleozoic rocks of England and Wales". Q. J. Geol. Soc. Lond. 8: 136–138. doi:10.1144/GSL.JGS.1852.008.01-02.20. 
  10. ^ Chambers 21st Century Dictionary. Chambers Dictionary (Revised ed.). New Dehli: Allied Publishers. 2008. p. 203. ISBN 978-81-8424-329-1. 
  11. ^ Orr, P. J.; Benton, M. J.; Briggs, D. E. G. (2003). "Post-Cambrian closure of the deep-water slope-basin taphonomic window". Geology. 31 (9): 769–772. Bibcode:2003Geo....31..769O. doi:10.1130/G19193.1. 
  12. ^ Butterfield, N. J. (2007). "Macroevolution and macroecology through deep time". Palaeontology. 50 (1): 41–55. doi:10.1111/j.1475-4983.2006.00613.x. 
  13. ^ Schieber, 2007, pp. 53–71.
  14. ^ Seilacher, A.; Hagadorn, J.W. (2010). "Early Molluscan evolution: evidence from the trace fossil record" (Submitted manuscript). Palaois. 25 (9): 565–575. Bibcode:2010Palai..25..565S. doi:10.2110/palo.2009.p09-079r. 
  15. ^ A. Knoll, M. Walter, G. Narbonne, and N. Christie-Blick (2004) "The Ediacaran Period: A New Addition to the Geologic Time Scale." Submitted on Behalf of the Terminal Proterozoic Subcommission of the International Commission on Stratigraphy.
  16. ^ M.A. Fedonkin, B.S. Sokolov, M.A. Semikhatov, N.M.Chumakov (2007). "Vendian versus Ediacaran: priorities, contents, prospectives. Archived 4 October 2011 at the Wayback Machine." In: edited by M. A. Semikhatov "The Rise and Fall of the Vendian (Ediacaran) Biota. Origin of the Modern Biosphere. Transactions of the International Conference on the IGCP Project 493, August 20–31, 2007, Moscow." Moscow: GEOS.
  17. ^ A. Ragozina, D. Dorjnamjaa, A. Krayushkin, E. Serezhnikova (2008). "Treptichnus pedum and the Vendian-Cambrian boundary". 33 Intern. Geol. Congr. 6–14 August 2008, Oslo, Norway. Abstracts. Section HPF 07 Rise and fall of the Ediacaran (Vendian) biota. P. 183.
  18. ^ A.Yu. Rozanov; V.V. Khomentovsky; Yu.Ya. Shabanov; G.A. Karlova; A.I. Varlamov; V.A. Luchinina; T.V. Pegel’; Yu.E. Demidenko; P.Yu. Parkhaev; I.V. Korovnikov; N.A. Skorlotova (2008). "To the problem of stage subdivision of the Lower Cambrian". Stratigraphy and Geological Correlation. 16 (1): 1–19. Bibcode:2008SGC....16....1R. doi:10.1007/s11506-008-1001-3. 
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Further reading

  • Amthor, J. E.; Grotzinger, John P.; Schröder, Stefan; Bowring, Samuel A.; Ramezani, Jahandar; Martin, Mark W.; Matter, Albert (2003). "Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman". Geology. 31 (5): 431–434. Bibcode:2003Geo....31..431A. doi:10.1130/0091-7613(2003)031<0431:EOCANA>2.0.CO;2. 
  • Collette, J. H.; Gass, K. C.; Hagadorn, J. W. (2012). "Protichnites eremita unshelled? Experimental model-based neoichnology and new evidence for a euthycarcinoid affinity for this ichnospecies". Journal of Paleontology. 86 (3): 442–454. doi:10.1666/11-056.1. 
  • Collette, J. H.; Hagadorn, J. W. (2010). "Three-dimensionally preserved arthropods from Cambrian Lagerstatten of Quebec and Wisconsin". Journal of Paleontology. 84 (4): 646–667. doi:10.1666/09-075.1. 
  • Getty, P. R.; Hagadorn, J. W. (2008). "Reinterpretation of Climactichnites Logan 1860 to include subsurface burrows, and erection of Musculopodus for resting traces of the trailmaker". Journal of Paleontology. 82 (6): 1161–1172. doi:10.1666/08-004.1. 
  • Gould, S. J.; Wonderful Life: the Burgess Shale and the Nature of Life (New York: Norton, 1989)
  • Ogg, J.; June 2004, Overview of Global Boundary Stratotype Sections and Points (GSSPs) https://web.archive.org/web/20060423084018/http://www.stratigraphy.org/gssp.htm Accessed 30 April 2006.
  • Owen, R. (1852). "Description of the impressions and footprints of the Protichnites from the Potsdam sandstone of Canada". Geological Society of London Quarterly Journal. 8: 214–225. doi:10.1144/GSL.JGS.1852.008.01-02.26. 
  • Peng, S.; Babcock, L.E.; Cooper, R.A. (2012). "The Cambrian Period". The Geologic Time Scale (PDF). 
  • Schieber, J.; Bose, P. K.; Eriksson, P. G.; Banerjee, S.; Sarkar, S.; Altermann, W.; Catuneau, O. (2007). Atlas of Microbial Mat Features Preserved within the Clastic Rock Record. Elsevier. pp. 53–71. 
  • Yochelson, E. L.; Fedonkin, M. A. (1993). "Paleobiology of Climactichnites, and Enigmatic Late Cambrian Fossil" (Free full text). Smithsonian Contributions to Paleobiology. 74 (74): 1–74. doi:10.5479/si.00810266.74.1. 

External links

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