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1.4.13.4: Pre-Cambrian Animal Life - Biology

1.4.13.4: Pre-Cambrian Animal Life - Biology


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Learning Outcomes

  • Describe the features that characterized the earliest animals and when they appeared on earth

The time before the Cambrian period is known as the Ediacaran period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure 1). In addition to their morphological similarity, molecular analyses have revealed similar sequence homologies in their DNA.

The earliest life comprising Ediacaran biota was long believed to include only tiny, sessile, soft-bodied sea creatures. However, recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and possibly even before the Ediacaran period.

Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years, and are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the main body (estimated about 5 cm long). Other fossils from the Ediacaran period are shown in Figure 2.

Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures. These fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Until this discovery, most scientists believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals may in fact have evolved during the Cryogenian period.


The Cambrian Period (542-488 Million Years Ago)

Before the Cambrian period, 542 million years ago, life on earth consisted of single-celled bacteria, algae, and only a handful of multicellular animals--but after the Cambrian, multi-celled vertebrate and invertebrate animals dominated the world's oceans. The Cambrian was the first period of the Paleozoic Era (542-250 million years ago), followed by the Ordovician, Silurian, Devonian, Carboniferous and Permian periods all of these periods, as well as the succeeding Mesozoic and Cenozoic Eras, were dominated by the vertebrates that first evolved during the Cambrian.


1.4.13.4: Pre-Cambrian Animal Life - Biology

The Cambrian Period marks an important point in the history of life on Earth it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion," because of the relatively short time over which this diversity of forms appears. It was once thought that Cambrian rocks contained the first and oldest fossil animals, but these are now found in the earlier Ediacaran (Vendian) strata.

Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Ediacaran Period immediately preceding the Cambrian. Some of the odd fossils of the biota from the Ediacaran may also have been animals representative of living phyla, although this remains a somewhat controversial topic. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years. Trace fossils made by animals also show increased diversity in Cambrian rocks, showing that the animals of the Cambrian were developing new ecological niches and strategies — such as active hunting, burrowing deeply into sediment, and making complex branching burrows. Finally, the Cambrian saw the appearance and/or diversification of mineralized algae of various types, such as the coralline red algae and the dasyclad green algae.

This does not mean that life in the Cambrian seas would have been perfectly familiar to a modern-day SCUBA diver! Although almost all of the living marine phyla were present, most were represented by classes that have since gone extinct or faded in importance. For example, the Brachiopoda was present, but greatest diversity was shown by inarticulate brachiopods (like the one pictured below, left). The articulate brachiopods, which would dominate the marine environment in the later Paleozoic, were still relatively rare and not especially diverse. Cambrian echinoderms were predominantly unfamiliar and strange-looking types such as early edrioasteroids, eocrinoids, and helicoplacoids. The more familiar starfish, brittle stars, and sea urchins had not yet evolved, and there is some controversy over whether crinoids (sea lilies) were present or not. Even if present, crinoids were rare in the Cambrian, although they became numerous and diverse through the later Paleozoic. And while jawless vertebrates were present in the Cambrian, it was not until the Ordovician that armored fish became common enough to leave a rich fossil record.


Left: Acrothele, a fairly common inarticulate brachiopod from the the Wheeler Shale of western Utah. Middle: Olenellus fremontii from the Latham Shale of southern California. Right: A hyolith, also from the Latham Shale.

Other dominant Cambrian invertebrates with hard parts were trilobites (like the one pictured above), archaeocyathids (relatives of sponges that were restricted to the Lower Cambrian), and problematic conical fossils known as hyolithids (like the one pictured above, right). Many Early Cambrian invertebrates are known only from "small shelly fossils" — tiny plates, scales, spines, tubes, and so on. Many of these were probably pieces of the skeletons of larger animals.

A few localities around the world that preserve soft-bodied fossils of the Cambrian show that the "Cambrian radiation" generated many unusual forms not easily comparable with anything today. The best-known of these sites is the legendary Burgess Shale (middle Cambrian) in the British Columbian Rocky Mountains. Sites in Utah, southern China, Siberia, and north Greenland are also noted for their unusually good preservation of non-mineralized fossils from the Cambrian. One of these "weird wonders", first documented from the Burgess Shale, is Wiwaxia, depicted at lower left. Wiwaxia was an inch-long, creeping, scaly and spiny bottom dweller that may have been a relative of the molluscs, the annelids, or possibly an extinct animal group that combined features of both phyla.

