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 4.5 meters long and weighing
more than 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 some things
about being a giant, skin-shelled sea turtle that just work, no matter where, or when,
you lived. 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 3 meters long. It had a shell made of tough skin over strut-like
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. 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 its 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 protostegids. 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 the Chelonioidea, may have first appeared during that explosion of sea turtle diversity
in the late Cretaceous. And this is the group that living sea turtles
belong to. Which means, somehow, some of them 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 protostegids 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 the 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 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 helped make these turtles more buoyant. And it’s possible that other factors could’ve
shaped the evolution of the ossicles, too. In any event, they were part of the specialized
body plan that made 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 another change in climate. 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 light-weight shell,
it has one of longest recorded migrations of any vertebrate animal. Plus, as a gigantotherm, the leatherback’s
size, coupled 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 1200-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 protostegids, 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. We’d like to give a special thanks to Dr.
George R. Zug at the Smithsonian National Museum of Natural History for his help with
this episode. And if you found yourself thinking - where
did turtles even come from? Then be sure to check out our episode called, “How
the Turtle Got Its Shell” for more info on nature’s home on legs. And big thanks to this month’s turtally awesome
Eontologists.....Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick
Seifert! You can become an Eonite at patreon.com/eons
and get fun perks like behind the scenes content and submitting a joke for us to read, like this one, which I’ve been told to prepare
myself for. This is from Matt Jenkins A Dinosaur was helping her friend pick an
outfit. Her advice was: "Try Sarah's Tops?" Let’s never speak of that again. And as always thanks for joining me in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more fantastic
fossils.
