Monday, September 21, 2015

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:





Mesosaurus and relatives


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!

Friday, September 11, 2015

Mandibles and scraps

I'm collecting pictures of all the mandible parts that we have found to date. A long time ago we found our first piece of mandible. This gave us the first clue that skull pieces would be found, even before the actual half-skull was found. Then we found an enigmatic piece which looks like an Ichthyosaur mandible piece, with a groove for tooth roots. I'm not sure now that is came from an Ichthyosaur since we have not found any other pieces like it. Here are pictures of both of them.

Here is the second definite mandible piece. It is split and the tooth roots are visible in the jawbone.

Latest, we have a more complete mandible in two pieces. One was excavated last year, the other piece a few weeks ago. They are both from the same bone, but were located in two blocks (block 1 and block 2) and were broken through when the blocks broke. This is the most complete fragment so far. It clearly shows that the front part of the jaw had teeth in sockets, but further toward the rear it transitioned to a grooved bone that may not have had teeth. Think of a horse, with cutting teeth in the front and molars in the rear separated by a vacant space. ( where the bit goes).  Other known thallatosaurs have weird tooth patterns so this is not surprising, I'd really like to find a whole lower jaw, or at least more toward the rear, but you get what you get on these beasts.

I wish you could see the detail in these teeth. They even have small pits in them. A retired dentist friend of mine ( Greg Gentry) from the NARG group says it's probably a case of the enamel wearing through and exposing the dentin, Since the dentin is softer, it wears away and forms a pit or depression. Anyway it's really cool and wonderful to think that these teeth were used 230 million years ago and were made the same way as current teeth, even though tey are reptile and our teeth are mammal.

 Last of all, I've been going through the back-log of bones removed from the block but not yet finished. This includes a lot of smaller fragments and backbones  - centrums. They don't tell us a lot scientifically so they have a lower priority, but we need to do them eventually. Here are a lot of rib fragments:

Ans here are a couple of centrums - the force-bearing part of the backbones. One of these is very unusual by being thin for it's diameter. We have one or two like this already. They both show the typical hourglass figure where the neural arch attached to them, so we know they are both from these thalattosaurs. 

Sincerely, Greg Carr