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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cambrian explosion | 6/11 | https://en.wikipedia.org/wiki/Cambrian_explosion | reference | science, encyclopedia | 2026-05-05T11:01:41.804996+00:00 | kb-cron |
The Burgess Shale and similar lagerstätten preserve the soft parts of organisms, which provide a wealth of data to aid in the classification of enigmatic fossils. It often preserved complete specimens of organisms only otherwise known from dispersed parts, such as loose scales or isolated mouthparts. Further, the majority of organisms and taxa in these horizons are entirely soft-bodied, hence absent from the rest of the fossil record. Since a large part of the ecosystem is preserved, the ecology of the community can also be tentatively reconstructed. However, the assemblages may represent a "museum": a deep-water ecosystem that is evolutionarily "behind" the rapidly diversifying fauna of shallower waters. Because the lagerstätten provide a mode and quality of preservation that is virtually absent outside of the Cambrian, many organisms appear completely different from anything known from the conventional fossil record. This led early workers in the field to attempt to shoehorn the organisms into extant phyla; the shortcomings of this approach led later workers to erect a multitude of new phyla to accommodate all the oddballs. It has since been realised that most oddballs diverged from lineages before they established the phyla known today—slightly different designs, which were fated to perish rather than flourish into phyla, as their cousin lineages did. The preservational mode is rare in the preceding Ediacaran period, but those assemblages known show no trace of animal life—perhaps implying a genuine absence of macroscopic metazoans.
== Stages == The early Cambrian interval of diversification lasted for about the next 20–25 million years, and its elevated rates of evolution had ended by the base of Cambrian Series 2, 521 million years ago, coincident with the first trilobites in the fossil record. Different authors define intervals of diversification during the early Cambrian different ways: Ed Landing recognizes three stages: Stage 1, spanning the Ediacaran-Cambrian boundary, corresponds to a diversification of biomineralizing animals and of deep and complex burrows; Stage 2, corresponding to the radiation of molluscs and stem-group Brachiopods (hyoliths and tommotiids), which apparently arose in intertidal waters; and Stage 3, seeing the Atdabanian diversification of trilobites in deeper waters, but little change in the intertidal realm. Graham Budd synthesises various schemes to produce a compatible view of the SSF record of the Cambrian explosion, divided slightly differently into four intervals: a "Tube world", lasting from 550 to 536 million years ago, spanning the Ediacaran-Cambrian boundary, dominated by Cloudina, Namacalathus and pseudoconodont-type elements; a "Sclerite world", seeing the rise of halkieriids, tommotiids and hyoliths, lasting to the end of the Fortunian (c. 525 Ma); a brachiopod world, perhaps corresponding to the as yet unratified Cambrian Stage 2; and Trilobite World, kicking off in Stage 3. Complementary to the shelly fossil record, trace fossils can be divided into five subdivisions: "Flat world" (late Ediacaran), with traces restricted to the sediment surface; Protreozoic III (after Jensen), with increasing complexity; pedum world, initiated at the base of the Cambrian with the base of the T.pedum zone (see Cambrian#Dating the Cambrian); Rusophycus world, spanning 536 to 521 million years ago and thus corresponding exactly to the periods of Sclerite World and Brachiopod World under the SSF paradigm; and Cruziana world, with an obvious correspondence to Trilobite World.