Ammonite, originally uploaded by elrina753.

This awesome replica of a turrilitid ammonite is from the Cretaceous Seas diorama at the American Museum of Natural History in New York City. I don’t know exactly which type of turrilitid this is, but my best guess is the genus Pseudhelicoceras.

Members of the ammonite family Turrilitidae are characterized by shells that are not typical tight spirals—a condition known to paleontologists as heteromorph. It isn’t clear what ecological niche the turrilitids filled, but at least some species are thought to have drifted up and down in the water column. They lived world-wide during the late Cretaceous period, but, like all ammonites, they went extinct in the same global catastrophe that killed off the dinosaurs 65 million years ago.