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Paleoworld-101

While looking at one of the shells in my collection that i had originally thought was a bivalve, from the stretch of beach between Lyme Regis and Charmouth, in Dorset (UK), another glance made me realize it is in fact a brachiopod: symmetry in plan view, asymmetrically sized valves in lateral view. 

So i dug out my British Mesozoic Fossils book and have identified it confidently as Cincta numismalis, which the book lists as occurring within the "Jamesoni Zone" of the Lower Lias at Radstock in Somerset. I am not familiar with the brachiopods of the Lower Lias at Lyme Regis in Dorset, but a quick search online using Fossilworks and plain google failed to show any other occurrences of this species from Lyme Regis. Do any of the Lias collecters here know if this is a common/widespread brachiopod taxon in both Somerset and Dorset? 

 

 

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It does look a dead ringer for it and it's true that the standard works on the coastal Lower Lias don't refer to C. numismalis Lamarck.

I'm guessing the NHM figure is based on the Ager monograph, 1956-67, which I don't have.

 

There are seven species of Cincta listed from the Dorset coast in Muir Wood, 1928, "Brachiopods from the Belemnite Marls" (part of a classic paper on the Belemnite Marls). At least some of them are very common there. 

This is reiterated in the Pal guide, 2010, "Fossils from the Lower Lias of the Dorset Coast". All are from the Jamesoni Zone.

These species are all based on S. S. Buckman's (1907) species from Radstock and other locations.

 

As the local specimens are frequently crushed, the Pal guide figures some of the uncrushed ones from Radstock first figured by S.S. Buckman in 1907. 

Here's the plate from the guide. Of these, yours perhaps most closely approaches fig. 4, which is Cincta nummosa S.S Buckman, 1907. 

 

So you may have a new species from Lyme/Charmouth, or there may be too many species described which should be regarded as variants.  I haven't seen Ager's monograph so I don't know if he was using morphometric analysis which is what would be needed. Buckman is renowned for creating an excess of ammonite species! 

 

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Edited by TqB
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Paleoworld-101

Thanks for the detailed response! I agree Fig. 4 in your image also looks like a pretty good fit. In which case it may be best to just call mine Cincta sp. 

The reason mine is uncrushed is because it was picked up on top of the soft muds/clays at shore-level, directly west of Charmouth once you hit the first major bend in the beach. It was ex-situ, but probably came straight from the soft beds in this vicinity. Which i think would be either the 'Shales with Beef' or 'Black Ven Marl Member'.

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1 minute ago, Paleoworld-101 said:

Thanks for the detailed response! I agree Fig. 4 in your image also looks like a pretty good fit. In which case it may be best to call mine Cincta sp. 

The reason mine is uncrushed is because it was picked up loose sitting on top of the soft muds/clays, directly west of Charmouth once you hit the first major bend in the beach. It was ex-situ, but almost certainly came straight from the soft beds in this vicinity. Which i think would be either the 'Shales with Beef' or 'Black Ven Marl Member'. 

You're welcome! The matrix is too pale for those beds, but is typical of the Stonebarrow Marl (Belemnite Marls). It occurs higher in the cliffs where you say, and does include some softer layers. Fallen belemnites and other fossils from it are often found there. It is a better specimen than most - I just call my few specimens Cincta sp. too. :) 

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