Missing Marine Life main content.

Missing Marine Life

Part of the Dinosaurs: Ancient Fossils, New Discoveries exhibition.

Large dinosaurs like T. Rex are the most famous creatures that went extinct some 65 million years ago. But a wide range of other species also perished around the same time. Some of the smallest victims lived in the oceans, where many groups of single-celled organisms went extinct, along with ammonites and some other invertebrates (animals without a backbone).

Fossil ammonite


  • Ammonites: Ammonites were among the most common invertebrate animals of the ancient seas.
  • Belemnites: The squidlike creatures called belemnites were especially abundant between 200 and 140 million years ago.

Hard Hit

  • Clams: two large groups of clams went extinct, while other groups survived.
  • Coccolithophorids: More than half of the single-celled marine organisms called coccoliths died out suddenly.
  • Planktonic Foraminifera: As the one-celled organisms called forams died off, animals that ate them likely became more vulnerable to extinction.
An ammonite of the Cretaceous seas.


Adios, Ammonites

The shells of the marine animals called ammonites ranged in size from a few inches to many feet in diameter. Ammonites could float and swim--and most were fierce predators with strong jaws and sharp vision. Their closest living relatives include modern squid and the chambered nautilus.

Bye-Bye, Belemnites

The ancient animals called belemnites had long, slender bodies, numerous tentacles and an internal shell--the only part of the animal that fossilized. Scientists look to the closest living relatives of belemnites, the squid, cuttlefish, and octopus, for information on how these now-extinct animals lived. Belemnites were likely fast swimmers with hooks (instead of suckers) on their tentacles.

Clam Calamity

Mollusks like oysters, clams and scallops still exist today. But two groups of clams died out during the mass extinction around 65 million years ago. These extinct clams were notably different than all modern clams. Rudist clams, for instance, built reefs similar to coral reefs. Inoceramid clams went extinct at the same time.

Ancient Seas

During the Cretaceous period, the ancient seas were filled with now-extinct animals like ammonites and mosasaurs. Mosasaurs were large marine reptiles and ammonites were mollusks related to modern squid and the chambered nautilus.