Actually a Polycotylid Vertebra?

[Opabinia Blues]

Not sure if this goes here or in Fossil ID but I’ll post here…
 

This is one of my latest online purchases, and one I had to snag because if this is what it’s supposed to be it’s a neat piece. But seeing as I can’t personally verify it myself, I thought I’d post here. This vertebra was being sold as Trinacromerum sp. While I doubt that just a centrum like this is identifiable down to genus, I would like to know whether or not this really is a Polycotilid vertebra. I think it’s likely because it’s a very sizable bone that’s not mosasaur, but just want to check. This fossil is, according to the seller, from the Niobrara Chalk of Kansas. And as can be seen, this fossils as pretty obviously been flattened by geology.

 

 

Thanks!

Edited December 3, 2021 by Opabinia Blues

[KansasFossilHunter]

I think this is a pretty interesting piece! It does seem consistent with the Smoky Hill member of the Niobrara. Here's an elasmosaur vertebra from near the middle of that member (~Marker Unit 10) for comparison. This one is especially 3-D. I see some similarities here, but I'm still not 100% able to pin down the taxonomy beyond "plesiosauria indet" from here. Your piece does seem to have some nice bite marks too!

 

 

 

 

 

 

[pachy-pleuro-whatnot-odon]

Yeah, I got me one of those as well (would've gone for more if they wouldn't have been so deformed), and can confirm that these are indeed not mosasaur vertebrae, as those would be procoelous, whereas these vertebrae are amphiplatean. So definitely plesiosauria.

 

(source)

 

Hard to say whether it could indeed be Trinacromerum sp., though, as I've got little reference material, plesiosaur vertebrae are generally not very diagnostic and these suffer from heavy crushing/plastic deformation. However, I believe the vendor to be rather knowledgeable on fossils from the Niobrara Formation, and would thus expect this to be a proper identification that they may have made either based on associated material or the specific unit and locality the specimen was found in. Whatever the case, one of the main reasons why I ended up buying my particular specimen over the others available is that it preserves one of the vertebral faces without too much deformation, showing it's original rounded form. This form is consistent with both pliosaur and polycotylid vertebrae, but less so with plesiosauromorph plesiosaurs, including elasmosaurs. So, based on this, I'd feel safe to narrow the identity of the specimen down to "polycotylidae indet.", and may be even Trinacromerum sp. (I'm not sure what other medium-sized polycoltylids would've been around in the unit these vertebrae were found in).