8 Extinct Ocean Animals 

Written by Becky Mathews
Updated: September 7, 2022
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Extinct ocean animals range from fish, crustacea, reptiles, mammals, and a whole host of others but what binds them together is the risk of extinction. From ocean dwellers that lived alongside dinosaurs to more modern-day animals, let’s take a look at 8 extinct ocean animals to find out what species we’ve lost and why.

1.     Megalodon (Otodus megalodon)

Megalodon hunting
Huge Megalodon sharks went extinct 2.6 million years ago

©Catmando/Shutterstock.com

Surely the most famous of all extinct ocean animals is the huge Megalodon.

Megalodons were massive sharks, so huge they were at least 20-50 times larger than today’s great whites. Experts estimate they were between 105,000 pounds to 227,500 pounds and 50-60 feet long.

Those kinds of measurements are hard to compute, so for comparison, the largest great white shark weighs about 5,000 pounds and is 15-20 feet long. A bowling alley is about 63 feet long and comparable with the mighty megalodon’s length.

Much to the relief of sailors, megalodon went extinct in the Pliocene era about 2.6 million years ago. This was a point at which the planet entered a cold phase, but recent research believes the great white shark sped up their extinction. That’s because they ate young megalodons and small whales that made up the bulk of its diet.

 2.     Short-Nosed Sea Snake (Aipysurus apraefrontalis)

The short-nosed sea snake hasn’t been spotted since 2000.

©Kate L. Sanders, Tina Schroeder, Michael L. Guinea, and Arne R. Rasmussen / Creative Commons – License

The short-nosed sea snake (known as Sahul reef snake) is a venomous snake endemic to the Ashmore Reef and Hibernia reefs on the northwestern Australian coastline.

It was up to 24 inches long with a blunt face that gave it the common name short-nosed sea snake.

They returned to the ocean surface to breathe, but could dive for up to two hours without taking a breath because their skin absorbed oxygen. Interestingly, they shed their skin more often than land snakes to rid themselves of barnacles or sea creatures that would cling on. This gave them more skin surface to absorb oxygen.

They foraged on coral reefs and for eels and fish, but experts think warmer seas, trawlers, increased boat traffic, and water contamination caused their decline.

No one has spotted one since 2000 and it’s not officially extinct. Possible reported sightings occur, but scientists think they are a different species to the Ashmore Reef residents.

It doesn’t look good for the short-nosed sea snake, but there’s hope it will turn up in a quiet undisturbed area.

3.     Mosasaurus (Mosasauridae)

mosasaurus
Mosasaurus

©Daniel Eskridge/Shutterstock.com

Another massive seafarer for our list of extinct ocean animals is the huge Mosasaurus. This ocean giant lived in the cretaceous period 145.5 to 65.5 million years ago, much earlier than the megalodon.

Mosasaurus wasn’t a shark, it was an enormous lizard that breathed air and was an apex predator of the age. Estimates suggest Mosasaurus hoffmannii, the largest known species of mosasaur, reached 56 feet, but this is debated by paleontologists who think 36 feet is more accurate.

Their massive heads contained 250 teeth and a snake-like double-hinged jaw so they could grab their surface prey from below and swallow them whole. Paleontologists have found fossilized ammonites with mosasaurus teeth marks imprinted in the shell that indicates they ate almost anything going.

Mosasaurus became extinct at the same time as land-based dinosaurs 65 million years ago, most likely through starvation caused by an asteroid strike.

4.     The Great Auk (Pinguinus impennis

The Great Auk (Pinguinus impennis) 

©Mike Pennington / CC BY-SA 2.0 – License

This flightless seagoing bird looked a lot like a penguin, but it’s not closely related. The Great Auk’s scientific name is Pinguinus impennis so when explorers discovered penguins they were given the same title due to their similarities.

The Great Auk was around 33 inches tall and weighed 11 pounds. It was black and white with a distinctive white patch between its eyes. Great Auks had hooked black beaks, small six-inch wings and they walked upright. They couldn’t fly but were incredible swimmers that preyed on fish.

Great Auk lived on the north Atlantic coast from Canada to the United States, Iceland, Great Britain, and the Iberian Peninsula. Humans preyed on them intensively for their fluffy down feathers and meat.

Great Auks bred on remote rocky islands with a slope into the sea. They were easy to catch and kill because their specific breeding ground requirements meant there were few suitable sites.