A lot can happen in 40 million years, the approximate length of the Cambrian Period. Animals showed dramatic diversification during this period of Earth's history. This has been called the "Cambrian Explosion". When the fossil record is scrutinized closely, it turns out that the fastest growth in the number of major new animal groups took place during the as-yet-unnamed second and third stages (generally known as the Tommotian and Atdabanian stages) of the early Cambrian, a period of about 13 million years. In that time, the first undoubted fossil annelids, arthropods, brachiopods, echinoderms, molluscs, onychophorans, poriferans, and priapulids show up in rocks all over the world.

Stratigraphic boundaries are generally determined by the occurences of fossils. For instance, the trace fossil Treptichnus pedum marks the base of the Cambrian. This boundary is an unusual case, since stratigraphic boundaries are normally defined by the presence or absence of groups of fossils, called assemblages. In fact, much paleontological work is concerned with questions surrounding when and where stratigraphic boundaries should be defined. At first glance, this may not seem like important work, but consider this: if you wanted to know about the evolution of life on Earth, you would need a fairly accurate timeline. Questions such as: "how long did something stay the same?" or,"how fast did it change?" can only be assessed in the context of time.

Tectonics and paleoclimate

The Cambrian follows the Ediacaran Period, during which time the continents had been joined in a single supercontinent called Rodinia (from the Russian word for "homeland", rodina). As the Cambrian began, Rodinia began to fragment into smaller continents, which did not always correspond to the ones we see today. The reconstruction below shows the rifting of Rodinia during the second stage (Tommotian) of the Cambrian . Green represents land above water at this time, red indicates mountains, light blue indicates shallow seas of the continental shelves, and dark blue denotes the deep ocean basins. (For clarity, the outlines of present-day continents have been superimposed on the map.)

World climates were mild there was no glaciation. Landmasses were scattered as a result of the fragmentation of the supercontinent Rodinia that had existed in the late Proterozoic. Most of North America lay in warm southern tropical and temperate latitudes, which supported the growth of extensive shallow-water archaeocyathid reefs all through the early Cambrian. Siberia, which also supported abundant reefs, was a separate continent due east of North America. Baltica — what is now Scandinavia, eastern Europe, and European Russia — lay to the south. Most of the rest of the continents were joined together in the supercontinent Gondwana, depicted on the right side of the map South America, Africa, Antarctica, India, and Australia are all visible. What is now China and east Asia was fragmented at the time, with the fragments visible north and west of Australia. Western Europe was also in pieces, with most of them lying northwest of what is now the north African coastline. The present-day southeastern United States are visible wedged between South America and Africa they did not become part of North America for another 300 million years. Tectonism affected regions of Gondwana, primarily in what are now Australia, Antarctica, and Argentina. The continental plate movement and collisions during this period generated pressure and heat, resulting in the folding, faulting, and crumpling of rock and the formation of large mountain ranges.

The Cambrian world was bracketed between two ice ages, one during the late Proterozoic and the other during the Ordovician. During these ice ages, the decrease in global temperature led to mass extinctions. Cooler conditions eliminated many warm water species, and glaciation lowered global sea level. However, during the Cambrian there was no significant ice formation. None of the continents were located at the poles so land temperatures remained mild. In fact, global climate was probably warmer and more uniform than it is today. With the retreat of Proterozoic ice, the sea level rose significantly. Lowland areas such as Baltica were flooded and much of the world was covered by epeiric seas. This event opened up new habitats where marine invertebrates, such as trilobites, radiated and flourished.

Plants had not yet evolved, and the terrestrial world was devoid of vegetation and inhospitable to life as we know it. Photosynthesis and primary production were the monopoly of bacteria and algal protists that populated the world's shallow seas.

Also during the Cambrian, the oceans became oxygenated. Although there was plentiful atmospheric oxygen by the beginning of the period, it wasn't until the Cambrian that there was a sufficient reduction in the number of oxygen-depleting bacteria to permit higher oxygen levels in the waters. This dissolved oxygen may have triggered the "Cambrian Explosion" — when most of the major groups of animals, especially those with hard shells, first appeared in the fossil record.

Aldan River, Siberia,: This early Cambrian fauna tells us about the early evolution of animals with skeletons.

Burgess Shale, British Columbia: Thousands of soft-bodied animal fossils paint us a picture of early marine life.

House Range, Utah: An array of Cambrian critters has been found in the Wheeler Shale and the Marjum Formation.

Marble Mountains, California: Olenellid trilobites and more are found in this Mojave Desert locality.

