Have you ever found yourself getting into a spiral when trying to identify your fossil snails? If I asked you to picture a snail in your mind you’d probably find yourself thinking of the pesky lettuce-munching variety you find down the garden. These common snails are only one member of a larger group of animals known as the gastropods which live in a wide range of ecosystems from the deep ocean to freshwater ponds - and of course the lettuce patch. In the marine realm gastropods display a broad range of ecologies (including burrowing through the sediment and being carnivorous!) which leads to an astonishing variety of shell shapes and sizes. If you've ever found fossils in the rocks around Malton and Pickering you've more than likely seen a fossil gastropod. The Coralline Oolite, an Upper Jurassic (160-million-year-old) limestone that is exposed around the Vale of Pickering, hosts a wonderful array of these fossils. Quite often these are seen as strange outlines in the rock depicted by lines of brown calcite crystals or just the sediment infills taking the shape of the shell. This can make identifying them a little tricky at times but we’ve put a little guide together to help you find your way! Palaeontology is rammed to the rafters with fiddly terminology, and when describing fossils with protoconchs and abapical striae it can all be a bit baffling. But fear not, the only ones you need to know here are: Spire – Just like a church spire, this is referring to the pointiness of the shell. Snails can be high-spired (tall and usually thin), or low-spired (board and squat). Ribs – thin projections of the shell that are linear, these can either run in the direction of shell coiling (spiral ribs) or from the top of the shell to the bottom, cross-cutting the coils of the shell (axial ribs). Internal mould – These are like natural plaster casts, they are made by the inside of the shell filling up with mud. Later on, as water flows through the rock, the shell dissolves away. Before we dive into the Late Jurassic ocean and identify some snails we just want to throw in a little ‘disclaimer’… the identifications we have used here are the ones that are used frequently in the scientific literature, but there has been no rigorous review of these for well over 100 years … Bourgetia This is one of the big ones - it can be over 10 cm tall! Bourgetia is a high-spired gastropod but it is also quite broad and chunky with each coil of the shell having a rounded outline. The pattern on the shell is normally best seen on the broadest coil towards the base and is a series of grooves that spiral in the same direction as the coil of the shell – this is one of the best features to identify it with but unfortunately it’s not always very well preserved… Two specimens of Bourgetia. Left: Specimen showing well-preserved spiral grooves on the outer surface of the shell. Specimen housed in The Rotunda Museum, Scarborough. Right: The spiral grooves in this specimen have not been preserved. Specimen from East Ayton, near Scarborough, and held by the Yorkshire Geological Society. Pseudomelania These must be the most common gastropod fossils in the Vale of Pickering. They are usually preserved as outlines of shells depicted in large calcite crystals, or just internal moulds. They are again a reasonably large, high-spired gastropod, usually around 5-7 cm to the top of their shells, but they are not as chunky as Bourgetia. Their spire has a much straighter outline and they lack the grooved pattern on the outer shell although neither of these are seen on an internal mould. A selection of images showing the range of preservation in Pseudomelania. Specimens from near Pickering. Left: Specimen is broken through to show the internal mould and shell preserved in large brown calcite crystals around the outside. Middle: Internal mould, nothing remains of the shell. Right: A large and complete example showing details of the outside of the shell, note the straight outline of the spire. Specimen on display in the Yorkshire Museum. Cylindrites The Swiss rolls of the gastropod world! These are maybe one of the stranger looking gastropods of the Oolite especially if they have been eroded at an angle. Each coil of the shell is quite tall but they don’t really have a spire, instead the younger coil of the shell wraps around the previous one making them look like a Swiss roll – especially when they have been broken across the middle. There are quite a few different species of Cylindrites but we’ve never found one preserved well enough to actually differentiate them. Broken specimens of Cylindrites from near Wrelton, Pickering. The image to the right is an example of a complete Cylindrites, although this is from older Jurassic rocks from Gloucestershire it nonetheless gives an impression of what these shells might look like if they were complete. This last specimen is held at the British Geological Survey, image credit GB3D Fossils. Nerinea Nerinea is a very high spired gastropod, it is also extremely thin and has lots of coils in the shell. What makes it really distinct from other snail shells is each coil tightly abuts the next and the shell slightly flares outwards at this point. The shell is typically covered with a series of faint grooves that run along with the coil direction of the shell. Nerinea have internal ribs that give the cross sections of these shells a very strange and irregular appearance! We first found one of these in Hodge Beck, near Kirkbymoorside, but they can be found all over the area when you look hard enough! Different styles of preservation of Nerinea. Left: Complete specimen showing clearly the shell pattern, specimen was collected near Pickering and is held in the Sedgwick Museum, Cambridge, image from GB3D Fossils. Top middle: Long-ways slice through a Nerinea shell, the internal spaces have been infilled by large calcite crystals, specimen collect from near Pickering. Top right: A polished slice through what is most likely a Nerinea. The white arrows highlight the internal ribs. This specimen was photographed at Kirkdale Cave, Kirkbymoorside. Bottom: A nearly complete Nerinea showing some of the fine patterns on the shell. This specimen was collected from Hodge Beck, near Kirkbymoorside.
This is the first in our series of fossil identification guides from inland Yorkshire! Have you found any fossil gastropods in the Vale of Pickering? Send us a picture! Author: Jed Atkinson
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