In 1913, a French naturalist described a poorly
preserved upper jaw from the Late Jurassic Period. The teeth in that jaw were large and serrated,
like steak knives - the kind of teeth that could only have come from a predator. And for decades, paleontologists thought this
fossil belonged to a carnivorous dinosaur called Megalosaurus. Which kind of made sense at the time. In the early years of paleontology, researchers
didn’t know how diverse ancient animals were, so a lot of fossils were just grouped
under the same few names. But then, in 1982, the real owner of that
jaw was identified. Turns out, experts had actually been dealing
with a completely different kind of apex predator - one that was as deadly in the water as Megalosaurus
was on land. That jaw belonged to a fully marine croc-relative
that was adapted for life in salt water, known as Aggiosaurus Now, yes, there are crocs today that do just
fine in salt water, like the very appropriately-named saltwater crocodile. But they still hang out in freshwater and
on land, too. But Aggiosaurus and its kin completely gave
that up. They lived out at sea, full time, their whole
lives. And in fact, the group that Aggiosaurus belonged
to, like many ancient marine reptiles, actually made this transition twice. But, let me tell you, taking that plunge wasn’t
easy - because, it turns out, it takes a very special set of traits to fully dedicate yourself
to life at sea. Now, Aggiosaurus was just one member of a
larger family of aquatic reptiles called metriorhynchids that lived from the Middle Jurassic to the
Early Cretaceous Period. While dinosaurs were dominating the land,
the metriorhynchids were thriving in the seas. And they might look pretty familiar, even
if you’ve never heard of them before. They probably remind you of animals like mosasaurs,
ichthyosaurs, and plesiosaurs. And while these four groups are all marine
reptiles, they actually don’t share an ancient marine reptile common ancestor. Instead, these groups all evolved independently
of one another to live in the water. Mosasaurs, for example, descended from terrestrial
lizards similar to modern day monitor lizards, while the ancestors of the metriorhynchids
were croc-relatives that lived on land! That makes all of these groups secondarily
aquatic. That means that, in each case, their terrestrial
ancestors originally evolved from aquatic animals millions of years earlier, making
this their second time around as marine animals. Which is weird, right? It sounds exhausting. So, if these animals each evolved separately,
why did they all look so similar? Well, if you’re an Eons super fan, and you probably are, you might
already know the answer. It’s another case of convergent evolution
- the same process that gave us carnivorous plants and way too many things that look like
crabs, but aren’t actually crabs. As aquatic predators, the different groups
of marine reptiles all independently developed similar adaptations to go with their similar
lifestyle. Now, for a while, even though we knew that
metriorhynchids were secondarily aquatic, we weren’t really sure how this transition
back to the water happened. But in 2018, a group of paleontologists described
a remarkable transitional fossil: It was an animal that they named Magyarosuchus. This 180-million-year-old animal had a unique
blend of terrestrial and marine traits, placing it midway between older, land-based croc-relatives
and their seafaring descendents. And this blend of traits highlights some of
the many obstacles that need to be overcome to transition from life on land to life in
the water. Like, how do you even move in the water? Meet Dakosaurus- a close relative of Aggiosaurus
and one from whom we have more complete fossils. This croc-relative had a sleek, streamlined
body that helped reduce drag in the water, making it much more hydrodynamic. It also had a tail fluke to propel itself,
as well as paddle-shaped limbs for steering and a little extra power. And even early transitional croc-relatives, like
that Magyarosuchus, had a tail fin! And, unlike their terrestrial relatives, true
metriorhynchids had no osteoderms, which are bony structures in the skin that help provide protection
and can help regulate body temperature. This helped give them maximum flexibility
for maneuvering in the water. But Magyarosuchus still had osteoderms - so
it looks like the tail fin was an earlier adaptation in the evolution of this group
than the loss of armor. Another thing land animals don’t have to
deal with? Controlling their buoyancy. Metriorhynchids had lighter skeletons that
allowed them to easily float at the surface of the water. And this was important for a couple key reasons. First: breathing - generally considered to
be important. Since these animals evolved from ones that
lived on land, they had lungs, not gills, so they needed to breathe air. Having a lighter skeleton helped them come
to the surface more easily to breathe. And second: thermoregulation. All reptiles are ectothermic, so they need
heat from an outside source to stay warm. The whole Mesozoic Era was generally a time
of warmer water and less extreme seasonal changes in temperature, which made the oceans
more hospitable for reptiles. But basking time at the surface of the water
would’ve still been important in cooler weather. Some experts also think that these marine
reptiles may have had higher metabolisms than reptiles today do, which would’ve helped
them stay warm. But these creatures had other problems to
