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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cambrian explosion | 5/11 | https://en.wikipedia.org/wiki/Cambrian_explosion | reference | science, encyclopedia | 2026-05-05T11:01:41.804996+00:00 | kb-cron |
Crustaceans, one of the four great modern groups of arthropods, are very rare throughout the Cambrian. Convincing crustaceans were once thought to be common in Burgess Shale-type biotas, but none of these individuals can be shown to fall into the crown group of "true crustaceans". The Cambrian record of crown-group crustaceans comes from microfossils. The Swedish Orsten horizons contain later Cambrian crustaceans, but only organisms smaller than 2 mm are preserved. This restricts the data set to juveniles and miniaturised adults. A more informative data source is the organic microfossils of the Mount Cap formation, Mackenzie Mountains, Canada. This late Early Cambrian assemblage (510 to 515 million years ago) consists of microscopic fragments of arthropods' cuticle, which is left behind when the rock is dissolved with hydrofluoric acid. The diversity of this assemblage is similar to that of modern crustacean faunas. Analysis of fragments of feeding machinery found in the formation shows that it was adapted to feed in a very precise and refined fashion. This contrasts with most other early Cambrian arthropods, which fed messily by shovelling anything they could get their feeding appendages on into their mouths. This sophisticated and specialised feeding machinery belonged to a large (about 30 cm) organism, and would have provided great potential for diversification: Specialised feeding apparatus allows a number of different approaches to feeding and development, and creates a number of different approaches to avoid being eaten.
=== Echinoderms === The earliest generally accepted echinoderm fossils appeared in the Late Atdabanian; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical. These provide firm data points for the "end" of the explosion, or at least indications that the crown groups of modern phyla were represented.
=== Burrowing ===
Around the start of the Cambrian (about 539 million years ago), many new types of traces first appear, including well-known vertical burrows such as Diplocraterion and Skolithos, and traces normally attributed to arthropods, such as Cruziana and Rusophycus. The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard exoskeletons, although they were not necessarily mineralised. Meiofaunal as well as macrofaunal bilaterians participated in this invasion of infaunal niches. Burrows provide firm evidence of complex organisms; they are also much more readily preserved than body fossils, to the extent that the absence of trace fossils has been used to imply the genuine absence of large, motile, bottom-dwelling organisms. They provide a further line of evidence to show that the Cambrian explosion represents a real diversification, and is not a preservational artefact.
=== Skeletonisation === The first Ediacaran and lowest Cambrian (Nemakit-Daldynian) skeletal fossils represent tubes and problematic sponge spicules. The oldest sponge spicules are monaxon siliceous, aged around 580 million years ago, known from the Doushantuo Formation in China and from deposits of the same age in Mongolia, although the interpretation of these fossils as spicules has been challenged. In the late Ediacaran-lowest Cambrian, numerous tube dwellings of enigmatic organisms appeared. It was organic-walled tubes (e.g. Saarina) and chitinous tubes of the sabelliditids (e.g. Sokoloviina, Sabellidites, Paleolina) that prospered up to the beginning of the Tommotian. The mineralized tubes of Cloudina, Namacalathus, Sinotubulites and a dozen more of the other organisms from carbonate rocks formed near the end of the Ediacaran period from 549 to 542 million years ago, as well as the triradially symmetrical mineralized tubes of anabaritids (e.g. Anabarites, Cambrotubulus) from uppermost Ediacaran and lower Cambrian. Ediacaran mineralized tubes are often found in carbonates of the stromatolite reefs and thrombolites, i.e. they could live in an environment adverse to the majority of animals.
Although they are as hard to classify as most other Ediacaran organisms, they are important in two other ways. First, they are the earliest known calcifying organisms (organisms that built shells from calcium carbonate). Secondly, these tubes are a device to rise over a substrate and competitors for effective feeding and, to a lesser degree, they serve as armor for protection against predators and adverse conditions of environment. Some Cloudina fossils show small holes in shells. The holes possibly are evidence of boring by predators sufficiently advanced to penetrate shells. A possible "evolutionary arms race" between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion. In the lowest Cambrian, the stromatolites were decimated. This allowed animals to begin colonization of warm-water pools with carbonate sedimentation. At first, it was anabaritids and Protohertzina (the fossilized grasping spines of chaetognaths) fossils. Such mineral skeletons as shells, sclerites, thorns and plates appeared in uppermost Nemakit-Daldynian; they were the earliest species of halkierids, gastropods, hyoliths and other rare organisms. The beginning of the Tommotian has historically been understood to mark an explosive increase of the number and variety of fossils of molluscs, hyoliths and sponges, along with a rich complex of skeletal elements of unknown animals, the first archaeocyathids, brachiopods, tommotiids and others. Also soft-bodied extant phyla such as comb jellies, scalidophorans, entoproctans, horseshoe worms and lobopodians had armored forms. This sudden increase is partially an artefact of missing strata at the Tommotian-type section, and most of this fauna in fact began to diversify in a series of pulses through the Nemakit-Daldynian and into the Tommotian. Some animals may already have had sclerites, thorns, and plates in the Ediacaran (e.g. Kimberella had hard sclerites, probably of carbonate), but thin carbonate skeletons cannot be fossilized in siliciclastic deposits. Older (~750 Ma) fossils indicate that mineralization long preceded the Cambrian, probably defending small photosynthetic algae from single-celled eukaryotic predators.
=== Burgess Shale type faunas ===