Article by: Adam Manning
Edited by: J. D. Dixon and Harry T. Jones
The Silurian Period spans the time in Earth’s history between 433.8 +1.5 million years (Ma) ago and 419.2 +3.2 Ma ago, and is the third period of the Palaeozoic Era. It was first established by Sir Rodrick Impey Murchison in 1839, in his book ‘The Silurian System’. The Silurian follows the Ordovician, which ended with one of the biggest mass extinctions in Earth’s history. This event saw the extinction of more than 85% of marine species. So how did the world’s ecosystems recover?
Global biodiversity in brachiopods, trilobites, anthozoans (corals) and bivalves had a long recovery period after the Ordovician-Silurian boundary. This took between 15-20 Ma to get back to pre-extinction levels of biodiversity, but this wasn’t the case everywhere. In Laurentia, a shallow ocean area that would eventually become part of North America, biodiversity rebounded to pre-extinction levels in approximately the first 5 million years of the Silurian. It is unclear why this is the case, but it is possible that immigration of taxa from other areas helped to replenish biotas. Other taxa likely followed a similar trend.
Image 1b: Anacaenaspis phasganis, from the Middle Silurian. Discovered in 1981 the Much Wenlock Limestone Formation of Wren’s Nest Hill, Dudley, West Midlands, England. The trilobite is 1.8 cm long (not including spines). Click Here.
During the Silurian, many groups of life continued to evolve. One of the most famous is the trilobites, which continued to diversify. Calymene blumenbachii, or the ‘Dudley Bug’, is a common and famous fossil from Dudley, England. These arthropods have been found curled up, a behaviour developed to help protect them from predators. Other trilobites developed amazing morphologies with long spines to protect themselves, like the case of Anacaenaspis phasganis, also found in Dudley, England.
Fish also continued to evolve and diversify during the Silurian Period. Many jawless fish (Agnatha) developed bony armour to help protect themselves against predators such as eurypterids (sea scorpions). But fish gained a new feature in the Silurian: jaws. The first jawed vertebrates (gnathostomes) were these jawed fish. The first gill arch evolved into a pair of jaws, and the gill slit became the spiracle. The ability to bite allowed fish to diversify into new niches by taking advantage of different food sources, as well as other useful things.
On land, the first instances of terrestrialisation occurred during the Silurian. The first fungi began to appear on land, and small vascular plants appeared on land for the very first time. Cooksonia is the most famous of these early pioneers, beginning to grow on the shores of the land in the Late Silurian. Whilst very small, about the length of a matchbox, and with a basic anatomy compared to modern plants, Cooksonia is an amazing example of the first plants that evolved on land. They had sporangia at the top of the plant, which was used for reproduction.
In conclusion, the Silurian Period is a testament to how life on Earth can bounce back after a mass extinction and continue to diversify and thrive. It was followed by the Devonian, where gnathostomes continued to diversify and dominate their ancient marine world.
Image References
[1] Image 1a: A curled up Calymene blumenbachii. Click Here. Image 1b: Anacaenaspis phasganis, from the Middle Silurian. Discovered in 1981 the Much Wenlock Limestone Formation of Wren’s Nest Hill, Dudley, West Midlands, England. The trilobite is 1.8 cm long (not including spines). Click Here.
[2] A: Full specimen of Cooksonia caledonica, scale bar = 2.1mm. B-D: Sporangia of the Cooksonia caledonica, scale bar = 0.5mm. Taken from Morel, Edwards, and Iñigez Rodriguez (1995).
Information References and Further Sources
[1] Chart drafted by K. M. Cohen, D. A. T. Harper, P. L. Gibbard, and 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. Accessed 23rd October 2019. Click Here.
[2] Clarkson, E. N. K. (1969). ‘A Functional Study of the Silurian Odontopleurid Trilobite Leonapsis deflexa (Lake)’, Lethaia, 2 (4), pp. 329-344. DOI: https://doi.org/10.1111/j.1502-3931.1969.tb01255.x. Accessed 23rd October 2019. Click Here.
[3] Dorling Kindersley. (2009). ‘Silurian’ in Prehistoric. Great Britain: Dorling Kindersley Limited. pp. 106-107.
[4] Edwards, D., Davies, K. L., and Axe, L. (1992). ‘A vascular conducting strand in the early land plant Cooksonia’, Nature, 357, pp. 683-685. DOI: 10.1038/357683a0. Accessed 23rd October 2019. Click Here.
[5] Krug, A. Z., and Patzkowsky, M. E. (2004). ‘Rapid recovery from the Late Ordovician mass extinction’, Proceedings of the National Academy of Sciences, 101 (51), pp. 17605-17610. DOI: https://doi.org/10.1073/pnas.0405199102. Accessed 23rd October 2019. Click Here.
[6] Morel, E., Edwards, D., and Iñigez Rodriguez, M. (1995). ‘The first record of Cooksonia from South Americain Silurian rocks of Bolivia’, Geological Magazine, 132 (4), pp. 449-452. DOI: https://doi.org/10.1017/S0016756800021506. Accessed 23rd October 2019. Click Here.
[7] Murchison, R. I. (1839). ‘The Silurian System Founded on Geological Researches in the Counties of Salop, Hereford, Randor, Montgomery Caermarthen, Brecon, Pembroke, Monmouth, Gloucester, Worcester, And Stafford: With Descriptions of the Coalfields and Overlying Formations, Part 1’, University of Minnesota: J. Murray. Click Here.
[8] Sherwood-Pike, M. A., and Gray, J. (1985). ‘Silurian fungal remains: probable records of the Class Ascomycetes’, Lethaia, 18 (1), pp. 1-20. DOI: https://doi.org/10.1111/j.1502-3931.1985.tb00680.x. Accessed 23rd October 2019. Click Here.
[9] Zhu, M., Yu, X., Ahlberg, P. E., Choo, B., Lu, J., Qiao, T., Qu, Q., Zhao, W., Jia, L., Blom, H., and Zhu, Y. (2013). ‘A Silurian placoderm with osteichthyan-like marginal jawbones’, Nature, 502 (7470), pp. 188-193. DOI: 10.1038/nature12617. Accessed 23rd October 2019. Click Here.