Fossils of the 2011 National Fossil Day Artwork

NFD 2011 artwork oval mosasaur
The 2011 National Fossil Day artwork features a mosasaur, a large marine reptile, preying upon an ammonite, a relative of the modern chambered nautilus. Though mosasaurs lived during the same time period as dinosaurs, they were not dinosaurs. Dinosaurs were land-dwelling; mosasaurs were aquatic. A fearsome predator of warm Cretaceous seas, the mosasaur would have dined regularly on these cephalopods, as indicated by numerous fossil ammonites found with mosasaur bite marks. Mosasaurs ate more than just ammonites. Fossil mosasaurs have been found with sea birds, fish, and even smaller mosasaurs in their guts. Mosasaur fossils have been found in rocks of Cretaceous age in many states across the country and around the world.
Cretaceous sea mural
Cretaceous seas teemed with life, many of these animal groups are now extinct. In the center of this mural is a mosasuar chomping on an ammonite. Note the ink being released by the ammonites, a characteristic of some modern cephalodpods. Another mosasaur is in the bottom right. Diving birds are in the upper right. A crocodilian is below the mosasaur. The animal in the bottom left is a dolphin-like marine reptile called an icthyosaur.

Image courtesy of the Smithsonian Institution, National Museum of Natural History.

High sea levels during the Cretaceous meant mosasaurs and ammonites had access to a large geographic area and their fossils have been found on nearly every continent. One of the best places to find ammonites in the United States is in North and South Dakota's Pierre Shale, where mosasaurs are also common. The Dakotas, as well as most of the other Great Plains states, were flooded by the Western Interior Seaway, a warm, shallow sea that split the North American continent in half. Also known as the Cretaceous Interior Seaway, this body of water was created as a result of tectonic forces pulling the middle of the continent down, which was then flooded by the globally high sea levels of that time period. Ammonites lived from the late Paleozoic through the Mesozoic eras (approximately 400 to 65 million years ago). Mosasaurs have only been found in rocks of late Cretaceous age (between about 100 and 65 million years ago).
ammonite display
This ammonite displays bite marks (line of small circles) from a mosasaur attack. Did the mollusk escape, or did it become dinner?

NPS photo of Smithsonian National Museum of Natural History specimen.

Fossil Facts:

  • Mosasaurs are reptiles, but were not dinosaurs.
  • Ammonites are members of a class of organisms known as cephalopods, meaning "head-foot". This group also includes the modern octopus.
  • Mosasaurs ranged in size from about 11 to 57 feet long, while ammonites could vary in size from an inch or two to 6 feet in diameter!
  • Ammonite shells increased in size and complexity through time, perhaps contributing to their extinction at the end of the Mesozoic.

Word origins:

  • Mosasaur: mosa refers to the Meuse River in Holland, where the type specimen was found. sauros is Greek for "reptile".
  • Ammonite: Early scientists referred to ammonites as ammonis cornua, meaning "Ammon's horn", referring to the Egyptian god Ammon. Ammon was commonly depicted with ram's horns, which are coiled like ammonites' shells.

Go See Them!

Ammonites are quite common in the fossil record and can be found in a number of locations across the country. Some National Park units where ammonite fossils have been found include Dinosaur, Carlsbad Caverns, Glacier, Glen Canyon, Grand Teton, Guadalupe Mountains, Mesa Verde, Niobrara, Yukon-Charley Rivers, and Zion, among others.

Mosasaurs have been found throughout the Great Plains region, including at partner sites Ladonia Fossil Park, Yellowstone National Park, and Mesa Verde. They also reside in the collections or exhibits of the Fick Museum, Kansas University Museum of Natural History, Museum of the Rockies, Sternberg Museum, South Dakota School of Mines Museum of Geology, Tate Museum, Texas Natural Science Center at the Texas Memorial Museum. Most of the states where these museums are located would have been covered by the Cretaceous Interior Seaway and thus, would have been home to these aquatic beasts. For more information on these partners, click on their links from our Partners tab.

Learn more about National Fossil Day and the NFD Logos and Artwork on the official National Fossil Day website.

Part of a series of articles titled National Fossil Day Logo and Artwork – Prehistoric Life Illustrated.

Last updated: September 8, 2020