Marine reptile bone from Charmouth, UK

[Fenguin1999]

Hello everyone, I found this piece of bone yesterday at charmouth, I have been told it is a bone from the back of the skull of an Ichthyosaur, however I not which bone specifically. After some research online my best guess would be the supraoccipital, however I could be way out as I am still new to this, so any further information would be great! 
 

Thanks!

 

Paper is 5mm grid paper

Left

Front 

 

Right

 

Back

 

Top

Under 

 

 

Edited May 6 by Fenguin1999

[Aman]

Hi, nice specimen you've got there! I know quite a bit about ichthyosaur anatomy, so I'll do my best to help. I agree that it is a bone from the back of the skull due to the overall shape, although I don't agree that this is a supraoccipital, as that is the bone that is found at the top of the foramen magnum (where the spinal cord connect to the brain) and so is normally arched. It is difficult to exactly tell what you have with 100% confidence due to the wear on the bone, but I believe that you have one of the two opisthotic bones. This is partly due to its strange, tri-lobed shape but also due to the fact that it is not symmetrical; this is especially evident in the 'top' and 'front' views. This could be due to deformation during fossilization, but the bone is so oddly shaped that I'm not sure what else it could be. I've attached a labelled diagram below to show the position of he bones in the skull. Hypothetically, you could tell what side of the skull the bone came from. but I think it's too water worn to be sure.

 

Once again, cool specimen. As you could probably tell from my answer, I'm a huge marine reptile nerd, and it always excites me when I see their fossils, especially when it's something that isn't the usual vertebra or rib section! Good job finding it!

[Fenguin1999]

15 hours ago, Aman said:

Hi, nice specimen you've got there! I know quite a bit about ichthyosaur anatomy, so I'll do my best to help. I agree that it is a bone from the back of the skull due to the overall shape, although I don't agree that this is a supraoccipital, as that is the bone that is found at the top of the foramen magnum (where the spinal cord connect to the brain) and so is normally arched. It is difficult to exactly tell what you have with 100% confidence due to the wear on the bone, but I believe that you have one of the two opisthotic bones. This is partly due to its strange, tri-lobed shape but also due to the fact that it is not symmetrical; this is especially evident in the 'top' and 'front' views. This could be due to deformation during fossilization, but the bone is so oddly shaped that I'm not sure what else it could be. I've attached a labelled diagram below to show the position of he bones in the skull. Hypothetically, you could tell what side of the skull the bone came from. but I think it's too water worn to be sure.

 

Once again, cool specimen. As you could probably tell from my answer, I'm a huge marine reptile nerd, and it always excites me when I see their fossils, especially when it's something that isn't the usual vertebra or rib section! Good job finding it!

Thank you so much for your detailed reply, and the ID! If I am understanding right would the ‘top’ view with the tall thin ridge from my post actually be this side of the bone from the diagram? 

 

I hadn’t actually noticed the asymmetrical aspects of the bone - I suppose my original thinking was that it was ‘symmetrical enough’ to have to be in the center back of the skull- but now that you mention it, there are definitely a few things, the underneath too seems to have impressions of different depths etc.

Again thanks very much for the help!

Edited May 12 by Fenguin1999