Oceans have teemed with fascinating, diverse marine animals throughout Earth’s history. Many creatures evolved and thrived, only to disappear in a natural disaster, climatic shift, or change in ocean chemistry. The oceans, which once provided shelter for these exceptional beings, now serve as the underwater museum of evolution, extinction, and survival.
In this article, we will dive into the history of some of the most intriguing marine animals that once dominated the oceans, understand the forces that led to their extinction, and reflect on the lessons they offer for today’s ocean ecosystems.
From ancient giants, such as Ichthyosaurus, to the more recent casualties, including the Caribbean monk seal and Steller’s sea cow, the list of extinct sea animals underscores the significance of nature conservation and the dangers facing marine life issues, such as ocean acidification, habitat loss, and entanglement through nets are causing problems to marine animals.
1. Dunkleosteus
Extinction date: Late Devonian Period, around 358 million years ago
Cause of extinction: Natural extinction possibly due to environmental changes, competition, and decrease in food supply during the Devonian period.
Location: Ancient oceans worldwide, primarily North America and Europe
It was an armored fish of the Devonian period, about 358 million years ago, as big as 33 feet long. Instead of traditional teeth, it had razor-sharp, bony plates in its jaw that acted like cutting edges to tear through the tough exoskeletons of other armored fish and prey on marine life. Its jaw muscles were strong enough to enable one of the strongest bites in the animal kingdom at that time. That would make it one of the best predators in the ocean during that era.
The rise of new, more efficient predators and environmental pressures such as ocean acidification and a decline in oxygen levels in the water may have all added to its extinction. Dunkleosteus and many other marine species could not adapt to ocean changes fast enough, thus disappearing from the fossil record.
This extinction event serves as a reminder of how vulnerable marine species are to shifts in the environment and highlights the importance of marine conservation today, as marine animals continue to face similar challenges from modern threats such as overfishing, climate change, and habitat loss.
2. Megalodon
Extinction date: Around 3.6 million years ago
Cause of extinction: Likely a combination of cooling ocean temperatures, habitat loss, declining food sources, and competition with other marine predators like early ancestors of the great white shark.
Location: Worldwide, in warm coastal waters and continental shelf regions.
Megalodon was considered to be one of the largest and most feared predators ever to reside on Earth, measuring an estimated 60 feet in length. This massive shark sported serrated teeth that were over 7 inches in length, easily tearing through the bone and flesh of its victims. As an apex predator, Megalodon victims included large marine mammals like whales, seals, and dolphins, plus other large fish. Its powerful jaws and very effective hunting strategies enabled it to remain the ocean’s top predator for millions of years.
Its extinction which happened about 3.6 million years ago is often assigned to a mosaic of causes with one of the main contributors being global cooling during the Pliocene Epoch. The combination of environmental shifts, food scarcity, and competition finally led to this once dominant predator extinct.
3. Helicoprion
Extinction date: Approximately 225 million years ago, in the early Triassic period.
Cause of extinction: Natural extinction likely due to environmental changes and shifts in marine ecosystems during the Permian-Triassic transition
Location: Ancient oceans, primarily around what is now North America, Eastern Europe, Asia, and Australia.
Helicoprion was an unusual prehistoric shark-like fish that lived 290 million years ago during the period of the Permian. Most famous for a bizarre, spiral-shaped jaw that has come to be called a “tooth whorl,” Helicoprion’s teeth were arranged in a spiral that may have helped it slice through soft-bodied prey like squid and other cephalopods. This jaw structure, unmatched in modern species, has long confused researchers because no other known animal shares a similar structure. The tooth whorl would be particularly effective in grabbing and cutting up prey, making Helicoprion a formidable predator when compared to any existing species.
Helicoprion roamed the oceans during a period of extreme environmental crisis known as the Permian-Triassic extinction event. This extinction event, which wiped out around 90% of Earth’s species, hit both marine animals and terrestrial life hard. The disaster was perhaps triggered by following factors, such as major climate shifts, ocean acidification, and changes in oceanic ecosystems. The drastic changes in marine environments, including the loss of habitat and disruption of food chains, contributed to the extinction of Helicoprion like many other extinct sea animals.
4. Archelon
Extinction date: Late Cretaceous Period, approximately 66 million years ago.
Cause of extinction: Likely due to climatic changes, habitat loss from shifting sea levels, and the mass extinction event that wiped out the dinosaurs.
Location: Ancient seas of North America, primarily around the Western Interior Seaway.
Archelon was the greatest known sea turtle. Such a creature could grow up to 15 feet long with a remarkable flipper spread that made this one of the most impressive marine reptiles of its day. This gigantic turtle had a leathery shell, instead of the hard shell of the modern sea turtle, which might have provided it with greater flexibility and hydrodynamics for swimming. Archelon primarily fed on jellyfish, mollusks, and other soft-bodied marine organisms, using its large beak to capture prey in the warm seas that once surrounded the ancient coastlines.
