Terminal Vertebra?

[ptychodus]

This was found in a creek with lots of late Cretaceous fossils. There were other vertebra nearby which approximately fit the convex face with their concave ends. They are approximately 5" long. I assume it is a marine reptile. Any thoughts?

[JohnJ]

It looks close to one of the four bones that make up a mosasaur atlas vertebra called the atlas intercentrum. Do you have photos of the other nearby vertebrae?

From Dale Russell's Systematics and Morphology of American Mosasaurs:

[Diceros]

I agree it's a mosasaur atlas element, but I'd have said the atlas centrum (at the back), not the atlas intercentrum (the bit on the bottom), or one of the two neural arches (the two side bits). It's too circular for the intercentrum. By the way, because it's so circular, I'd guess it's a mosasaurine mosasaur, like Mosasaurus or Globidens. The plioplatecarpine centra are more oval. Five inches is pretty big, so maybe even a tylosaurine.

Crocodilians today still have the atlas divided into four parts, in the old way.

[ptychodus]

Here are some other vertebra and a partial jaw which were found nearby.

[ptychodus]

Opposite sides

[Diceros]

With all mosasaur vertebral centra photographs, an ant. (the concave side) or post. (the convex side) view should be given, as the shape of the cotyle (ant.) or condyle (post.) is a lot of help with identification. They're probably easiest to set up on the concave side, but where that side's broken (as on one of yours), some wads of clay might be needed to prop it up. If possible, a ruler set on a box or book the lenght of the centrum (to be at the same level as the surface photographed, and set next to it) would help. Come to think of it, the ant. or post. ends could also be photographed by setting them on their ventral side at the edge of a table, then crouching down at that level.