Appearance of metazoans with mineralized skeletons, “explosion” in biotic diversity and disparity, infaunalization of the substrate, occurrence of metazoan Konservat Fossil-lagerstatten, establishment of most invertebrate phyla, strong faunal provincialism, dominance of trilobites, generally warm climate but with possible glacial–interglacial cycles in the later part, opening of the Iapetus Ocean, progressive equatorial drift and separation of Laurentia, Baltica, Siberia, and Avalonia from Gondwana characterize the Cambrian Period.

The Cambrian Period

Advances in Cambrian stratigraphy and paleontology: Integrating correlation techniques, paleobiology, taphonomy and paleoenvironmental reconstruction

Data on the first appearances of major animal groups with mineralized skeletons on the Siberian Platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the Cambrian radiation (popularly known as the ‘Cambrian explosion’) with a higher precision and provide a basis for the definition of Cambrian Stages 2 to 4. The Lophotrochozoa and, probably, Chaetognatha were first among protostomians to achieve biomineralization during the Terreneuvian Epoch, mainly the Fortunian Age. Fast evolutionary radiation within the Lophotrochozoa was followed by radiation of the sclerotized and biomineralized Ecdysozoa during Stage 3. The first mineralized skeletons of the Deuterostomia, represented by echinoderms, appeared in the middle of Cambrian Stage 3. The fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late Neoproterozoic, but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians. The distribution of calcium carbonate skeletal mineralogies from the upper Ediacaran to lower Cambrian reflects fluctuations in the global ocean chemistry and shows that the Cambrian radiation occurred mainly during a time of aragonite and high-magnesium calcite seas.

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Chronology of early Cambrian biomineralization

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Fossil calibrations for the arthropod Tree of Life

Cambrian Evolutionary Radiation: Context, correlation, and chronostratigraphy—Overcoming deficiencies of the first appearance datum (FAD) concept

Important steps have been made toward global subdivision of the Cambrian System. The consensus of opinion is that the system should be subdivided into four series representing subequal spans of time. The lower two series will correspond approximately to the traditional Lower Cambrian, the third series will correspond approximately to the traditional Middle Cambrian, and the fourth series corresponds approximately to the traditional Upper Cambrian. The fourth series, called the Furongian Series, has been ratified by the International Union of Geological Sciences (IUGS). Selection of horizons representing evolutionary events that can be correlated on a global scale, and which can be used as stage or series boundaries, is much more difficult for the lower two series intervals of the Cambrian than it is for the upper two series intervals of the Cambrian. The most practical concept for subdividing the system into stages involves the establishment of two stages each for the first and second series, and establishment of three stages each for the third and fourth series. The lowermost stage of the Furongian, called the Paibian Stage, has been ratified by the IUGS, and the International Subcommission on Cambrian Stratigraphy (ISCS) is nearing decisions on the levels that will used to mark the bases of the upper two stages of the Furongian. Concerning stage boundaries within the proposed third series of the Cambrian, the ISCS is close to decisions on two levels that will be used to mark boundaries.

Loren E. Babeock

Papers

Changing perspectives on Cambrian chronostratigraphy and progress toward subdivision of the Cambrian system