Archelon went extinct when the dinosaurs of the non-avian group became extinct at the end of the Cretaceous Period, about 66 million years ago. This was caused by extreme environmental shifts such as a catastrophic asteroid collision and volcanic activity, resulting in rapid cooling of the earth, ocean acidification, and loss of habitat.
5. Shonisaurus
Extinction date: Late Triassic Period, approximately 210 million years ago
Cause of extinction: Likely due to environmental changes and shifts in ocean ecosystems towards the end of the Triassic Period.
Location: Ancient oceans, primarily around North America (notably Nevada, USA).
Shonisaurus was one of the largest ichthyosaurs, a group of marine reptiles that resembled today’s dolphins, and could reach up to 50 feet in length. This elongated, dolphin-like body and strong tail make Shonisaurus an animal specially designed for swift swimming in the open sea, an agile and efficient predator in ancient oceans. It mostly preyed on fish and cephalopods, so it likely utilized its long snout and sharp teeth to attack fast-moving prey.
This mass extinction is believed to have been caused by major environmental changes, including climate change, changing ocean temperatures, and ocean acidification, which disturbed marine ecosystems. Such variations likely reduced food sources and altered the type of food web the ocean had and may have contributed to Shonisaurus and many other species going extinct.
6. Pterichthyodes
Extinction date: Devonian Period, approximately 380 million years ago
Cause of extinction: Likely due to environmental changes and competition with other evolving species during the Devonian Period
Location: Scotland and northern Europe.
Pterichthyodes was a tiny armored fish from the placoderm category. It was an interestingly winged-fin fish with its body covered by heavy protective plates. This ancient fish lived in fresh water and shallow ocean areas as a small sea animal without a backbone.
7. Steller’s Sea Cow
Extinction date: circa 1768
Cause of extinction: Overhunting by humans, particularly for meat, blubber, and hides.
Location: Commander Islands near Alaska and Russia.
The steller’s sea cow was one of two contemporary members of the Dugongidae family. It was the largest non-cetacean mammal, reaching sizes of up to 30 feet in length. This docile animal lived in shallow coastal waters and primarily fed on kelp and other sea plants, and helped the ecology by regulating marine plant life. It is a slow-moving, docile species that makes it very easy for humans to exploit.
They were discovered in 1741 by Europeans on an expedition headed by Georg Wilhelm Steller. It was hunted to extinction in only 27 years. As it was slow-moving, it could not flee from hunters, who wanted its meat, oil, and hide that the early European settlers dearly cherished. Their population was largely depleted from overhunting and their slow breeding cycle.
8. Thalassomedon
Extinction date: Late Cretaceous Period, around 94 million years ago
Cause of extinction: Likely due to environmental changes and competition with other marine predators.
Location: Ancient oceans North America.
Thalassomedon was a big plesiosaur with a long neck, growing up to 40 feet long. It had strong flippers for swimming well. This marine reptile was at the top of the food chain, eating fish and smaller sea creatures. Thalassomedon became extinct near the end of the Cretaceous period.”
9. Livyatan
Extinction date: Miocene Epoch, approximately 12-13 million years ago
Cause of extinction: environmental changes, competition with other marine predators, and the decline in prey species.
Location: Ancient oceans Peru.
Livyatan was a super-predatory whale of enormous size, estimated to be 60 feet long. Its huge size and teeth place it as one of the largest animals ever to be known. Livyatan hunted large marine mammals, most probably sperm whales. The most prevailing theory of how it went extinct is that the result was a match of fluctuating ocean temperatures with the rise of competing predator species such as contemporary sperm whales and a reduction during the Miocene Epoch of food availability.
10. Trilobites
Extinction date: End of the Permian Period, approximately 252 million years ago).
Cause of extinction: Permian-Triassic extinction event, which caused drastic environmental changes and ecosystem disruptions.
Location: North America, Europe, and Asia.
These arthropods had strong exoskeletons and segmented anatomies, which helped protect and support the structure throughout their journey along the sea floor. Trilobites existed in quite diverse species, with some well-suited to specific ecological niches of the old marine environments. This group formed one of the most successful and diversified assemblages of early marine organisms, flourishing in the global oceans and undergoing extensive evolutionary diversification into all sorts of forms.
Paleontologists believe that trilobites vanished during the Permian-Triassic extinction event. Most probably, these extinctions were caused by significant changes in the environment, such as global warming, ocean acidification, and changes in the chemical composition of oceans, affecting the type of ecosystems trilobites had become adjusted to. As ocean temperatures increased and oxygen decreased, the sudden changes killed many sea animals, including the trilobites. The Permian-Triassic extinction changed life on Earth, leading to the loss of many sea and land species.
