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.

The three-tipped femur, Trinacromerum (1888)

Phylum : Chordata
Class : Reptilia
Order : Plesiosauria
Suborder : Plesiosauroidea
Family : Polycotylidae
Genus : Trinacromerum
Species : T. bentonianum, T. latimanus, T. kirki, T. bonneri, T. osborni

  • Late Cretaceous (90 Ma)
  • 3 m long and 150 kg (size)
  • Kansas, United States (map)

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.

Trinacromerum skeleton