White-Inyo Mountains, California: Visit ancient Cambrian reefs in these mountains of eastern California.

  • Find out more about the Cambrian paleontology and geology of North America at the Paleontology Portal.
  • See the Wikipedia page on the Cambrian.

* Dates from the International Commission on Stratigraphy's International Stratigraphic Chart, 2009.

Page content written and completed by Ben M. Waggoner and Allen G. Collins, 11/22/1994 tectonics and paleoclimate material added by Karen Hsu, Myun Kang, Amy Lavarias, Kavitha Prabaker, and Cody Skaggs as part of a Biology 1B project for Section 112 under Brian R. Speer, 5/1/2000 Ben M. Waggoner revised the Life content, 9/2001 Sarah Rieboldt updated the pages to reflect the Geological Society of America (GSA) 1999 Geologic Timescale, 11/2002 Dave Smith recombined the content into a single page, adapted it to the new site format and made some content updates, 7/6/2011 Acrothele and hyolith photos by Ben M. Waggoner Olenellus photo by Dave Smith source of Tommotian map unknown


References

Sperling, E. A. et al. Nature 523, 451–454 (2015).

Zhang, S. et al. Proc. Natl Acad. Sci. USA http://doi.org/bcgg (2016).

Sahoo, S. K. et al. Geobiology (in the press).

Sperling, E. A. et al. Proc. Natl Acad. Sci. USA 110, 13446–13451 (2013).

Wood, R. A. et al. Precambrian Res. 261, 252–271 (2015).

Bengtson, S. &amp Zhao, Y. Science 257, 367–369 (1992).

Seilacher, A., Buatois, L. A. &amp Mángano, M. G. Palaeogeog. Palaeoclimatol. Palaeoecol. 227, 323–356 (2005).

Vannier, J., Calandra, I., Gaillard, C. &amp Zylinska, A. Geology 38, 711–714 (2010).

Carbone, C. &amp Narbonne, G. M. J. Paleontol. 88, 309–330 (2014).

Mángano, M. G. &amp Buatois, L. A. Proc. R. Soc. B 281, 20140038 (2014).

Buatois, L. A., Narbonne, G. M., Mángano, M. C. Carmona, N. B. &amp Myrow, P. Nature Commun. 5, 3544 (2014).


Precambrian Microbial Life

The earliest evidence of life comes from chemical fossils formed only 100 million years after the end of the Hadean period of intense bombardment by meteorites and planetesimals (3.8 bya), during which huge impacts by planetesimals would periodically vaporize the oceans and sterilize the Earth. This earliest life is inferred because the carbon 13:carbon 12 ratio ( 13 C/ 12 C) of carbonaceous deposits is characteristic of biochemical processes which preferentially use 12 C. The earliest records of cells are microfossils of cyanobacteria from about 3.5 bya, with recognizable stromatolite fossils found in Australia by 3.45 bya.

  • Shallow sea bottoms appear to have been covered by mats of cyanobacteria (engraving) due to a general lack of grazing animals prior to the Cambrian. A cast of a bacterial mat (sometimes called "Elephant skin") from the White Sea region of Russia is displayed.
  • Stromatolites are the dominant fossil type for most of the Precambrian, with the oldest identified examples going back at least 3450 million years. During the Precambrian stromatolites formed reefs comparable in extent and magnitude to the great coral reefs of recent and modern times. Stromatolites became rare during the Cambrian with the advent of multicellular grazing animals from which they have little natural protection. Modern examples occur only in protected environments such as high salt lagoons where animals don't exist (e.g. the famous Shark's Bay examples in Australia), and in sand fields (such as in the photograph to the left taken in the Bahamas) where they are periodically covered in sand, denying access to grazing animals.

      (Proterozoic, 1200 Mya, Mount Nelson Formation, British Columbia) (Proterozoic, ca. 1800 Mya, Biwabik Formation, Minnesota). The typical layered structure of stromatolites is obvious in this specimen. The structure of this specimen shows something of the growth and ecology of these biostructures. Photosynthetic cyanobacteria on the surface released oxygen that then precipitated red iron oxide during the day, adding layer after layer ( see the closeup image showing the fine structure of these layers). As the layers build up the buried layers the oxygen was consumed and anerobic bacteria thrive, some metabolizing iron oxide and sulfur to create crystals of iron pyrite seen as golden flecks deeper in the stromatolite. Two crystals are also visible in the closeup . The white quartz layer is a later intrusion as the rock was buried and fractured over time.