11. Ichthyosaurus
Extinction date: Triassic to Early Jurassic Period, approximately 90 million years ago).
Cause of extinction: Competition with other marine reptiles, such as plesiosaurs and mosasaurs.
Location: North America, Europe, and Asia.
Ichthyosaurus was a marine reptile that had a swimming-in-dolphins way of body conformation, large eyes, and a snout like it was a long fish with full-tooth infestation. It was an apex predator that roamed the oceans hunting fish and cephalopods. The reasons for this creature’s extinction occurred over time during the Triassic to Early Jurassic Period, involved competition with other marine reptiles and shifts in environmental conditions, thereby changing the conditions and availability of prey within the ocean.
12. Caribbean Monk Seal
Extinction date: Circa 1952 but declared extinct in 2008 by the IUCN Red List
Cause of extinction: Overhunting, habitat loss, and human-induced disturbances such as fishing and tourism.
Location: The Caribbean Sea, the Gulf of Mexico, and the southern coast of the United States.
The Caribbean Monk Seal was also known as the “West Indian Monk Seal“. Its geographic spread included the Caribbean Sea, Gulf of Mexico and the western Atlantic. The Caribbean Monk Seal was last sighted in 1952 between Jamaica and Nicaragua. They fed primarily on fish, crustaceans, and mollusks.
This sub-tropical seal was driven to extinction from unsustainable hunting practices. Primarily targeted for its valuable oil and meat, these large marine mammals reached a length of up to 8 feet (2.4 meters). Sadly, their docile nature made them an easy kill while resting, birthing and nursing their pups. Its relatives, the Hawaiian Monk Seal and the Mediterranean Monk Seal are now endangered species.
13. The Great Auk
Extinction date: Mid-19th century (1844)
Cause of extinction: Over hunting, egg collection, and habitat destruction
Location: North Atlantic, including Canada, Greenland, Iceland, and Northern Europe
The Great Auk is one of the best-known cases of a marine extinction. Dubbed the “Original Penguin” for its striking black and white plumage, Great Auks were found across the North Atlantic, from Newfoundland to Norway. These large flightless seabirds were just under 3 feet (92 centimeters) and were prized not only for their meat and feathers, but also their eggs, fats and oils.
As early as 1542, European sailors were reported to indiscriminately harvest their eggs and meat. By 1785, explorer George Cartwright had predicted their imminent extinction due to unsustainable harvesting. Sadly, in June 1844 the final pair of Great Auks were killed while incubating their egg in Iceland. Incidentally, this last egg was crushed under a fisherman’s boot while trying to capture the breeding pair.
14. Mosasaurus
Extinction date: End of the Cretaceous Period, approximately 66 million years ago
Cause of extinction: Cretaceous-Paleogene (K-Pg) mass extinction event, environmental changes, and collapse of marine ecosystems
Location: Global, with fossils found in North America, Europe, Africa, and Antarctica
Mosasaurus was a massive marine reptile, reaching lengths of up to 56 feet. It was an apex predator with powerful jaws and sharp, conical teeth, preying on fish, sharks, marine reptiles, and even smaller mosasaurs.
Thriving in warm, shallow seas, Mosasaurus dominated prehistoric oceans. However, it went extinct during the Cretaceous-Paleogene K-Pg mass extinction, likely due to the dramatic climate shifts, food shortages, and ecological collapse following the asteroid impact that marked the end of the age of dinosaurs.
15. Labrador duck
Extinction date: Late 19th century (last confirmed sighting in 1878)
Cause of extinction: Habitat destruction, declining food sources, and possible over hunting
Location: Coastal waters of North America, from Canada to the northeastern United States
The Labrador Duck was a medium-sized sea duck, growing up to 23 inches long. It had distinctive black-and-white plumage and a specialized bill adapted for feeding on mollusks and small marine invertebrates. Last seen in 1878 in New York, these sea ducks migrated during the winter but preferred the coasts of Labrador, where it bred.
Unlike many extinct birds, the exact reason for the Labrador Duck’s disappearance remains uncertain. While it was not heavily hunted for meat due to its reportedly poor taste, human activities may have contributed to its decline by depleting its primary food sources. This included unsustainable egg and bird harvesting, the feather trade, a decline in food sources (i.e. shellfish), and loss of habitat due to human encroachment along the coast.
16. Sea Mink
Extinction date: circa 1860.
Cause of extinction: Highly prized for the mink fur trade.
Location: Coast of New England, New Brunswick and Newfoundland
The Sea Mink was first described by Daniel Webster Prentiss in 1903 from skull fragments, just over 40 years after it was declared functionally extinct around 1860. The last individuals, however, were speculated to have died out by 1920.
Unfortunately, not much is known about the Sea Mink. The primary cause of its extinction was its increasing desirability in the fur trade. The pelts of Sea Minks were considered high-value products used to make fur coats. Its closest living relative is the American Mink, a species half its size that occupies inland riparian habitats.
