April 6. Silurian fossils in Great Britain

In the April 4 episode, I mentioned that the supercontinent Gondwana was situated in the southern hemisphere, partly on the South Pole, during the Silurian. The continents of Laurentia – which is North America – Baltica (Europe) and Siberia were all converging to some degree, and all were pretty much in the tropics, near the Silurian equator. Once the glacial epoch at the start of the Silurian was over, warm, shallow seas prevailed on those continents. A lot of the sediments that were deposited in those seas became limestone, and many of them are remarkably fossiliferous.

In Wales, where the Silurian got its name, and southwest England, the Silurian Much Wenlock limestone is one of the richest fossil assemblages in the world. It’s named for the village of Much Wenlock in Shropshire, so called to distinguish that village from the nearby Little Wenlock. More than 600 different species have been found, and most of them are invertebrates typical of warm, shallow seas. Crinods, brachipods, corals, bryozoans, and trilobites make for a diverse collection – it must have been something like a modern tropical lagoon, supporting lots of life. We’ll talk more about corals, which began to make reefs during the Silurian, tomorrow.

Wenlock limestone

It probably won’t surprise you to hear that the Wenlock limestone was laid down during the Wenlock Epoch, the second subdivision of the Silurian Period. The time span for the Wenlock is about 423 to 428 million years ago, a total of 5 million years. The Wenlock limestone was deposited in the early part of that time. The geological term for a package of rocks with similarities in time and composition is a formation, which is often, but not always, one particular kind of rock like limestone. The Wenlock is subdivided into three members that reflect slightly different environmental conditions. Some parts of it contain silt, finer grained than sand, which would probably mean that there was an influx of sediment being eroded off a land area at some distance from the site of deposition. Coarser sediments, like sand or gravel, would have been dumped closer to the source area, such as along a beach. So the silty sediment in the Wenlock limestone would be a ways out into the sea, perhaps a half mile or a few miles from shore, maybe carried there by storm action.

A study by Ratcliff and Thomas in 1999, linked on the blog, determined that the Wenlock was deposited in water with a variety of energies operating – parts of it were laid down below any wave action, while some was within the zone where storm waves could operate. Some of it was deposited well above the wave base, meaning the water was in motion almost continuously. Lots of living things like crinoids, fastened to the water bottom, like that kind of water, where fresh nutrients are moved in all the time. Such water would be the opposite of stagnant – the waves and moving water would be oxygenating it as well as bringing in food particles. A great place for animals that can’t move around.

Because of the abundant diverse fossils, some have called the Much Wenlock the most famous rock formation in England. It was the fossils in these rocks, together with additional work, that led Murchison to define the Silurian. Today, it’s considered to be a lagerstätten, a German term meaning ‘fossil ore body’ because of its richness and completeness. The Burgess shale is another lagerstatten.

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April 6, 1916, is the birthday of Vincent McKelvey, in Huntington, Pennsylvania. He joined the US Geological Survey in 1941, and served as its 9th director, from 1971 to 1977. He was well known for his scientific studies of phosphates and deep-sea mineral deposits.

—Richard I. Gibson

Photo by Rept0n1x via Wikimedia Commons, under Creative Commons license.


Lapworth Museum collections 

Ratcliff and Thomas (1999)

Much Wenlock limestone