"To see a world in a grain of sand,
And a heaven in a wild flower,
Hold infinity in the palm of your hand
And eternity in an hour."

From "Auguries of Innocence"

by William Blake

Friday, 18 March 2011

Fossils - Part 7: Brachiopods, Corals, Wenlock Limestone and Bryozoans


Brachiopods form a separate Phylum - Brachiopoda.

In most bivalves the 2 shell valves which enclose the soft body tissues of the animal are of equal size and shape with the plane of symmetry usually passing between the valves but in brachiopods one valve is usually bigger than the other and the plane of symmetry passes through both valves.

Brachiopods first appeared in the early Cambrian around 550 million years ago and species are still found today. They were far more abundant in Palaeozoic and Mesozoic Eras (542 - 65 million years ago) than in the Cenozoic Era (65 million years ago until present).

They are wholly marine living on the sea-floor where they are attached to a solid object such as a rock by a pedicle (fleshy stalk). Pedicles do not readily fossilize but the hole in the shell where the stalk would have passed into the valve can just be seen to the left of the fossil in the photo below.

Although brachiopods are usually between 2 - 5 centimetres long, they can range in size from a few millimetres up to 30 centimetres. The shells are mainly made of calcite.

They are often known as "lamp-shells" because of their similarity to old Roman oil lamps.

Brachiopod Fossil(100 million years old from the Cretaceous Period)


Corals are animals closely related to sea anemones and jelly fish and belonging to the same Phylum Cnidaria

They are entirely marine and consist of an individual animal (polyps) that secretes a hard skeleton (the corallite). They are predators with stinging cells on their tentacles to trap prey.

Modern day corals belong to a group called the Scleractinians which first appeared in the Triassic Period around 240 million years ago and their skeletons are composed of aragonite.

Fossil corals before this time belong to one of two different groups. They are either Tabulate Corals which are always colonial or Rugose which can occur as individual animals or form colonies. Both of these groups first appeared in the Ordovician Period and became extinct in the Mass Extinction at the end of the Permian. These earlier corals had skeletons made of calcium carbonate.

Ordovician (500 - 435 million years ago) fossil coral

Wenlock Limestone

420 million years ago England and Wales were part of a large land mass located about 25 degrees south of the Equataor. Tropical reefs occurred in the warm shallow seas off the coastline and the remains of many organisms living in and around the reef eventually created the rock formation known today as Wenlock Limestone. This limestone is found today in some regions of Central England and the Welsh Borders especially around Wenlock Edge, Shropshire and Dudley, West Midlands.

Below is a photo of Wenlock Limestone which contains various species of brachiopod - Atrypa sp, Sphaervnchia sp, Dolerorthis sp, Gypidula sp together with a species of coral called Favosites. The rock also contains Bryozoa (sea-mats), Polyzoa and crinoid (sea-lily) fragments.

Bryozoans first appeared in the Ordovician and are still found today. All species are aquatic and many live in shallow seas although a few can be found in freshwater. The tiny animals with calcium carbonate skeletons are colonial, filter feeding animals.

This piece of Wenlock Limestone is one of my favourites - the more you examine it, especially with a hand lens, the more fossils you can find. I find it amazing that I am holding something that was formed 420 million years ago and I love the way it demonstrates some of the different organisms living at one particular moment in time.

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