CAMBRIAN PERIOD - 550 million years ago
Cambrian Period. The picture is printed on premium photo quality super B paper 17 inches x 22 inches.
The Cambrian period was characterized by an interesting diversity of life in the seas. Life had not yet evolved upon land. The highest forms of life existing at this time were the trilobites, which reached lengths up to two feet. Here is shown a view of the ocean floor during this period with a gathering of denizens from its shadowy depths. Species include Paradoxides gracilis (trilobite), Hydrocephalus minor (trilobite), Peronopsis integra (trilobite) Stromatocystis pentangularis, & Acadocrinus nuntius.
The Cambrian period follows after the Neoproterozoic and is followed by the Ordovician period. The Cambrian is divided into three epochs � the Early Cambrian (Caerfai or Waucoban), Middle Cambrian (St Davids or Albertian) and Furongian (also known as Late Cambrian, Merioneth or Croixan). Rocks of these epochs are referred to as belonging to the Lower, Middle, or Upper Cambrian.
The time range for the Cambrian has classically been thought to have been from about 500 mya to about 570 mya. The lower boundary of the Cambrian was traditionally set at the earliest appearance of early arthropods known as trilobites and of primitive reef-forming animals known as archeocyathids.
The end of the period was eventually set at a fairly definite faunal change now identified as an extinction event. Fossil discoveries and radioactive dating in the last quarter of the 20th century have called these dates into question. Date inconsistencies as large as 20 Ma are common between authors. Framing dates of ca. () 545 to 490 mya were proposed by the International Subcommission on Global Stratigraphy as recently as 2002.
A radiometric date from New Brunswick puts the end of the first stage of the Cambrian around 511 mya. This leaves 21 Ma for the other two stages of the Cambrian.
A more precise date of 542 � 0.3 mya for the extinction event at the beginning of the Cambrian has recently been submitted. The rationale for this precise dating is interesting in itself as an example of paleological deductive reasoning. Exactly at the Cambrian boundary there is a marked fall in the abundance of carbon-13, a "reverse spike" that paleontologists call an excursion. It is so widespread that it is the best indicator of the position of the Precambrian-Cambrian boundary in stratigraphic sequences of roughly this age. One of the places that this well-established carbon-13 excursion occurs is in Oman. Amthor (2003) describes evidence from Oman that indicates the carbon-isotope excursion relates to a mass extinction: the disappearance of distinctive fossils from the Precambrian coincides exactly with the carbon-13 anomaly. Fortunately, in the Oman sequence, so too does a volcanic ash horizon from which zircons provide a very precise age of 542 � 0.3 Ma (calculated on the decay rate of uranium to lead). This new and precise date tallies with the less precise dates for the carbon-13 anomaly, derived from sequences in Siberia and Namibia. It is presented here as likely to become accepted as the definitive age for the start of the Phanerozoic eon, and thus the start of the Paleozoic era and the Cambrian period.
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