Mandible or Maxilla - it's complicated!

I posted a few pictures of a skull fragment and a piece of bone that I thought was a mandible. The mandible, of course, is the lower jaw. These both came out of block 1. Here is the skull piece:

Here is the other side, the probable inside

And here is what I thought was a mandible piece:

We also have a little more of this second bone that was in another block (block 2). It was prepared last year and the two pieces have now be re-united. Block 1 and block 2 broke naturally, and the bone spanned the gap across the two blocks. The smaller piece is closer to the body, so it is the proximal part. Fitting these two bones together we can clearly see the teeth at the  distal end of the jaw bone, while it apparently transitions to a groove without teeth on the proximal end:

After re-assembly is is clear that it is not clear how this bone - the supposed mandible - fits into the curved jaw. Thalattosaurs have weird jaws and tooth structures, but this is just plain not working. Here's a whole bunch of pictures from the web illustrating their strange mouths:

Vancleavea

Xinpusaurus

Askeptosaurus

Bamaolinensis

Mesosaurus and relatives

Endennasaurus

I told you their jaws were strange!

Worst of all, the best way it fits is as part of the upper jaw. This would make it another Pre-Maxilla, or the Maxilla. But is looks nothing like the two other pieces of upper jaw we have - one on the whole skull, one excavated last month! These pictures are taken of the accurate 3D printed copies as the originals have now been sent to Alaska for study.

However, I think this picture clears up some confusion. 

It is the inside of the front of the skull and the similar part of the unknown bone. They look very similar, except the new bone is smaller. There is a clear depressed notch on the right end (proximal) of the bones. I measured the distance from the left end of the bones to the beginning of this notch. The whole skull is 95 mm - the new bone is 85 mm. So we either have a younger animal, or the other sex. This might also explain the differences in profiles of the nose - sexual dimorphism? Age Progression? Natural Variation? 

With a Puzzled Perspective, Greg Carr

PS I sure like working with 3D printed reference materials - they don't break when I drop them!