Ammonites were extinct shelled sea animals related to modern squids, octopuses, and nautiluses. They lived in ancient oceans for millions of years and are famous for their beautiful spiral shells. Ammonites disappeared at the end of the Cretaceous Period, about 66 million years ago.

Anomalocaris was a strange ocean predator from the Cambrian Period. It was not a dinosaur, fish, or reptile, but an early arthropod relative with large eyes, swimming side flaps, two grasping front appendages, and a round mouth. It was one of the biggest predators of its time.

Archelon was a giant extinct sea turtle from the Late Cretaceous seas of North America. It was not a dinosaur and not a land tortoise. This enormous marine turtle had powerful flippers, a leathery shell-like covering, a hooked beak, and swam through the Western Interior Seaway while mosasaurs, plesiosaurs, sharks, and toothed diving birds shared the water.

Basilosaurus was a long-bodied ancient whale from the Late Eocene. It was not a dinosaur or a lizard, even though its name means king lizard because early scientists first misunderstood its fossils. Basilosaurus lived in warm seas, had sharp teeth, tiny external hind limbs, and was a top ocean predator before modern whales evolved.

Cladoselache was an early shark-like fish from the Late Devonian Period. It was not a dinosaur or marine reptile, but an ancient cartilaginous fish with a streamlined body, forked tail, smooth grasping teeth, and famous fossils from the Cleveland Shale of Ohio.

Dakosaurus was a marine crocodyliform from the Jurassic and Cretaceous seas. It was not a dinosaur, but a sea-going relative of crocodiles with a deep skull, sharp serrated teeth, flipper-like limbs, and a body adapted for swimming after prey in ancient oceans.

Dorudon was an ancient whale from the Eocene seas. It was not a dinosaur, fish, or modern dolphin. This basilosaurid whale had a streamlined body, sharp teeth, small hind limbs, and swam through warm oceans such as the ancient Tethys Sea, where it lived alongside the much larger Basilosaurus.

Dunkleosteus was a giant armored fish from the Late Devonian Period. It was not a dinosaur, shark, or marine reptile, but a placoderm with a heavy armored head and sharp bony jaw plates. This prehistoric predator lived long before dinosaurs appeared.

Elasmosaurus was a long-necked marine reptile from the Late Cretaceous Period. It was not a dinosaur, but a plesiosaur with a tiny head, many neck bones, a broad body, and four flippers. It lived in ancient seas that once covered parts of North America.

Eurypterids were extinct aquatic arthropods often nicknamed sea scorpions. They were not true scorpions, dinosaurs, fish, or reptiles, but relatives of the chelicerate group that includes spiders, scorpions, and horseshoe crabs. Some eurypterids were small, while a few giant species became some of the largest arthropods ever.

Hallucigenia was a tiny spiky animal from the Cambrian Period. It was not a dinosaur, fish, or reptile, but an extinct worm-like animal related to velvet worm relatives. Hallucigenia became famous because early scientists were not sure which side was up, which end was the head, or how its strange body worked.

Helicoprion was a strange shark-like cartilaginous fish from the Permian Period. It was not a dinosaur or true modern shark, but a prehistoric relative of chimaeras with a famous spiral tooth whorl in its lower jaw. That tooth spiral puzzled scientists for many years.

Hesperornis was an extinct toothed bird from the Late Cretaceous seas of North America. It was not a dinosaur in the everyday “T. rex” sense, but birds are part of the dinosaur family tree. Hesperornis could not fly, had tiny wings, used powerful legs and feet for diving, and chased fish in the Western Interior Seaway.

Ichthyornis was an extinct toothed bird from the Late Cretaceous seas of North America. It was not a pterosaur and not a modern gull, though it probably lived a seabird-like life. This flying bird had sharp teeth in its jaws, a small beak at the tip, strong wings, and hunted over the Western Interior Seaway.

Ichthyosaurus was a fish-shaped marine reptile from the Early Jurassic Period. It was not a dinosaur, but an ocean-living reptile with a streamlined body, flippers, sharp teeth, and large eyes. Fossils from Europe help scientists study how some reptiles became fast swimmers in ancient seas.

Kronosaurus was a giant marine reptile predator from the Early Cretaceous Period. It was not a dinosaur, but a short-necked pliosaur with a massive head, powerful jaws, sharp teeth, four flippers, and fossils famous from Australia. It hunted in ancient oceans and inland seas.

Leedsichthys was a gigantic prehistoric fish from the Jurassic Period. It was not a dinosaur, shark, or marine reptile, but a huge bony fish that likely filter-fed on tiny plankton. Its fossils are often fragmentary, which makes its exact size hard to measure.