The last colony of Great Auk lived on a rocky outlet near Iceland that was inaccessible to humans. A volcanic eruption destroyed that site, so the Great Auks moved to the island of Eldey. There they were hunted until the last breeding pair was killed on 3rd June 1844.

penguin in water with open beak
Great Auk looked very similar to penguins but were a different species

©iStock.com/anyaberkut

 5.     Eel Grass Limpet (Lottia alveus)

The eel grass limpet was a marine gastropod mollusk that lived in the western Atlantic Ocean from Labrador in Canada to New York.

It was a common little species of sea snail, often called the bowl limpet, that was easily spotted in northeastern North America’s eelgrass beds.

It lived and ate exclusively on the blades of eel grass, Zostera marina, which is the reason for its extinction. Slime mold disease decimated eelgrass in the 1930s and without a habitat the eel grass limpet became extinct. This article from the National Library of Medicine explains that eelgrass was infected with a disease that re-appeared in the 1980s.

Sadly, the 1980s was much too late to save the eel grass limpet. The last known limpet was collected in 1929, but the extinction wasn’t noticed for 60 years.

6.     Caribbean Monk Seal (Neomonachus tropicalis)

Carribean Monk Seal (Neomonachus tropicalis)

©New York Zoological Society / public domain – License

Caribbean monk seals were officially declared extinct in 2008, 50 years after the last one was seen. The reason for their extinction was overhunting by humans.

This large grey-brown seal was native to the Caribbean Sea. At eight feet long and weighing 600 pounds it was almost the same size as a grizzly bear or mountain zebra.

Christopher Columbus made the first documented mention of a Caribbean monk seal in 1494. They lived around Hispaniola, but their meat, oil, and docile nature meant they were hunted relentlessly. Hunting, coupled with human competition for fish, meant they were wiped out.  

The last one was spotted between Nicaragua and Jamaica in 1952. Officials hunted for five years, but no others were found.

Its close relatives the Hawaiian monk seal and the Mediterranean monk seal are now endangered too.

What a sad story for this sweet-natured seal family.

7.     Steller’s sea cow (Hydrodamalis gigas)

Steller's Sea Cow
Steller’s sea cow was a herbivore that ate kelp and became extinct in the 1800s

©Liliya Butenko/Shutterstock.com

Stellar’s sea cow was discovered in 1791 in the Bering Sea off Alaska’s coast.

It was a large animal and named after the man who first described it – Georg Steller. Much like today’s endangered manatee, Steller’s sea cow was a herbivore that grazed on aquatic plants such as kelp.

It was large at about 30 feet with a downwards snout and a thick layer of blubber to keep it warm in the frigid seas. Its forked tail propelled it forward like a whale and it communicated through grunts and snorts.

This docile, slow-moving creature was extinct within 27 years of its discovery. it was hunted to excess for its fat and oil.

8.     New Zealand Grayling (Prototroctes oxyrhynchus)

New Zealand Grayling

©Frank Edward Clarke / public domain – License

The New Zealand grayling is an extinct fish that migrated between fresh and saltwater habitats on New Zealand’s coast.

It was a beautiful fish that began life silver, darkened to brown with a lighter belly, and on occasion turned gold. It measured up to 17 inches and was called ‘Pokororo’ by native Maori who relied on it as a food source.

New Zealand graylings were numerous in the 1800s but declined in the 1900s. They were last seen in 1923. In 1951 it became the first native freshwater fish to receive legal protection, but due to a lack of sightings, it was officially declared extinct in 2018.

Scientists don’t know for sure but they suspect modern methods of overfishing, freshwater pollution, and the effects of deforestation on waterways led to its extinction.

That concludes our list of 8 extinct ocean animals, but it could have been much longer.

We’ve lost a lot of ocean species over the past century alone. There are 72 marine animals on today’s critically endangered list, such as Hawksbill turtles whose coral reef habitats are so threatened that their future is looking very dim.

Whereas we can’t go back in time (yet!) to save the dinosaurs, this list is a clear indication that human hunting and environmental destruction can cause devastation to today’s animals and the environment.


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About the Author

I’ve been a professional writer since 2014 with special interests in the environment, particularly endangered animal and plant species. I graduated from the University of Reading and the University of Oxford, UK with qualifications in history and archaeology. Outside work I rehabilitate injured wildlife, grow heritage plants and wildflowers in my allotment garden, and play the piano badly.

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