The Ordovician Period spans the time in Earth’s history from 485.4 ±1.9 Ma to 443.8 ±1.5 Ma, and is the second period of the Paleozoic Era. The period was first named in 1879 by Charles Lapworth, who assigned rocks with common characteristics of both Adam Sedgwick’s Cambrian System and Roderick Murchison’s Silurian System to a new period between these existing ones. In doing so, Lapworth ended the argument between Sedgwick and Murchison regarding which rocks are representative of which period. The new Ordovician Period was still contested until the end of the 1890s, but it wasn’t until 1960 that the name finally became official. Four major continents (Baltica, Gondwana, Laurentia, and Siberia) and three major oceans (Iapetus, Paleo-Tethys, and Panthalassic) existed during this period.
In the wake of the end-Cambrian extinction, lifeforms in the Early Ordovician were able to occupy many vacant niches. However, it wasn’t until the Great Ordovician Biodiversification Event (GOBE) in the Middle Ordovician that life started to fully flourish once more. The GOBE shows an exponential increase in the biodiversity of marine populations, with radiations in many groups, such as arthropods (particularly trilobites), bivalves, bryozoans, cephalopods, echinoderms, gastropods, and graptolites, along with the development of reef communities. Accompanied by this exponential increase in invertebrate species was the emergence of the initial jawless fish (e.g. arandaspids), which exhibited vertebrate characteristics such as bony plates, scales, and potentially even teeth for the first time.
This is argued to have resulted from a combination of many geological and biological processes. For example, marine transgressions (increases in global sea level) caused flooding of the continental shelf, so these areas could be colonised by a wide variety of aquatic fauna. Other causes may include global cooling, elevated atmospheric oxygen concentrations, and a rise in oceanic nutrient levels from increased upwelling. Some research correlates the onset of an asteroid shower with the GOBE, however, in this case, the former is less likely to have caused the latter.
At the end of the Ordovician there was a catastrophic extinction event, the first of the five major mass extinctions in Earth’s history. The event is said to have occurred in two pulses, and has been linked to the formation of ice sheets covering part of Gondwana as the continent moved over the South Pole, with evidence of such ice sheets found in Arabia, Canada, and South America. The first pulse occurred due to reductions in both oxygen levels and global temperatures, as well as simultaneous sea level regression (fall) and subsequent loss of shelf habitat. The second pulse occurred due to anoxia caused by a Late Hirnantian sea level transgression. The first phase affected deeper water communities, while the second phase affected shallow water and mid-shelf communities. There was an overall loss of greater than 85% of marine species.
The Ordovician was a key period for the development of life, and while much of this became extinct, the survivors continued to adapt in the following Silurian.
 Illustration of Megalograptus ohioensis, an Ordovician eurypterid (sea scorpion), by Jack Wood.
 Ordovician trilobite fossils recovered from Gilwern Quarry, Pembrokeshire. Image by Daniel Cashmore. Available at: geology_bham.
Information References and Further Sources:
 Armstrong, H. A., and Harper, D. A. T. (2014). ‘An earth system approach to understanding the end Ordovician (Hirnantian) mass extinction’, in Keller, G., and Kerr, A. C., Volcanism, impacts, and mass extinctions: causes and effects. Boulder, Colorado: The Geological Society of America. pp. 287-300. Available at: http://dro.dur.ac.uk/13146/1/13146.pdf. Accessed 19th July 2019.
 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 19th July 2019.
 Coenraads, R. R. (2004) “Ancient Worlds” in Rocks & Fossils: A Visual Guide. The Reader’s Digest Association Limited. pp. 72-75
 Dorling Kindersley. (2009). “Ordovician” in Prehistoric. Great Britain: Dorling Kindersley Limited. pp. 81-93.
 Edwards, C. T. (2019). Links between early Paleozoic oxygenation and the Great Ordovician Biodiversification Event (GOBE): A review, Palaeoworld, 28 (1-2), pp. 37-50. doi: https://doi.org/10.1016/j.palwor.2018.08.006. Available at: https://www.sciencedirect.com/science/article/pii/S1871174X1830043X. Accessed 19th July 2019.
 Harper, D. A. T., Hammarlund, E. U., and Rasmussen, C. M. Ø. (2014). End Ordovician extinctions: A coincidence of causes, Gondwana Research, 25 (4), pp. 1294-1307. doi: https://doi.org/10.1016/j.gr.2012.12.021. Available at: https://www.sciencedirect.com/science/article/pii/S1342937X13000154. Accessed 19th July 2019.
 Lindskog, A., Costa, M. M., Rasmussen, C. M. Ø., Connelly, J. N., and Eriksson, M. E. (2017). Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification, Nature Communications, 8. Available at: https://www.nature.com/articles/ncomms14066. Accessed 19th July 2019.
 Rasmussen, C. M. Ø., Ullmann, C. V., Jakobsen, K. G., Lindskog, A., Hansen, J., Hansen, T., Eriksson, M. E., Dronov, A., Frei, R., Korte, C., Nielsen, A. T., and Harper, D. A. T. (2016). Onset of main Phanerozoic marine radiation sparked by emerging Mid Ordovician icehouse, Scientific Reports, 6. Available at: https://www.nature.com/articles/srep18884. Accessed 19th July 2019.
 Schmitz, B., Harper, D. A. T., Peucker-Ehrenbrink, B., Stouge, S., Alwmark, C., Cronholm, A., Bergström, S. M., Tassinari, M., and Xiaofeng, W. (2007). Asteroid breakup linked to Great Ordovician Biodiversification Event, Nature Geoscience, 1. Available at: https://darchive.mblwhoilibrary.org/bitstream/handle/1912/2272/Naturegeosciences.pdf?sequence=1. Accessed 19th July 2019.
 Webby, B. D. (1998). Steps toward a global standard for Ordovician stratigraphy, Newsletters on Stratigraphy, 36 (1), pp. 1-33. Available at: https://researchers.mq.edu.au/en/publications/steps-toward-a-global-standard-for-ordovician-stratigraph. Accessed 19th July 2019.