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| Precambrian |
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Home | Exhibits | Life Through Time Exhibit | Cambrian |
| Precambrian |
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The Cambrian* Period begins the Phanerozoic Eon, the last 542 million years during which fossils with hard parts have existed, is the first division of the Paleozoic Era (251 - 542mya). Marine animals with mineralized skeletons make their first appearance in the shallow seas of the Cambrian, though only "small shelly organisms" are preserved for the first ten million years. In the "Cambrian explosion” of metazoan diversity most animal groups appear over the short span of the next ten million years. All of the invertebrate phyla as well as the chordates are established by the end of the Period.
Cambrian animals were organized into a unique marine fauna, one of three recognized during the Phanerozoic. This faunal ecosystem was mostly deposit feeders with nearly all animals living near the surface of the sea bottom. Thus trilobites dominated Cambrian seas, comprising about 90% of the skeletonized remains, and most benthic1 trilobites were apparently epifaunal2 deposit feeders. Fossil deposits including soft-bodied organisms, such as the Burgess Shale, are still dominated by trilobites. Most of the other Burgess Shale organisms were also deposit feeders, though a few soft-bodied predators were preserved. Suspension feeders, such as sponges, primitive echinoderms and brachiopods exist, but are uncommon. Overall, Cambrian animals are skewed towards epifauna or infauna3 with even suspension feeders clustered close to the sea bottom.
The Cambrian Period may be divided into three divisions: Lower (Early), Middle, and Furongian (Late). As noted above, trilobites are the most common fossil types, and these dominant animals of the Cambrian seas characterize this Period. Each division of the Cambrian is identified with particular trilobite genera. The Cambrian Period lasted nearly 54 million years, ending with a mass extinction. Nearly 75% of trilobite families and 50% of sponge families disappeared at this time. The unique Cambrian evolutionary fauna continue through the Paleozoic though the Paleozoic fauna quickly come to dominate. The few remaining organisms of the Cambrian fauna are extinguished at the end of the Permian.
Globally, the Cambrian was a time of warm climate, while exhibiting strong provincialism among its fauna. Tectonically the Cambrian saw the opening of the Iapetus Ocean and the separation of the Launtentia, Baltica and Siberia plates (see tectonic reconstruction, linked above).
* Cambrian comes from the word Cambria, the Roman name for North Wales. It was named for some exposed strata by Sedgewick in 1835.
Cambrian Animal (Metazoan) Fossils |
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Trilobites (ToL: Trilobites<Arthropoda<Ecdysozoa<Bilateria<Metazoa<Eukaryota) |
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Trilobites first appear about 525 Mya. They are the most common and best known of Cambrian fossils (typically 90% of skeletonized fossils), dominating the seas for most of this period. Humboldt's Cambrian display includes specimens from the first two epochs. Lower Epoch (542-513 Mya): Wanneria walcottana, Olenellus fremonti. Middle Epoch (513-501 Mya): Asaphiscus wheeleri, Bathyriscus fimbriatus, Paradoxides pradoanus (top & bottom), Elrathia kingi , Modocia typicalis, Peronopsis intersitcus. Two trilobite assemblages from this epoch are also displayed: Ogygopsis sp and Baltagnostus eurypyx . |
Echinoderms (ToL: Echinodermata<Deuterostomia<Bilateria<Metazoa<Eukaryota) |
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| Echinoderms were not common in the Cambrian. In this case we see a specimen of Gogia spiralus, one of the earliest stalked echinoderms. Formerly considered in a separate group, the eocrinoids, they are now placed within the cystoids. These animals lived attached to the sea floor by a plate covered stalk, but did not have stems. Gogia is at the base of the cystoids. Different species gave rise to three distinct Paleozoic lineages. |
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Brachiopods (ToL: Brachiopoda<Lophotrochozoa<Bilateria<Metazoa<Eukaryota) |
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A relatively common Cambrian fossil is the brachiopod. Next to trilobites, inarticulate brachiopods comprise the most common fossil type, representing 5-7 percent of skeletonized remains. A single species is displayed in this case: Acrpthele subsidua . |
Archeocyathids (ToL: Metazoa<Eukaryota) |
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Archeocyathids: These unusual organisms have very distinctive skeletons. They were possibly related to the sponges. Archeocyathids are good indicators of Lower and Middle Cambrian rocks. |
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Trace fossils (ToL: Metazoa<Eukaryota) |
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Trace fossils of burrows mark the existence of digging organisms. One type of burrow from worms, the Skolithos burrows often mark the boundary between Cambrian and Pre-cambrian deposits. |
Cambrian Prokaryote Fossils |
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Cyanobacteria (ToL: Eubacteria) |
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| Lower Cambrian Chambless Limestone bearing Girvanella (dark oval nodules) precipitated by an extinct genus of cyanobacteria. | |
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Stromatolite (early Cambrian, >530 Mya, Midwestern US). The typical layered structure of stromatolites is obvious in this specimen. Stromatolites, the dominant fossil type for most of the Precambrian, began to disappear with the advent of grazing lifestyles for animals in the Cambrian and soon became rare as fossils. |
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Last modified 7 February 2008| ©1998, HSU NHM
