During the time of the dinosaurs there lived a giant, leathery sea turtle that dined on massive molluscs and swam alongside plesiosaurs and mosasaurs…
What is Archelon?
At 4.6m from head to tail and 4m from flipper to flipper, Archelon is - by far - the largest sea turtle that has ever lived.
This prehistoric giant is more than twice the size of today’s largest living sea turtle, the leatherback sea turtle (Dermochelys coriacea).
What did Archelon look like?
Like living leatherbacks, Archelon didn’t have a hard shell. Instead, it was covered in leathery skin that, when stretched over a framework of bones, created a protective carapace. Archelonalso had a beak, like other living sea turtles, but its was hooked and reminiscent of a crude crowbar.
As well as being the largest sea turtle ever, Archelon was also one of the earliest and lived shortly after the clade (Panchelonioidea) evolved in the Early Cretaceous.
Archelon lived in the Late Cretaceous (from 80 to 74 million years ago), which was the time that many of the most iconic dinosaurs and their ancestors - Triceratops, Ankylosaurus, Edmontosaurus, and T.rex - were beginning to dominate on land.
Where did Archelon live?
Archelon is only known from fossils found in a rock formation known as the Pierre Shale. This formation stretches all the way from Pembina Valley, Canada, to New Mexico, US, and is believed to have been deposited in the Western Interior Seaway during the Campanian Age (84 to 72 million years ago) of the Late Cretaceous.
Where was the Western Interior Seaway?
The Western Interior Seaway was a large, inland sea that once split North America into two distinct landmasses, Laramidia to the west and Appalachia to the east. The land surrounding this sea was populated by many species of dinosaurs, but life underwater was just as diverse.
From other discoveries made in the Pierre Shale, it’s understood that Archelon lived alongside other giant marine reptiles such as plesiosaurs and mosasaurs, as well as a fanged, bus-sized fish known as Xiphactinus.
Based on its thick and robust belly armour (also known as a plastron), palaeontologists think Archelon was likely a slow-moving bottom feeder that preferred shallow coastal waters. Its cousins, the smaller Protostega and Protosphargis, were built quite similarly and were also leathery, but they’re thought to have spent more of their time cruising the open ocean.
Why did Archelon grow so large?
By the Early Cretaceous, ichthyosaurs and plesiosaurs were on the decline and much less dominant than they were during their heyday in the Jurassic Period. This left many niches vacant and opened the door to several new types of marine reptiles, including mosasaurs and - you guessed it - sea turtles.
Archelon belongs to a group of sea turtles known as the protostegids. This group first appeared in the Early Cretaceous and were characterised not only by their large size and leathery carapaces, but flattened bodies, short tails, and huge, oar-like flippers.
As a group, protostegids had a worldwide distribution and fossils of them have been found in Brazil, the UK, and Australia.
It’s unclear exactly why protostegids evolved to be giants, but there are several hypotheses. A large body is one of, if not the best deterrents against predation. It’s also thought larger bodies made protostegids better long distance swimmers. This effectively increased the size of their territories, allowing them to seek out more food and - importantly - potential mates.
Archelon is the largest known protostegid and is thought to have reached its gargantuan size as a result of a ‘predator-prey arms race’ in the Western Interior Seaway. As apex predators such as the 10m-long Tylosaurus - a contemporary mosasaur - grew in size, so did Archelon.
An adult Archelon may have been a difficult catch for a fully-grown Tylosaurus, but juveniles and sub-adults would have definitely been on the menu. From the moment Archelon were born, it was probably a race to see who could grow the largest the fastest - only those select few were likely to survive into adulthood.
What did Archelon eat?
Archelon was an obligate carnivore, meaning most of its nutrition came from meat.
Exactly what Archelon ate is somewhat of a mystery, but based on the shape of its beak and the makeup of its jaws, it’s thought it specialised in crushing hard-shelled prey, such as large ammonites, crabs, and other crustaceans. It has also been hypothesised that its beak was adapted for shearing flesh, like the beak of living leatherback sea turtles. This means it may have dined on jellyfish and other soft-bodied invertebrates, as well as bony fish.
How fast was the Archelon?
Compared to other protostegids and its living, leathery analogs, Archelon had relatively weak limbs and was thus a slower swimmer. That said, while it may not have been capable of sustaining high speeds, it’s thought it was at least fast enough to pursue the active prey that made up its diet.
Did anything prey on Archelon?
You’d think a seafaring creature the size of a car and the weight of an adult rhinoceros would be at the top of the food chain. Normally that would be the case, but during the Late Cretaceous the world’s oceans were home to bloodthirsty predators even larger than Archelon, and many of these monsters had a particular taste for giant sea turtles.
There are several fossils of Protostega - a close cousin of Archelon - that display bite marks made by sharks, as well as an extraordinary specimen that has a shark’s tooth still lodged in its carapace.
The fossil that Archelon was described from in 1895 also displays signs of possible predation. This giant, virtually complete specimen is missing a hind flipper and the remaining bits of bone demonstrate evidence of stunted growth, suggesting the individual sustained such an injury early in its life.
The most likely explanation behind the missing flipper is that it was bitten off by a bird while the individual was a tiny hatchling and trying to make its way from its nesting grounds to the sea. The flipper may have also been bitten off by a larger predator, such as a mosasaur, while the individual was a juvenile or sub-adult.
Whatever happened to this particular Archelon, it survived the ordeal and lived well into adulthood.
The largest Archelon ever found, a three-ton colossus known as ‘Brigitta’, is thought to have lived for more than 100 years. Just like the largest and oldest leatherback sea turtles living today, it’s likely that Brigitta and other especially large Archelon had few, if any natural predators.
Why did Archelon become extinct?
Archelon faced extinction roughly 74 million years ago, just before the end of the Cretaceous Period.
There’s evidence that ocean temperatures dropped worldwide during this time. It’s possible that giant sea turtles like Archelon weren’t able to adapt to these cooler waters quick enough and became extinct as a result.
The Western Interior Seaway, where Archelon lived, also shrunk and progressively moved southwards during the Late Cretaceous before it eventually disappeared entirely in the earliest Paleocene Period. This dramatic reduction in habitat was a death knell for Archelon and likely sealed its fate.
While several other protostegids survived deeper into the Cretaceous, the group was ultimately wiped out during the asteroid-induced mass extinction at the end of the period.
The disappearance of Archelon and other protostegids in the Late Cretaceous seems to have coincided with the increasing size of dermochelyids - a group that includes the extant leatherback sea turtle. These large, similar-looking turtles likely filled the niches left by the protostegids and prospered in their absence.
Is Archelon an ancestor of today’s leatherback sea turtles?
For a long time, protostegids and dermochelyids were thought to be sister groups belonging to the superfamily Chelonioidea. By this understanding, the leatherback sea turtle would be Archelon’s closest living relative.
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This makes a lot of sense when you look at the two giants side by side, but more recent phylogenetic studies have found they’re not closely related at all and that protostegids represent a completely separate, long-extinct lineage that originated in the Late Jurassic.
Instead of being a sister group to dermochelyids, protostegids are part of a separate branch that emerged before sea turtles (members of Chelonioidea), at least as we know them, evolved.
Yes, Archelon may look like an oversized leatherback, but it’s as far removed from this living lookalike as we are from gibbons. The similarities they share - large body size, leathery carapaces, oar-like flippers etc. - are more likely as a result of convergent evolution than having evolved from a common ancestor.
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