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.
Trinacromerum dates from the stage of the late Cretaceous period, about 90 million years ago, when the last plesiosaurs and pliosaurs were trying to hold their own against the better-adapted marine reptiles known as mosasaurs.
As you might expect, given its fierce competition, Trinacromerum was
sleeker and faster than most plesiosaurs, with long, powerful flippers
and a narrow snout suited to snapping up fish at high speeds. In its
overall appearance and behavior, Trinacromerum was very similar to the
later Dolichorhynchops, and was once thought to be a species of this better-known plesiosaur.