Remember the Ordovician – An important part of the Palaeozoic

Sandwiched between the geological periods of the Cambrian and the Silurian comes the Ordovician, (pronounced “Or-doe-viss-ian”).  The period lasted approximately 50 million years 495 to 443 million years ago.  Like the Silurian period that followed it, the Ordovician was named after an ancient British tribe, a Romano-British hill tribe the Ordovices.  Evidence from rock strata indicates that during the early Ordovician, marine transgressions (sea levels rising and flooding land) were at their greatest, much of the continents around at the time became flooded.  Rising seas are a problem today, a result of global climate change; but the rises experienced in the Holocene are fortunately, not likely to be on the scale as seen in the early Ordovician.

Marine life continued to diversify with a huge number of different types of animals and algae becoming established.  However, this abundance of life was not to last.  Geological data suggests that the global climate became increasingly wetter and colder.  At the end of the Ordovician much of the Earth experienced an Ice Age and the subsequent locking up of vast amounts of water led to a global reduction in sea level.  Estimates as to the extent of the sea level fall vary, some scientists claim that it fell by as much as 300 metres but others put forward a more conservative estimate of about 100 metres (still an immense change in climate and environment).  Many of the shallow seas dried out leaving behind salt and other minerals.  These changes led to a mass extinction with many forms of marine life, especially sedentary ones being killed off.

Rocks of Ordovician age show trace fossils indicating that during this period the first animals ventured out onto land.  Sets of strange parallel trackways, only 10 mm wide or so have been found in upper Ordovician strata in northern England.  These trackways, trace fossils, are believed to have been made by segmented arthropods as they moved over mud by the side of freshwater pools.  These are some of the first signs of animals beginning to migrate onto the land.

Despite the fact that in geological terms the Ordovician comes before the Silurian it was named and described as a period of geological time after both the Cambrian and Silurian had been named.  A Scottish schoolmaster and geologist (born in England; he settled in Scotland), Charles Lapworth made a detailed study of the strata in the Southern Uplands hills and mountains of Scotland.  He mapped the complex succession of ancient marine strata using fossils to identify the relative age of strata, this is called biostratigraphy.  In 1879, he published his work helping to resolve a controversy regarding the age of rocks that had raged for sometime within the scientific world.  Ever since the English professor of geology, the Reverend Adam Sedgewick and his co-worker Sir Roderick Murchison named the Cambrian and Silurian periods in 1835, the actual age of ancient rocks and the order in which some of them had been laid down was debated.

Using Graptolite fossils, Charles Lapworth was able to correlate successive rock strata and work out the correct chronological sequence of deposition.  He identified three distinguishable and observable lower Palaeozoic faunas.  It was this work that led to the recognition of the Ordovician as a distinct geological period separating the older Cambrian strata from the younger Silurian.

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