Gar or Champsosaur jaw

[DAS_Rex]

Received this piece lately and was a little stumped. Origin is Hell Creek Fm. in Harding County, SD.  It seems to be either a large gar or champsosaur jaw section.  I first thought it was indeed gar. Now I’m not so sure.
 

The teeth are on the outer, natural edge of the bone that curves to the under side. Also I see no sign of secondary tooth rows along the larger teeth, which have a more oval base than I’m used too for far.  The ones I’ve always seen (both fossil and living) have many secondary teeth lining the larger conical teeth. Also the large teeth were not along the extreme edge of the jaw. I wondered it this could actually be a section of Champsosaurus jaw instead?
 

Any opinion would be appreciated. 

[ThePhysicist]

I believe you have a partial Melvius thompsoni jaw, it was a large amiid and somewhat closely related to the gar. The lack of a secondary tooth row and size of the aveoli I believe comfortably rules out lepisosteids. Champsosaurus has circular aveoli.

 

Dentary fragment, scale = 1 cm. Note also the placement of the foramina.

 

Laurie J. Bryant (1988) A new genus and species of Amiidae (Holostei; Osteichthyes) from the Late Cretaceous of North America, with comments on the phylogeny of the Amiidae, Journal of Vertebrate Paleontology, 7:4, 349-361, DOI: 10.1080/02724634.1988.10011669

Edited August 30, 2024 by ThePhysicist

[DAS_Rex]

I hadn’t even thought about Melvius. I have some teeth from that species from the same area.  I do believe you are correct. The shape, details, and size would match very nicely.  The bone just screams fish to me.