Liopleurodon was a powerful marine reptile from the Jurassic Period. It was not a dinosaur, but a short-necked pliosaur with a large head, sharp teeth, strong jaws, and four flippers. It hunted in ancient seas and is one of the best-known pliosaur predators.

Livyatan was a gigantic extinct sperm whale relative from the Miocene seas. It was not a dinosaur and not the same as a modern sperm whale. Livyatan melvillei had massive upper and lower teeth, a huge skull, and likely hunted large marine animals, making it one of the ocean’s most powerful prehistoric predators.

Megalodon was a gigantic prehistoric shark, not a dinosaur or marine reptile. It lived millions of years after the non-bird dinosaurs went extinct. Megalodon is famous for enormous serrated teeth, powerful jaws, and fossil clues that show it hunted large ocean animals.

Mosasaurus was a giant marine reptile from the Late Cretaceous Period. It was not a dinosaur, but a sea-living relative of lizards and snakes. Mosasaurus had powerful jaws, sharp teeth, flippers, and a long body built for hunting in ancient oceans.

Nothosaurus was a semi-aquatic marine reptile from the Triassic Period. It was not a dinosaur, but an older sea-going reptile with a long neck, long jaws, pointed teeth, strong limbs, and webbed feet. Scientists think it may have lived a bit like a seal, hunting in water and resting near shore.

Onchopristis was an extinct sawskate, a shark-and-ray relative with a long saw-like snout called a rostrum. It lived during the Cretaceous Period in rivers, coastal waters, and shallow habitats. Fossils show its rostrum carried barbed tooth-like structures along the sides.

Opabinia was a strange Cambrian sea animal from the Burgess Shale of Canada. It was not a dinosaur, fish, or reptile, but an extinct arthropod relative with five eyes, a flexible front proboscis, side swimming flaps, gills, and a tail fan. Its odd body helped make it one of the most famous animals of the Cambrian Period.

Orthoceras was a straight-shelled cephalopod from ancient seas. The name is often used for fossils of long, straight nautiloid shells. These animals were related to modern nautiluses, squids, and octopuses, but they lived in hard cone-shaped shells instead of soft bodies alone.

Pelagornis was a giant extinct seabird with long wings and strange bony tooth-like spikes along its beak. It was not a pterosaur and not a dinosaur. The famous species Pelagornis sandersi may have had the largest wingspan of any known flying bird and glided over ancient oceans around 25 million years ago.

Plesiosaurus was a long-necked marine reptile from the Early Jurassic Period. It was not a dinosaur, but a sea reptile with a small head, sharp teeth, broad body, short tail, and four flippers. Fossils from England helped make Plesiosaurus one of the classic prehistoric sea animals.

Protostega was a giant extinct sea turtle from the Late Cretaceous. It was not a dinosaur and not the same as Archelon, though both were enormous prehistoric turtles. Protostega gigas swam through the warm Western Interior Seaway of North America using long flippers like underwater wings.

Sea scorpions were extinct aquatic arthropods called eurypterids. They were not true scorpions, dinosaurs, fish, or reptiles. Some lived in seas, while others lived in brackish or freshwater habitats. A few giant kinds became among the largest arthropods ever known.

Shonisaurus was a giant marine reptile from the Late Triassic Period. It was not a dinosaur, but an ichthyosaur, a group of ocean reptiles with streamlined bodies and flippers. Fossils from North America show that some ichthyosaurs grew to whale-like sizes in ancient seas.

Stethacanthus was a shark-like prehistoric fish that lived before and during the Carboniferous Period. It was not a dinosaur, but a cartilaginous fish famous for a strange flat dorsal fin on mature males. That fin looked a bit like an ironing board covered with tiny tooth-like spikes.

Tiktaalik was an extinct lobe-finned fish from the Late Devonian Period. It was not a dinosaur, but a famous transitional fossil with fish features and some limb-like traits. Tiktaalik lived in shallow water and helps scientists study how some vertebrates moved toward life on land.

Trilobites were extinct marine arthropods that lived in ancient oceans for hundreds of millions of years. They were not dinosaurs, fish, or reptiles. Trilobites had hard outer shells, segmented bodies, and three lengthwise lobes that gave them their famous name.

Tylosaurus was a giant marine reptile from the Late Cretaceous Period. It was not a dinosaur, but a mosasaur related to lizards and snakes. Tylosaurus had a long body, powerful jaws, sharp teeth, flippers, and a pointed snout for hunting in ancient seas.

Xiphactinus was a large predatory fish from the Late Cretaceous Period. It was not a dinosaur, shark, or marine reptile, but a huge bony fish with sharp teeth, a powerful body, and fossils famous from the ancient Western Interior Seaway of North America.