Ichthyosaur or Plesiosaur Bone for ID (Lyme Regis, UK)

[Paleoworld-101]

This is a marine reptile bone that i found nearly 10 years ago between Lyme Regis and Charmouth, which is famous for its Early Jurassic marine fossils (about 195 million years old). Although it is worn there is some clear shape that should indicate what bone it is, although i have so far been unable to figure it out. Realistically, it is going to be ichthyosaur (most likely) or plesiosaur in origin. 

 

Two ovular depressions/joint surfaces are clearly visible on one side of the bone. Furthermore, the flatness of the bone is real and not just due to wear (both main faces are the edge/surface of the bone). The side with the two suspected joint surfaces is the thickest side, and it slopes down to become progressively thinner opposite to them. 

 

A paddle bone of some sort is my suspicion but i am yet to see a clear match. Any ideas? Thanks!

 

 

 

 

 

 

 

 

Edited February 9 by Paleoworld-101

[rocket]

reminds me to a phalangebone (paddlebone) from an Ichthyosaur, bit weathered and eroded

interesting find!

Edited February 9 by rocket

[pachy-pleuro-whatnot-odon]

I'm afraid the specimen's state of preservation quite hampers identification, since it's hard to determine how much of the specimen has weathered away. At the same time, the oblong shape and uneven thickness make it hard to imagine this having been a vertebra. I therefore agree that a paddle bone is more likely. Notwithstanding, the depressions observed in your example are unusual. And unless they were caused by other phalangi pressing into this one through plastic deformation, I don't think these would naturally occur on any plesiosaur or ichthyosaur paddle bone I've seen. In terms of distinction between plesiosaur and ichthyosaur phalanges, only the bones in a plesiosaur extremity that don't follow the traditional tetropod elongated phalangial bone plan would be the lower arm and leg bones (radius, ulna, tibia, fibula, intermediare) and carpalia/tarsalia (radiale, ulnare, tibiale, etc.). These, however, would be more geometric than your specimen. In contrast, in ichthyosaurs, paddle bones are more uniform in shape, often being round, square or rectangular/ovate, as is the case here. I'm therefore inclined to agree with Frank in this most likely being an ichthyosaur paddle bone.

[Paleoworld-101]

Thanks for the input, guys. A paddle bone does seem most likely, but at the same time, those depressions (and the thin ridge of bone between them) look so prominent, i just get the feeling like they are real features of the anatomy rather than some kind of deformation. Which begs the question, which phalanx has these? Like you said pachy, i don't think these are naturally found on any bone from the paddle. Which is why i am still confused. 

 

Looking at these images below (both Lyme Regis ichthyosaurs), realistically, which paddle bone would it be?

Presumably one of the more proximal bones. But again, why the prominent paired depressions if they are real features of the anatomy? And should a paddle bone be so sloped in thickness? The only other remotely possible option in my mind would be a bone from the skull. 

 

 

Edited February 9 by Paleoworld-101

[pachy-pleuro-whatnot-odon]

Actually, one of my earlier thoughts concerning this bone is that it's actually a very worn ichthyosaur vertebra, where the depressions are part of the attachment site for the neural arch, and the slope of the bone is just where it takes the typical ichthyosaurian hourglass-like shape. It's not uncommon to find halved ichthyosaur vertebrae at Lyme Regis. Hence my mentioning it being a shame that the specimen is so rolled, as this obfuscates estimation of how much of the bone has been lost, and thus what its original shape would've been.

[Paleoworld-101]

1 hour ago, pachy-pleuro-whatnot-odon said:

Actually, one of my earlier thoughts concerning this bone is that it's actually a very worn ichthyosaur vertebra, where the depressions are part of the attachment site for the neural arch, and the slope of the bone is just where it takes the typical ichthyosaurian hourglass-like shape. It's not uncommon to find halved ichthyosaur vertebrae at Lyme Regis. Hence my mentioning it being a shame that the specimen is so rolled, as this obfuscates estimation of how much of the bone has been lost, and thus what its original shape would've been.

 

Another interesting idea.

Correct me if i am wrong, but is this image below what you are seeing?

 

 

 

I can see this too, but there are still a number of issues that make me skeptical:

 

[pachy-pleuro-whatnot-odon]

13 minutes ago, Paleoworld-101 said:

Another interesting idea.

Correct me if i am wrong, but is this image below what you are seeing?

 

 

Yeah, I guess something along those lines. Though it's just another guess. Like I said, the specimen is rather worn to really drawn any easy conclusions...