    History of Life: The Cambrian Explosion

    Life can get tricky sometimes. I mean the history of life, of course… The Cambrian Explosion is a great example. Evidence for the first bacterial forms living on Earth date back to 4 billion years ago. It then took nearly 2 billion years for these unicellular organisms to get it together, as they say, and become multicellular. Then, after only a billion and a half more years (that’s 500 million years ago for those playing at home) nearly every animal phylum that exists today had evolved and diversified to some extent. This was quick jump for a slow and gradual process.

    The sudden appearance of diverse life in the fossil record baffled scientists and even caused Darwin to question his theory on evolution by natural selection. They called this the Cambrian Explosion. Today, scientists have several theories to explain the phenomenon. One theory is that life may have been able to evolve quickly because the Earth had changed so much that it created enough new niches that species could rapidly fill them. The second theory is that the explosion was really a dud that some lineages can be traced farther back than the Cambrian and so there was no real ‘explosion’. Like a lot controversial science topics, the answer probably lies somewhere in the middle.

    In the last 542 million years there have been three eras of Earth’s history, the Paleozoic, the Mesozoic, and the Cenozoic. Each of these eras is broken up into two or more periods. The Cambrian period is the first period of the Paleozoic era, the period in Earth’s history when we can see more evidence for diverse life than anytime before that. This period began 542 million years ago with an explosion of life and ended with a mass extinction 52 million years later.

    Building a Bomb

    Earth was changing drastically in the millions of years before the Cambrian Explosion. Multicellular animals had taken some strange forms, and many of them have not been fully classified by scientists because they are too weird to fit any one category. Some animals, like flatworms, segmented worms and maybe cnidarians and sponges (though this is up for debate) made their appearance just before the Cambrian. These animal-wierdos are known as the Ediacaran Fauna. Unfortunately, these animals were nearly all soft bodied and didn’t fossilize well. This is one of the arguments against the explosion theory that the fossil record is too incomplete to say that animals diversified as fast as it appears. It is possible that we evolved from stranger beings than we thought.

    Life on Earth today would probably have been much different if the Ediacaran Fauna had persisted and lived on. But, fortunately for us, they did not. Geologists agree that, before the Cambrian, in the late Proterozoic era, Earth’s temperature dropped insanely low and glaciers covered almost the entire surface of the planet. Temperatures got so low that even the equatorial areas were a slushy mess. This is known as the Snowball Earth theory. The massive glaciers that covered Earth may explain why Earth gave the cold shoulder (get it?!) to the Ediacaran Fauna and many went extinct. Thankfully, the ice thawed and life was able to move on.

    The late Proterozoic was changing shape as well as temperatures. A massive supercontinent called Rodinia began to fracture and split into separate continents. This allowed shallow seas to dominate a significant chunk of Earth’s surface. Today, shallow seas are biodiversity hotspots for a reason. They are warm, they have plenty of sunlight, and plenty of food. Surviving Ediacaran fauna would have naturally migrated to such a place. Because many ecological niches were empty after the last major extinction, these animals would have been able to radiate easily and speedily.

    Who came to the big bang?

    As we have said, some phyla like flatworms, segmented worms, sponges and cnidarians, hung around from the Proterozoic. Others, like mollusks, arthropods and chordates evolved in the Cambrian.

    Mollusks are represented by what scientists call ‘small shelly fauna’. Basically, they are just like they sound: small (a couple of millimeters long), soft bodied organisms encased in a calcium carbonate shell. Later in the Cambrian, more shelled fossils appear.

    Arthropods are represented by everybody’s favorite fossil, Trilobites, among other animals that evolved a hard chitin exoskeleton, instead of a calcium carbonate shell.

    The earliest Chordates, our own illustrious phylum, even made their appearance in the Cambrian! This animal was called Pikaia, and was one of the earliest true chordates.

    The list goes on to include nearly every animal phyla that currently exists! The only phylum that was tardy to the party were the Bryozoans (moss animals), who evolved in the early Ordovician period, right after the Cambrian.

    Fat Man and Little Boy

    Whether the phyla that appear are a result of long, gradual evolution or fast radiation, we can see that the fossil record becomes much more crowded in the Cambrian. Much of this is due to animals finally evolving hard parts like exoskeletons and shells, which fossilize very well.

    Many scientists call the Cambrian Explosion an ‘evolutionary arms-race’ to account for such a fast radiation. The adaptations that arose in the Cambrian were mainly for protection and predation. If one animal evolved a hard exoskeleton, another would have to evolve better predation strategies to survive. Some evolutionary marvels like eyes, brains and ears may have come out of such arms races.


