The Cambrian Period

Article by: Adam Manning
Edited by: Harry T. Jones,  
J. D. Dixon, and Jack Wood

Artwork: DPA Cambrian Marine Environment by Karen Carr
Available at: www.karencarr.com/portfolio-images/Marine-animals-and-fish/Cambrian/Discovery-Park-of-America/DPA-Cambrian-Marine-Environment/635

The Cambrian Period began 541.0 ±1.0 million years (Ma) ago and ended 485.4  ±1.9 Ma, and is the first period of the Palaeozoic Era. It was first named in Sedgwick and Murchison, 1835. The name Cambrian is derived from “Cambria”, the classical name for Wales, latinised from “Cymru”. It is an extremely important period in Earth’s history due to the Cambrian Explosion, an event which revolutionised the marine ecosystems of the time, causing them to begin to resemble modern ecosystems.

The period preceding the Cambrian, the Ediacaran, did have life. In fact, life had existed for billions of years before the Cambrian. For example, in Western Australia, stromatolites have been dated to 3.45 billion years (Ga) ago. However, for much of Earth’s history, life had been more simple, single-celled organisms. It wasn’t until around the Ediacaran that the first metazoans (multicellular animals) began to appear in the fossil record. These organisms, such as Charnia, had little in common with modern organisms, but they may have provided the groundwork for an evolutionary bloom later in time.

In the Cambrian Period, there was a major diversification of marine animals into many of the modern phyla that exist today, where animals began to take on characteristics that we recognise more easily. But what triggered this explosion in life?

Artwork: Anomalocaris canadensis by Jack Wood.
Though predatory animals were around before Anomalocaris, it represents one of the first large predators.

At approximately 541 Ma there was a rise in oceanic oxygen levels, correlating to just before the Cambrian Explosion. Research in recent years suggests that this increase was not as a major as once thought, but it would have taken oxygen levels up to approximately 10% of modern oceanic concentrations of oxygen at the sea surface. Past this critical level, predation begins to appear in modern deep-sea ecosystems. So it is thought that this increase may have been enough for new organisms (some of which would have been predatory) to appear in the Cambrian.

Many other factors contributed to the Cambrian Explosion. During the Cambrian, the climate was warm, with instances of sea level rise creating epicontinental seas. This provided more ecospace for life, and also caused increased continental weathering, adding more nutrients and minerals to marine ecosystems. This meant organisms had more resources and so could grow larger and develop calcified hard parts, seen in fossils such as Cloudina.

So we know how animals could evolve into more complex organisms, but why did they in the first place? After all, creating a mineralised exoskeleton takes a lot of energy, so why invest so much into it? This ability evolved in several groups of unrelated organisms. It is thought that new adaptations, like the first compound eyes, allowed for predation and higher levels of competition between organisms, which drove the production of exoskeletons as a means of protection. Organisms could then diversify into new niches, such as burrowing or free-swimming, for the first time.

The Cambrian ended with the Cambrian-Ordovician mass extinction. This was an event which killed off many mollusc and arthropod species, and is thought to have been caused by a decline in oxygen.

Image References:
[1] Artwork: DPA Cambrian Marine Environment by Karen Carr
Available at: www.karencarr.com/portfolio-images/Marine-animals-and-fish/Cambrian/Discovery-Park-of-America/DPA-Cambrian-Marine-Environment/635
[2] Artwork: Anomalocaris canadensis by Jack Wood.

Information References and Further Sources:
[1] Antcliffe, J. B., and Brasier, M. D. (2007). Charnia and sea pens are poles apart, Journal of the Geological Society, 164, pp.49-51. doi: https://doi.org/10.1144/0016-76492006-080. Available at: https://jgs.lyellcollection.org/content/164/1/49. Accessed 6th June 2019.
[2] Babcock, L. E., Peng, S-C., Brett, C. E., Zhu, M-Y., Ahlberg, P., Bevis, M., and Robison, R. A. (2015). Global climate, sea level cycles, and biotic events in the Cambrian Period, Palaeoworld, 24 (1-2), pp. 5-15. doi: https://doi.org/10.1016/j.palwor.2015.03.005. Available at: https://www.sciencedirect.com/science/article/pii/S1871174X15000177?via%3Dihub. Accessed 6th June 2019.
[3] Chart drafted by K.M. Cohen, D.A.T. Harper, P.L. Gibbard, J.-X. Fan (c) International Commission on Stratigraphy, August 2018. To cite: Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated). The ICS International Chronostratigraphic Chart. Episodes 36: 199-204. URL: http://www.stratigraphy.org/ICSchart/ChronostratChart2018-08.pdf. Accessed 6th June 2019.
[4] Fox, D. (2016). What sparked the Cambrian explosion?, Nature, 530 (7590). Available at: https://www.nature.com/news/what-sparked-the-cambrian-explosion-1.19379. Accessed 31st May 2019.
[5] Hofmann, H. J., Grey, K., Hickman, A. H., and Thorpe, R. I. (1999). Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia, The Geological Soceity of America Bulletin, 111 (8), pp. 1256-1262. doi: https://doi.org/10.1130/0016-7606(1999)111%3C1256:OOGCSI%3E2.3.CO;2. Available at: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/111/8/1256/183537/origin-of-3-45-ga-coniform-stromatolites-in?redirectedFrom=PDF. Accessed 6th June 2019.
[6] Peng, S., Babcock, L.E., and Cooper, R.A. (2012). ‘Chapter 19 The Cambrian Period’, in Gradstein, F. M.,  Ogg, J. G., Schmitz, M., and Ogg, G (1st ed.) The Geologic Time Scale 2012. Elsevier B.V. pp. 437-488. Available at: https://tinyurl.com/yyastatu .Accessed 6th June 2019.
[7] Smith, M. P., and Harper, D. A. T. (2013). Causes of the Cambrian Explosion, Science, 341 (6152), pp. 1355-1356. doi: 10.1126/science.1239450. Available at: https://science.sciencemag.org/content/341/6152/1355. Accessed 6th June 2019.