5.9 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cambrian explosion | 4/11 | https://en.wikipedia.org/wiki/Cambrian_explosion | reference | science, encyclopedia | 2026-05-05T11:01:41.804996+00:00 | kb-cron |
Changes in the abundance and diversity of some types of fossil have been interpreted as evidence for "attacks" by animals or other organisms. Stromatolites, stubby pillars built by colonies of microorganisms, are a major constituent of the fossil record from about 2,700 million years ago, but their abundance and diversity declined steeply after about 1,250 million years ago. This decline has been attributed to disruption by grazing and burrowing animals. Precambrian marine diversity was dominated by small fossils known as acritarchs. This term describes almost any small organic-walled fossil—from the egg cases of small metazoans to resting cysts of many different kinds of green algae. After appearing around 2,000 million years ago, acritarchs underwent a boom around 1,000 million years ago, increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation. Other groups of small organisms from the Neoproterozoic era also show signs of antipredator defenses. A consideration of taxon longevity appears to support an increase in predation pressure around this time.
In general, the fossil record shows a very slow appearance of these lifeforms in the Precambrian, with many cyanobacterial species making up much of the underlying sediment.
=== Ediacaran organisms ===
At the start of the Ediacaran period, much of the acritarch fauna, which had remained relatively unchanged for hundreds of millions of years, became extinct, to be replaced with a range of new, larger species, which would prove far more ephemeral. This radiation, the first in the fossil record, is followed soon after by an array of unfamiliar, large fossils dubbed the Ediacara biota, which flourished for 40 million years until the start of the Cambrian. Most of this "Ediacara biota" were at least a few centimeters long, significantly larger than any earlier fossils. The organisms form three distinct assemblages, increasing in size and complexity as time progressed. Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses—one paleontologist proposed that the strangest organisms should be classified as a separate kingdom, Vendozoa.
At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs (Kimberella), echinoderms (Arkarua) and arthropods (Spriggina, Parvancorina, Yilingia). Still, debate exists about the classification of these specimens, mainly because the diagnostic features that allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the ediacarans. However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal. These organisms are central to the debate about how abrupt the Cambrian explosion was. If some were early members of the animal phyla seen today, the "explosion" looks a lot less sudden than if all these organisms represent an unrelated "experiment", and were replaced by the animal kingdom fairly soon thereafter (40 million years is "soon" by evolutionary and geological standards). The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period, about 565 million years ago. They were probably made by organisms resembling earthworms in shape, size and how they moved. The burrow-makers have never been found preserved, but, because they would need a head and a tail, the burrowers probably had bilateral symmetry—which would in all probability make them bilaterian animals. They fed above the sediment surface, but were forced to burrow to avoid predators.
== Cambrian life ==
=== Trace fossils === Trace fossils (burrows, etc.) are a reliable indicator of what life was around, and indicate a diversification of life around the start of the Cambrian, with the freshwater realm colonized by animals almost as quickly as the oceans.
=== Small shelly fauna ===
Fossils known as "small shelly fauna" have been found in many parts on the world, and date from just before the Cambrian to about 10 million years after the start of the Cambrian (the Nemakit-Daldynian and Tommotian ages; see timeline). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, archeocyathids (sponge-like animals) and small shells very like those of brachiopods and snail-like molluscs—but all tiny, mostly 1 to 2 mm long.
While small, these fossils are far more common than complete fossils of the organisms that produced them; crucially, they cover the window from the start of the Cambrian to the first lagerstätten: a period of time otherwise lacking in fossils. Hence, they supplement the conventional fossil record and allow the fossil ranges of many groups to be extended.
=== Cnidarians === The first cnidarian larvae, represented by the genus Eolarva, appeared in the Cambrian, although the identity of Eolarva as such is controversial. If it does represent a cnidarian larva, Eolarva would represent the first fossil evidence of indirect development in metazoans in the earliest Cambrian. Medusozoans developed complex life cycles with a medusa stage during the Cambrian explosion, as evidenced by the discovery of Burgessomedusa phasmiformis.
=== Trilobites ===
The earliest trilobite fossils are about 530 million years old, but the class was already quite diverse and cosmopolitan, suggesting they had been around for quite some time. The fossil record of trilobites began with the appearance of trilobites with mineral exoskeletons—not from the time of their origin.
=== Crustaceans ===