Studies from old photos that I took at the Natural History Museum in D.C. on my birthday in 2005. There are a Basilosaurus, a Dolichorhynchops (the plesiosaurid), and a…turtle. Of some kind. Sorry, I got lucky with the second one, as the Wiki page on Plesiosauroidea has a picture of the same skeleton. The Smithsonian hasn’t done a great job at making it easy to look this stuff up.

AH! Protostega. That’s it. A lot of the critters from that exhibit apparently came from the Niobrara Formation.