- In 1896, a paleontologist working on fossils from South Dakota described a spectacular new discovery.

It was a giant sea turtle, the biggest fossil turtle ever described.

It lived between 83 million and 72 million years ago in the Late Cretaceous period when the interior of North America was covered by a huge sea, and at around four and a half meters long and weighing more two metric tons, Archelon, as it's known, was more than twice the size of the largest sea turtle today, the leatherback.

Like the leatherback, Archelon didn't have a hard upper shell or carapace.

Instead, it just had a framework of ribs covered by rubbery, flexible skin, but here's the thing: Archelon wasn't an ancestor of today's leatherback turtles or any other living sea turtles.

In fact, Archelon didn't even make it out of the Cretaceous, but as the Cenozoic Era started, a completely different group of sea turtles was ready to take over the world's oceans: the true leatherbacks, and they would share a lot of the same adaptations that the ancient giant had, why?

Well, it looks like there are just some things about being a giant skin-shelled sea turtle that just work no matter where or when you lived.

(mysterious music) We can start our story of the sea turtle back in the Cretaceous period about 120 million years ago, when the first true marine turtles appeared.

The planet was really active tectonically back then as the supercontinent Gondwana was breaking up.

As new, smaller continents formed, lots of new coastlines appeared, creating plenty of sea turtle habitat.

Taking advantage of these new marine habitats, turtles may have transitioned from land to sea multiple times in different parts of the world.

One of these transitions included the group of sea turtles that Archelon belonged to, the Protostegids, whose fossils have been found in Europe, Australia, and South America, and some of the earliest Protostegids had bodies like Archelon's: big, with lightweight shells, which may have let them travel long distances and spread out around the world.

Their large bodies might also have helped them maintain a more stable body temperature than most other reptiles, a phenomenon called gigantothermy.

Basically, being big means your body has more volume relative to surface area, so you lose less heat to the environment, and big changes were in the works that would give these turtles a bit of a boost.

Right in the middle of the Cretaceous, the climate warmed, and sea levels rose, creating even more sea turtle habitat.

By this time, North America was literally split down the middle, from the Gulf of Mexico to the Arctic Ocean, by a body of water known as the Western Interior Seaway, and this warm, kinda brackish water, turned out to be a great place for sea turtles, hosting an explosion of their diversity, including some of the giants like another huge cousin of Archelon, Protostega gigas.

It was smaller than Archelon, but not by much.

It was still more than three meters long.

It had a shell made of tough skin over strutlike ribs, like the leatherback, and it was more widespread, but then, well, you know how the world is, change is the only constant.

So in the Late Cretaceous, the climate cooled again, which made sea levels drop, and this, combined with the geological uplift of the Great Plains, meant that by 70 million years ago, the Western Interior Seaway had disappeared, its water now frozen in the poles.

Most of the North American sea turtles went extinct, disappearing from the fossil record.

Both Archelon and his cousin Protostega were gone before the Cretaceous period even ended, and that was the end of that first group of giant, skin-shelled turtles, the Protostegans, but a new, separate lineage was gaining a foothold, or a flipper-hold?

Anyway, this one would eventually lead to the leatherback.

These marine reptiles, a superfamily known as Chelonioidea, may have first appeared during that first explosion of sea turtle diversity in the Late Cretaceous, and this the group that living sea turtles belong to, which means somehow, some of them must have made it through one of the world's biggest changes: the End Cretaceous mass extinction 66 million years ago.

For example, fossils of a North American sea turtle called Euclastes show that it was around both before and after the extinction event.

It had a nearly complete, bony shell, and it looks to be related to today's hard-shelled sea turtles.

As for the leatherbacks, the family they belong to is called Dermochelyidae, which literally means skin shells, and this family likely split off from the harder-shelled ancestors of other living sea turtles around the Late Cretaceous.

Soon after, the superfamily Chelonioidea appeared.

Now, we're still not sure where geographically the leatherback's family came about, but there's some genetic evidence that it originated in the Indo-Pacific region, and we do know that these new skin-shelled turtles totally took off after the End Cretaceous extinction, I mean, as fast as turtles can, you know, take off.

Just like the Protostegans did 50 million years earlier, this group diversified and spread around the world, their fossils found in deposits from Japan to the UK, and this worldwide expansion of the leatherback's family showed how successful their body plan, that is, big and with a skin-covered shell, can be in the right conditions, and over time, that body plan gradually became even more specialized with a new adaptation: bits of bone embedded in a leathery shell called ossicles.

Ossicles first show up in a turtle from Saudi Arabia around 56 million years ago called Arabemys.

Then other species with ossicles kept popping up all over, from Africa to New Zealand and even Antarctica.

Ossicles may provide some degree of protection, but some researchers think that they might also play a role in calcium regulation.

Modern turtles release calcium from their shells to keep their bloodstreams from getting too acidic, and we do know that over time, these ossicles became thinner and lighter, which help make these turtles more buoyant, and it's possible that other factors could've shaped the evolution of ossicles too.

In any event, they were part of the specialized body plan that made the long migrations possible, but the early success of the leatherback's family wouldn't last long.

Only three species of these skin-shelled turtles are found in the Oligocene epoch, two in the Miocene, and just one in the Pliocene, so what happened to them?

Well, it was probably yet another change in climate.

(air whooshing) Between about 45 million and 23 million years ago, the climate got a lot cooler and drier, making the world much less favorable for sea turtles, and today, the leatherback is the only skin-shelled sea turtle left.

Like Arabemys and later skin shells, it has hundreds of ossicles in its shell, and maybe thanks to its lightweight shell, it has one of the longest recorded migrations of any vertebrate animal, plus as a gigantotherm, the leatherback's size, along with an extra layer of insulating brown fat, keeps its body temperature stable, even while migrating from its tropical breeding grounds to subpolar areas, where it forages, and the leatherback's suite of adaptations, including its thick, oily skin, allow it to dive more than 1,200 meters.

Today's leatherback is the last member of its family and the only surviving part of that second great radiation of skin-shelled sea turtles, and while it isn't related to Archelon and the Protostegans, it does resemble them in a lot of important ways, yet another reminder that body plans that just work, like being a huge, skin-shelled turtle, can show up in the history of life again and again.

(mysterious music)