    Contents

    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 since 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 Edit

    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). Trilobite zones allow biostratigraphic correlation in the Cambrian. Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper 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:

    International Series 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
    Adelaidean

    *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, 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 Edit

    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]

    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. 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 sections, ranging from 544 or 542 Ma, are more suitable. [21]

    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]

    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.

    The Cambrian flora was little different from the Ediacaran. The principle taxa were the marine macroalgae Fuxianospira, Sinocylindra, and Marpolia. No calcareous macroalgae are known from the period. [31]

    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., [13] and microbes forming microbial Earth ecosystems, comparable with modern soil crust of desert regions, contributing to soil formation. [32] [33]

    The Cambrian explosion was a period of rapid multicellular growth. 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 with 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. 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]

    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 showed 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] Fossils of the track-maker of Climactichnites have not been found however, fossil trackways and resting traces suggest a large, slug-like mollusc. [43]

    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 [44] and mineralizing animals were rarer than in future periods, in part due to the unfavourable ocean chemistry. [44]

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

    The United States Federal Geographic Data Committee uses a "barred capital C" ⟨Ꞓ⟩ character to represent the Cambrian Period. [45] The Unicode character is U+A792 Ꞓ LATIN CAPITAL LETTER C WITH BAR . [46] [47]


    The Institute for Creation Research

    Trilobites (a type of arthropod) appear in sedimentary rocks as part of the Cambrian Explosion. 1 In this episode, all the major animal groups first appear as fossils. They appear suddenly, fully-formed, and functional, and the older rock layers below them contain no ancestors. 2,3

    Creation scientists explain the sudden appearance of these Cambrian-system fossils as evidence of the first major advance of water across the continents at the onset of the Flood. Imagine huge tsunami-like waves overtaking millions of creatures. It seems likely that shallow marine organisms were the first types of animals buried by these raging floodwaters. 1

    A recent report, published in the Proceedings of the National Academy of Sciences, details a comprehensive new study of the global Ediacaran sediments&mdashthose found just below the Cambrian. 4 The goals of the research included searching for arthropod fossils in Ediacaran sediments and explaining why there are few, if any, arthropods below the Precambrian/Cambrian boundary. 4

    Allison Daley from the University of Lausanne, Switzerland, and her colleagues from the University of Oxford even considered the trace fossil (tracks and burrows) record of the arthropods and biases that may have been caused by various modes of preservation. 4 They said that &ldquoTo search for euarthropod fossils [trilobites, insects, crustaceans] earlier in the rock record requires comparable fossil preservation modes in the Precambrian, where exceptional preservation of soft-tissue abounds.&rdquo 4

    It has been argued that the absence of euarthropods from the Precambrian Period, earlier than around 540 million years ago [the secular age assigned to the base of the Cambrian], is the result of a lack of fossil preservation. But the new comprehensive study suggests this isn&rsquot the case. 5

    The scientists found there were similar opportunities for preservation in both the Cambrian and Precambrian rocks, and yet arthropod fossils and trace fossils are found only in the Cambrian sediments. &ldquoThe lack of euarthropod body fossils in the Ediacaran biota is mirrored in all other preservational regimes in the Precambrian, including BSTs [Burgess Shale Type deposits], phosphatised microfossils, and chert deposits.&rdquo 4

    They also found that euarthropod trace fossils are &ldquostrikingly absent&rdquo during the Ediacaran period. 4

    Daley and her co-authors concluded by saying,

    This constrains the appearance of the euarthropod stem lineage to no older than 550 Ma [about the time of the onset of the Cambrian system by secular estimates]. While each of the major types of fossil evidence (BSTs, trace fossils, and biomineralized preservation) have their limitations and are incomplete in different ways, when taken together they allow a coherent picture to emerge of the origin and subsequent radiation of total group Euarthropoda during the Cambrian. 4

    This study clearly demonstrates that the arthropods, like the trilobites, have no earlier ancestors in the rock record. They show up suddenly as part of the Cambrian Explosion just like all other major animal groups. As Daley and her colleagues have shown, the preservation opportunities in the Cambrian and the Ediacaran were exactly the same, but no arthropods appear in the rock record until the Cambrian. This study deepens the Cambrian Explosion enigma for secular scientists.

    The global Flood remains the best explanation for the fossil record and the Cambrian Explosion. Animal fossils merely appear in rocks in their order of burial. Evolutionary scientists can continue to search for ancestors in the rock record, but they will always come up empty-handed.

