Object of the Month:
Portland Screw snail fossil

 

Ellen, receptionist at the Lapworth Museum of Geology , describes her choice of Object of the Month.; a Portland Screw snail shell fossil.

Video transcript here

Hi, my name is Ellen. I work here on reception at the Lapworth Museum and my Object of the Month is the Portland Screw.

Now, you know, we all go on campus for a coffee and I'm always fascinated by the shapes that the bakers come up with for their cakes that are on sale. For example, the cinnamon swirl always reminds me of the curly snail. And one of my favourites is the cream horn and we just got obviously a pastry case here but it shows you the shape of the Portland Screw.

We have one here which is a modern day snail shell but it gives you an idea of the shape that was created. Now what's fascinating about these is how they create the spiral. The mollusc inside would have secreted more aragonite on one side than the other, and therefore create the equiangular spiral of the shell. And, as it grew, then shell grew with it.

These over time obviously sank to the sea bed and the acid waters and the carbonite muds replaced the shell itself. And that's what's fascinating about this particular rock sample we have here is that actually it leaves a cast, an echo of the shell inside, and you can see here, and here those forms.

What was unique about the Portland screw, and why it gets its name, is that it had an extra ridge around the edge of the shell which allows it to burrow into or screw into the sand in the shoreline which gave it an advantage over its other gastropod relatives as it was more difficult for a scavenger just to get it out of the sand.