Tag Archives: Dorset

Identification – Ammonite in sandstone

One of the most interesting parts of working in museums is helping people discover something new (and I usually learn something new myself). A really important way for museums to do their job as a welcoming public source of information is by identifying mystery objects that you might find on a walk, on a seaside holiday or even in your garden or attic.
Anyone can bring in an item for us to identify, for free, and you should have an answer within a few weeks. It might look a bit like this:

Ammonite in sandstone

A close-up of the stone showing the surface pores and a fossil, with ruler for scale. © Saffron Walden Museum.
A close-up of the stone showing the surface pores and a fossil, with ruler for scale. © Saffron Walden Museum.
The whole stone showing surface texture, a fossil and a ruler for scale. © Saffron Walden Museum.
The whole stone showing surface texture, a fossil and a ruler for scale. © Saffron Walden Museum.
A close-up shot of a second fossil, with ruler for scale. © Saffron Walden Museum.
A close-up shot of a second fossil, with ruler for scale. © Saffron Walden Museum.
Close-up shot showing what seems to be fine layering in the stone. © Saffron Walden Museum.
Close-up shot showing what seems to be fine layering in the stone. © Saffron Walden Museum.
The whole piece of stone showing surface pores. © Saffron Walden Museum.
The whole piece of stone showing surface pores. © Saffron Walden Museum.

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This piece of stone is a Jurassic fine-grained sandstone or sandy limestone, which may be from the Lias Group rock unit found on the Dorset coast, although it has a sandier appearance and rougher texture than the rocks usually found in this formation. If it is from the Dorset Lias formation, the rock is roughly 195 to 200 million years old, and the fossils it contains would be a species of Promicroceras ammonite, which are common along the Dorset coast.

Fossil of a Promicroceras ammonite.
Image: Ammojoe CC BY-SA 3.0 (Wikimedia Commons)

The bristleworm, Polydora ciliata. Image: Yale Peabody Museum of Natural History [CC0] (Wikimedia Commons)

 

 

 

 

 

 

The surface pattern of pores in the rock was made much more recently. They were probably made by a species of Polydora worm, probably Polydora ciliata. P. ciliata is a small, rock- or shell-boring worm which can grow up to 30mm (1 1/8 in.) long, and is also known as a bristleworm.

P. ciliata burrows in stone. Image: Rosser1954 CC BY-SA 3.0 (Wikimedia Commons)

Bristleworms are thought to burrow into rock or shell by scraping away at the surface using specialised bristles on the fifth segment of its body, although it may also secrete chemicals such as weak acid to help. It digs a U-shaped burrow, which appears on rocks as distinctive small slots or a ‘sunglasses’ shape.

 – James Lumbard, Natural Sciences Officer.