    1. Clarey, T. 2014. Trilobites: Sudden Appearance and Rapid Burial. Acts & Facts. 43 (2): 13.
    2. Thomas, B. Is the Cambrian Explosion Problem Solved?Creation Science Update. Posted on ICR.org December 12, 2011, accessed May 23, 2018.
    3. Thomas, B. Cambrian Fossil Intensifies Evolutionary Conundrum. Creation Science Update. Posted on ICR.org September 26, 2014, accessed May 23, 2018.
    4. Daley, A. et al. 2018. Early fossil record of Euarthropoda and the Cambrian Explosion. PNAS. 115 (21): 5323-5331.
    5. University of Oxford. 2018. Major fossil study sheds new light on emergence of early animal life 540 million years ago. Posted on eurekalert.org on May 21, 2018, accessed on May 23, 2018.

    *Dr. Timothy Clarey is Research Associate at ICR and earned his doctorate in geology from Western Michigan University.


    Author information

    Affiliations

    Ludwig-Maximilians-Universität München, Department of Earth & Environmental Sciences, Palaeontology & Geobiology, Richard-Wagner-Str. 10, 80333, Munich, Germany

    Martin Dohrmann & Gert Wörheide

    GeoBio-CenterLMU, Ludwig-Maximilians-Universität München, Richard-Wagner Str. 10, 80333, Munich, Germany

    SNSB – Bavarian State Collections of Palaeontology and Geology, Richard-Wagner Str. 10, 80333, Munich, Germany


    The Institute for Creation Research

    A fossil creature from the phylum Entoprocta (invertebrate animals that have tentacles and lacking a mineralized skeleton) was found in marked abundance (over 400 individuals) in Burgess Shale. The Burgess is a sedimentary layer that's purportedly part of the Cambrian period about a half-billion years ago, according to evolutionists. 1 The problem for paleontologists is that the supposedly 520 million year old creature looks exactly like its living counterparts, only up to 8 eight times larger.

    The Cambrian geologic system is an enigma for the evolutionary paradigm. If evolution is true, life would have started out simple and then evolved in complexity over time. The Cambrian system is one of the oldest geological layers containing billions of sophisticated fossils, supposedly formed after the Precambrian system. The Precambrian layers contain "simple" single-celled life, but also have jellyfish and annelids (worms).

    In the Cambrian system, the fossils represent an explosion of complex multicellular life forms&mdashhence the term "Cambrian Explosion." The problem for evolution is that the Cambrian explosion occurs suddenly with no transitional forms preceding it. Furthermore, many of the types of fossils found in the Cambrian layer are represented by modern organisms, such as entoprocts, that are alive and well today. For a summary of the Cambrian geological system and Burgess Shale, see the excellent review by Dr. John Morris in Acts & Facts. 2

    This species of entoproct, Cotyledion tylodes, was identified previously, but the specimens were not well preserved and difficult to characterize. The discovery of hundreds of new extremely well preserved fossils in much older strata was a huge surprise. Another shock was the extremely well defined detail of the fossil's mouth, anus, and digestive tract, proving that the previous classification of C. tylodes as a cnidarian (a jellyfish-like creature) was wrong. In fact, not only was the creature postulated to be much older than previously estimated, it was incredibly more complex.

    Interestingly, the fossils of C. tylodes also appear to have somewhat more complex features than modern entoprocts. Unlike living entoprocts, the stem and flowerlike feeding cup of the "ancient" version was covered by tiny hardened protuberances (sclerites), and the creatures were much larger.

    Clearly, a majority of the fossil record was formed as a result of the year-long global Flood recorded in Genesis, making it one of evolution's greatest enemies. 3 The original diversity of organisms were created by God to reproduce "after their kind," which is why fossils like the entoproct are complex, fully formed, and similar to their modern living counterparts.

    1. Pappas, S. 2013. 500-Million-Year-Old Animal Looked Like a Tulip. LiveScience. Posted on livescience.com, January 17, 2013, accessed January 23, 2013.
    2. Morris, J. 2008. The Burgess Shale and Complex Life. Acts & Facts. 37 (10): 13.
    3. Morris, J. and F. Sherwin. 2010. The Fossil Record: Unearthing the History of Life. Dallas, TX: Institute for Creation Research.

    Image credit: Copyright © 2013 LiveScience. Adapted for use in accordance with federal copyright (fair use doctrine) law. Usage by ICR does not imply endorsement of copyright holders.


    Watch the video: Precambrian Creatures: The First Animals (July 2022).


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