17. Eelgrass Limpet
Extinction date: circa 1929.
Cause of extinction: Diminishing food source and habitat i.e. Eelgrass plant.
The Eelgrass Limpet was a species of sea snail that had a very narrow environmental tolerange. It was native to the western Atlantic Ocean and could be found along the coast between Labrador, Canada, and New York. It was the first marine invertebrate to go extinct in an Ocean basin.
The collapse of the Eelgrass Limpet’s habitat and primary food source led to its ultimate extinction. The Eelgrass (Zostera marina), a type of seagrass, was affected by eelgrass wasting disease, caused by a pathogenic strain of Labyrinthula. The eelgrass eventually bounced back from refuges in brackish water, however the Eelgrass Limpet did not.
18. Japanese Sea Lion
Extinction date: circa 1951.
Cause of extinction: Unsustainable hunting for its skin, whiskers, oil, and internal organs.
The Japanese Sea Lion likely went extinct in the 1950s. Their geographic range spread across the northwestern Pacific Ocean along the coastlines of Japan, North and South Korea, as well as parts of Russia.
They were unsustainably hunted for their skin (clothes, shoes, and blankets), whiskers (pipe cleaners), oil (lamps), and internal organs (natural remedies). They were also captured for the circus trade. In addition to this, submarine warfare during WWII also contributed to their decline through habitat loss and degradation.
What We Can Learn from Extinct Marine Animals
As we look back at the causes of extinct sea animals, we can learn vital lessons to protect the remaining species, such as the North Atlantic right whale, bluefin tuna, and hawksbill turtle, which continue to face threats from human activity and environmental changes. Wildlife conservation is a priority for the future of our oceans, and by focusing on threatened species and protecting their habitats, we can help preserve these magnificent creatures for future generations.
Two species that are currently on the brink of extinction are the Vaquita and the Hawaiian monk seal. The Vaquita, one of the smallest living cetaceans, is a rare porpoise native to the northern Gulf of California, with fewer than 20 individuals remaining due to bycatch in illegal fishing nets.
The Hawaiian monk seal, one of the world’s most endangered seal species, is struggling to survive due to habitat loss, entanglement in marine debris, and human disturbance.
By taking action now, we can help prevent these species from suffering the same fate as the Labrador Duck and other extinct marine animals.
Ocean Insights: Hear From Our Experts
By Rida Nasir
Researcher and Conservationist, with an MSc in Environmental Science
Our modern world often distances us from the natural world, leading to a disconnect that allows us to forget the consequences of our actions on the environment. We become desensitized to extinction, viewing it as an abstract concept rather than a devastating loss. This disconnect can be compounded by a lack of historical awareness, where past mistakes and their repercussions fade from collective memory.
To combat this, we need to actively cultivate a sense of environmental history, recognizing the impact of human actions on the planet and learning from past extinctions. Preserving our natural heritage goes beyond simply protecting endangered species - it offers an opportunity to maintain a connection to our past and understand our place in the web of life.
FAQs
What is the largest extinct sea animal in the world?
An extinct species, named Perucetus colossus, a whale that swam the oceans some 38 million years ago and likely weighed almost 400,000 pounds.
How many sea animals have died?
A planetary crisis with over 100 million marine animal lives getting lost every year, and the decay of the ocean’s ecosystem.
What role do marine protected areas (MPAs) play in conservation efforts?
MPAs help protect critical habitats, support biodiversity, and allow marine populations to recover by restricting harmful human activities such as fishing, drilling, and tourism.
Why are marine species more vulnerable to extinction compared to terrestrial species?
Marine species are more vulnerable due to their reliance on specific water conditions, slow reproductive rates, and human impacts such as ocean acidification and warming temperatures.
What is the IUCN Red List, and how does it help track endangered marine species?
The International Union for Conservation of Nature (IUCN) Red List is a global database that assesses the conservation status of species, tracking their population trends and threats to help guide conservation efforts.
What is the difference between extinct and extirpated marine species?
Extinct species are those that no longer exist anywhere, while extirpated species have disappeared from specific regions but still exist elsewhere in the world.
How are species classified as endangered?
The Endangered Species Act (ESA) classifies a species as endangered if it is at risk of extinction throughout all or a significant portion of its range due to threats such as habitat destruction, overexploitation, and climate change. The process includes scientific assessments and public input.
Why are sea turtles endangered?
Sea turtles are endangered due to human activities like fishing, habitat destruction, and climate change. These threats, coupled with hunting and egg collection in some areas, have significantly reduced their populations.
Are walruses endangered?
The IUCN describes the status of walruses as unknown. But, the biggest threat facing this animal is the loss of sea ice due to climate change. They are being forced to change their behavior due to the loss of their hunting territory.
