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Cambrian painting

Cambrian Explosion

© G. Paselk, 2011

Artist’s impression of mid-Cambrian life showing many soft-bodied animals as seen in the Burgess shale. Animals include, from left: Dinomischus, Ottoia, Halucinoginia, Wiwaxia, Trilobites, Archeocyathans, Microdictyon, Canadia, and Pikia. Two larger Anomolocarus are seen in the distant background.

 Precambrian

Cambrian

542.0 to 488.3 Million years ago

Richard Paselk

 Ordovician

techtonic map icon
Plate Tectonic Reconstructions

The Cambrian* Period begins the Phanerozoic Eon, the last 542 million years during which fossils with hard parts have existed. It is the first division of the Paleozoic Era (542Ma -251Ma). Marine animals with mineralized skeletons make their first appearance in the shallow seas of the Cambrian, though only "small shelly fossils" (tiny shells, spines and scales from early metazoans) and trace fossils are preserved for the first ten million years or so. In the "Cambrian explosion” of metazoan diversity most animal groups appear over the short span of the following ten million years. All of the invertebrate phyla as well as the chordates are apparently established by the end of the Period.

A major early Cambrian event was the transformation of the seabed. Early Cambrian sea floors, like late Proterozoic (Ediacaran) seafloors, were covered in microbial mats with an oxygen-free, sulfide-rich, hard layer of mud just below the surface, as shown in the first frame of the illustration below:

cartoon of ediacaran vs. Cambrian seafloor

public domain image via Wikipedia Creative Commons

Early metazoans fed on and burrowed just under these mats. Around the Ediacaran-Cambrian transition burrowing animals, initially near-microscopic “worms” in the interstices of sand etc., began to mix the seafloor below them, gradually oxygenating and softening the mud below the mats. The softened and oxygenated mud provided new niches that larger burrowing organisms could evolve to exploit. Evolving bottom feeding animals gradually destroyed the mats themselves, by their activities creating more new niches. And, of course all of these new animals became themselves new food sources for evolving predators. All of this early evolution over the first ten million years or so of the Cambrian presaged the amazing diversification to come in the Cambrian Explosion.

The animals (metazoans) of the Cambrian Explosion were organized into a unique marine Cambrian fauna, one of three recognized marine fauna of the Phanerozoic. This faunal ecosystem was mostly deposit feeders with nearly all animals living near the surface of the sea bottom. Most of these metazoans are living on, attached to, or making shallow borrows in the sea bottom. Even suspension feeders, which were uncommon, such as brachiopods, echinoderms and the reef-building archeocyathids, make their livings near the seafloor. Trilobites dominate from the Cambrian explosion to the endof the Cambrian, comprising 80-90% of the skeletonized remains. Most benthic1 trilobites were apparently epifaunal2 deposit feeders.

The Cambrian is unique in the fossil record in the number of Lagerstätten, deposits where soft-body parts and soft-bodied organisms are preserved. These deposits provide a unique view of the extraordinary diversity of the Cambrian fauna, as only 5-10% of the organisms preserved in them would have been fossilized under normal conditions. Cambrian lagerstätten are preserved in part due to the unique chemical conditions resulting from the minimal levels of burrowing during ths Period. Fossil deposits including soft-bodied organisms, such as the Lower Cambrian Chengjiang deposits in China and the Middle Cambrian Burgess Shale in Canada, are still dominated by trilobites. Most of the other Chengjiang and Burgess Shale organisms were also deposit feeders, though a few soft-bodied predators were preserved. 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. Nearly 75% of trilobites and other animals, including the reef-building Archeocyathids, vanished in a great mid-period extinction event when shallow seas withdrew. When the shallow seas returned an even greater diversity of Cambrian animal life resulted, again filling the oceans with a wide variety of exotic organisms. After nearly 54 million years, The Cambrian ends with another major extinction event. Nearly 75% of trilobite families and 50% of sponge families disappeared at this time. The unique Cambrian evolutionary fauna continues through the Paleozoic, but the Paleozoic fauna quickly come to dominate. The few remaining organisms of the Cambrian fauna are finally lost in the great Permian extinction events.

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.

  1. Sediment occupied by organisms in fresh or marine aqueous environments.
  2. Epifauna: Animals living on or attached to the sea bottom.
  3. Infauna: Animals living in the sediment.

Cambrian Prokaryote Fossils

Cyanobacteria (ToL: Eubacteria)

stromatolite engraving thumbnail

Bacterial stromatolite

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 (possibly protozoan?) in the late Proterozoic of the Precambrian. They reappeared after the end Precambrian extinction events and the extinction of the Vendian fauna, soon becoming rare again as fossils with the evolution of the Cambrian fauna and grazing metazoans.

fossil image icon Stromatolite (early Cambrian, >530 Mya, Midwestern US). Lower Cambrian Chambless Limestone bearing Girvanella (dark oval nodules) precipitated by an extinct genus of cyanobacteria.

 

Cambrian Animal (Metazoan) Fossils

Trace fossils (ToL: Metazoa<Eukaryota)

trace fossil engraving thumbnail

Worm burrows

Burrowing animal trace fossils mark the existence of digging organisms, and the beginning of the Phanerozoic. One type of burrow from worms, the Skolithos burrows often mark the boundary between Cambrian and Pre-cambrian deposits.

fossil image icon Trace fossils of burrows

Archeocyathids (ToL: Metazoa<Eukaryota)

These unusual organisms have very distinctive skeletons. They were possibly related to the sponges.The archeocyathids were the first reef building animals, originating in the Cambrian explosion around 525 Ma. Archeocyathids are good indicators of Lower and Middle Cambrian rocks. Click on these links for an artist's interpretation of some varieties of live archeocyathids and a diagram of their anatomy.

fossil image icon Archeocyathids

Trilobites (ToL: Trilobites<Arthropoda<Ecdysozoa<Bilateria<Metazoa<Eukaryota)

trilobite engraving thumbnail

Trilobites

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.

Echinoderms (ToL: Echinodermata<Deuterostomia<Bilateria<Metazoa<Eukaryota)

Echinoderms were primative an uncommon 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.

Gogia spiralus

Brachiopods (ToL: Brachiopoda<Lophotrochozoa<Bilateria<Metazoa<Eukaryota)

brachiopod engraving thumbnail

Brachiopods

A relatively common Cambrian fossil is the brachiopod. Next to trilobites, inarticulate brachiopods (brachiopods with untoothed hinges) comprise the most common fossil type, representing 5-7 percent of skeletonized remains. A single species is displayed in this case, a plate with over a dozen small (< 1 cm) shells of an inarticulate brachiopod:

fossil image icon Acrpthele subsidua .

 

The engravings of Cambrian fossils are from Dana, James D. (1870) Manual of Geology or H. Alleyne Nicholson (1876) The Ancient Life-History of the Earth.



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