Archelon: The biggest turtle to ever live

Archelon is the biggest turtle discovered to date. Find out what we know about this goliath of the shallow Cretaceous seas and discover why turtles have never grown so gargantuan again.

What was Archelon?

Archelon is part of an extinct family of prehistoric turtles called Protostegidae. Though once thought to be related to modern leatherback sea turtles, they actually belong in a group of their own.

Protostegids are superficially similar to the turtles we see today – both have large, flat front flippers for fast swimming, a small tail and a broad, shallow shell. However, unlike modern turtles, shells of protostegids were more of a frame than a fully fused structure. They also have nasal bones – the bones just above the nostrils – that have been lost in all other sea turtles.

How big was Archelon?

Archelon was the largest turtle that ever lived! The biggest specimen was discovered in South Dakota in the United States. It measures 4.6 metres long, with a flipper-to-flipper span of four metres. As well as being the biggest, it was also the heaviest at an estimated 2.2 to 3.2 tonnes.

When did Archelon live?

Archelon lived during the later stages of the Cretaceous Period, between 80 million and 74 million years ago.

The Cretaceous is the last period of the Mesozoic Era, which also includes the Triassic and Jurassic. This was the age of dinosaurs and giant marine reptiles, which means Archelon swam in the same seas as formidable predators such as Mosasaurus.

Archelon may have fought back with flipper-strikes and bites or by using its large shells to obstruct an attack. However, we know these tactics weren’t always successful, as protostegid bones have been discovered in the stomach contents of mosasaurs.

Where did Archelon live?

During the Late Cretaceous, a vast shallow sea, known as the Western Interior Seaway, cut through the middle of North America. It ran from the Arctic Ocean in the north to the Gulf of Mexico to the south, at its peak it reached 3,200 kilometres long and 1,000 kilometres wide.

Its maximum depth, however, was thought to be only 800 to 900 metres. This is shallow in terms of oceans, given the Atlantic has an average depth of more than 3,000 metres.

The whole world was warmer in the Cretaceous and the Western Interior Seaway was no exception, and its tropical waters hosted abundant marine life.

As well as mosasaurs, Archelon would have swum alongside plesiosaurs, sharks and the early crocodile-like Deinosuchus. Fish were also large at this time, such as the massive Xiphactinus, which reached four to five metres long. This diverse collection of massive creatures fed on the ocean’s abundant smaller animals too, including bivalve molluscs, ammonites, sponges and jellyfish, as well as plankton that formed the base of the food chain.

It’s likely Archelon laid eggs on beaches, like turtles do today. When they hatch, most modern sea turtles immediately go out into the open ocean. But we don’t know when this behaviour evolved.

Dr Ray Chatterji, who studies prehistoric turtles, says, “if we find evidence that baby sea turtles were hanging out in the shallows 70 million years ago, that tells us that the life history of sea turtles once looked very different, and that we have to look later for evidence of open-ocean childhoods”.

How did Archelon get so big?

The large territory of the Western Interior Seaway and the diverse range of potential prey provided perfect conditions for turtles to thrive. With a variety of larger predators swimming around too, turtles large enough to escape them had an advantage.

Dr Marc Jones, our Curator of Fossil Reptiles and Amphibians, says, “there are a lot of advantages to being big, such as increasing egg production capacity and reducing the risk of predation”.

However, he adds, “something to remember is that sea turtles should be a lot larger today, but they’ve been extensively hunted by humans. Historically, we removed the largest individuals whereas the smaller ones left more descendants.”

What did Archelon eat?

It’s hard to know exactly what ancient animals ate without fossilised stomach contents. The shape of their skulls often gives us clues, however. For example, the ancient turtle Alienochelys has a blunt, crushing-style skull, whereas Ocepechelon had an elongated snout that may have been used to suck prey out of crevices.

One smaller protostegid species in Australia was preserved with stomach contents that suggest they ate bottom-dwelling shellfish, such as bivalves and clams. However, Marc notes they may also have eaten fish, seagrass and jellyfish when they could.

He says, “some modern sea turtles, such as loggerheads, have very broad, opportunistic diets. It’s also possible that not all protostegid turtles ate the same thing – some may have specialised on sponges, like the hawksbill turtle does today.”

Ray adds, “I would really love to find stomach contents from the giants like Archelon and Protostega because they have features that make me think they might have been doing things a bit differently than their smaller relatives”.

A nest and some hatchlings would also be an amazing find, Ray adds. This is because as they grow up, turtles’ skull shape changes, reflecting adaptations to different diets.

When and why did Archelon go extinct?

Archelon went extinct at the end of the Cretaceous, along with an estimated 75% of all species at that time. This was the end of the age of dinosaurs – when an infamous giant asteroid strike spelled doom for almost all larger animals.

However, there are a few factors that meant the protostegids may have been doomed with or without the asteroid strike. This is supported by the fact that while the protostegids perished, other large marine turtles managed to survive.

Some researchers have suggested that protostegids’ frame-like shell made them more vulnerable to predation than other large turtles. However, Marc notes that some sharks today are still able to tackle sea turtles that have fused shells.

“It seems more likely that protostegids were very closely adapted to their extensive shallow sea environment, whereas the ancestors of modern sea turtles were able to make use of the open ocean – like six of the seven modern sea turtles do today,” he says.

Ray adds, “their large size probably made it hard to adapt to a dramatic drop in food availability – a big turtle has to eat a lot of food! They might have also been facing some competition from the early members of the modern sea turtle groups.”

Meet the family: What’s related to Archelon?

Several other turtle species reached giant proportions, though none quite matched the mass of Archelon.

Leviathanochelys aenigmatica

Largest specimen: 3.7 metres long

When it lived: 83.6 million to 72.1 million years ago

Fun fact: Leviathanochelys aenigmatica is Europe’s largest-ever turtle. Before its discovery, the largest fossils from the continent were only around 1.5 metres in length.

Protostega gigas

Largest specimen: 4.2 metres long

When it lived: 89.8 million to 80 million years ago

Fun fact: Protostega was the first protostegid species discovered. It was described in the 1870s by pioneering palaeontologist Edward Drinker Cope.

Alienochelys

Largest specimen: Known only from a partial skull measuring 41.5 centimetres long, giving an estimated body length of 2–2.5 metres.

When it lived: 70 million to 66 million years ago.

Fun fact: The name Alienochelys means ‘strange turtle’. This references this species’s oddly blunt skull, which suggests it may have used its powerful jaws to crush prey.

Ocepechelon bouyai

Largest specimen: Currently only known from a 70-centimetre-long skull, giving an estimated body length of around 2.4 metres.

When it lived: 67 million years ago

Fun fact: Ocepechelon had one of the most unusual skulls of any turtle ever discovered. It’s elongated with a pipette-like snout, making it a bit like the mouths of seahorses and pipefishes, which evolved for suction feeding – scientists think this may be how Ocepechelon fed too.

Does Archelon have any living relatives?

Sadly not – and turtles never got so big again! But two other key groups of sea turtles arose in the Mesozoic Era that still have relatives alive today – the hard-shelled turtles and leatherback turtles.

Despite having survived the last great mass extinction, sea turtles are now unfortunately under threat from human activity. They face many perils, including entanglement in ocean trash, pollution, strikes from boats, hunting and climate change.