deal with besides chilly water - like, salt. Reptiles today that live in saltwater, like
crocodiles and sea turtles, have salt glands in their eyes, nose, or mouth that remove
excess salt from their blood and prevent an overload of sodium. Metriorhynchids and other extinct marine reptiles
had especially large nasal salt glands, which we know because paleontologists have been
able to study these structures in a few incredibly rare internal casts of their snouts. These glands allowed them to efficiently get
rid of all the extra salt from taking in so much seawater and eating their salty prey. And catching that prey in the ocean, rather
than on land, meant even more specialized anatomy. Now many land predators rely on their sense of
hearing, but listening for prey in the water is an entirely different process. When we hear a sound, we’re able to tell
where that sound is coming from, based on the difference in time that it takes for the
sound to reach one ear versus the other. Ever have a moment when you can’t tell where
a sound is coming from? It's confusing, right? But sound travels much faster through water
than air, because water’s more dense! And our heads contain a lot of tissues that
are also full of water. This means that sounds reach us faster in
water, and we kind of pick them up with our whole head, not just our ears. Which makes it really hard for animals like
us to tell what direction a sound is coming from underwater. So this is where yet another adaptation came
in for marine reptiles like the metriorhynchids. In 2020, a team of researchers studied the
shapes of the inner ears of several groups of crocs and their close relatives, and found
that these shapes were distinct, depending on what kind of environment the animals lived
in. For example, crocs that were fully terrestrial
generally had long, narrow inner ear canals, while marine crocs had short, broad ones. And semi-aquatic crocs, like those alive today,
tended to be somewhere in between. The compact inner ear found in metriorhynchids
is actually very similar in some ways to what we see in modern-day whales. It’s thought that shorter ear canals may
have helped them better deal with stimuli caused by things like variations in pressure
and the movements of their heads while swimming and diving, though there is still some debate
about this. But by sorting out all these distractions,
the metriorhynchids may have been able to hear more clearly underwater and zero-in on
their targets. Ok, now that the metriorhynchids have solved
allll the problems of swimming, breathing, and eating, what if they wanted to have kids
-- yknow little metriorhyn-kids? [laughs] I couldn't resist, I'm sorry Well these guys had short limbs and small
hips, so they wouldn’t have had much muscle in these areas and likely couldn’t walk
on land at all -- so coming ashore to mate and have babies was out of the question. Instead, they probably gave birth to live
young while out at sea, just like some ichthyosaurs and mosasaurs probably did. But very few marine reptiles do this today. Now, as you can tell by now, becoming secondarily
aquatic required a lot of changes to many different parts of the body. So, why did so many reptiles do it? One simple answer? Dinosaurs. I mean, when in doubt, the answer for me is
always dinosaurs, no matter what the question is. Dinosaurs were everywhere on land, taking
up a lot of the food resources, so heading into the water meant more open niches for
reptiles to fill. And not getting eaten by a hungry dinosaur
was a plus, too. And this worked for them for over 50 million
years. But, even though the metriorhynchids and other
large marine reptiles like mosasaurs aren’t around anymore, there are still marine reptiles
alive today; there are just fewer of them and they’re less diverse than they used
to be. Out of about 12,000 reptile species, only
around 100 are considered marine, including marine iguanas, sea turtles, saltwater crocodiles,
and sea snakes. But many of these species are only found in
shallow waters and still spend some time on land, unlike the truly marine reptiles of
the past. So what happened to these sea monsters of
the Mesozoic? Well, the last of the metriorhynchids lived
in the Early Cretaceous. And we still don’t know exactly why they
disappeared -- but it may have had something to do with a drop in oxygen levels in the
oceans that was triggered by volcanic eruptions about 120 million years ago. This would make some sense, since we know
that one of the final nails in the coffin for the other groups of Mesozoic marine reptiles
-- the ones that made it all the way through the Cretaceous -- was rapidly changing ocean
chemistry after the K-Pg impact event. This opened up a niche that marine mammals,
like whales and dolphins, were happy to fill. Because, yes, they’re secondarily aquatic,
too! They became the new big marine predators,
developing similar adaptations as the marine reptiles of the past, with the added bonus
of being able to generate their own body heat. So from the Jurassic Period to today, the
problems that come with becoming secondarily aquatic seem to have had the same kinds of
solutions. And what we’ve learned is that it’s no
easy task to get your sea legs, but once you do, it’s smooth sailing. (Until you go extinct) So do you want even more science content? Then you’ll want to check out PBS’ newest show Animal IQ. Hosted by Trace Dominguez, Animal IQ features deep dives on animal minds to find out just how smart the animal kingdom really is. Like, we know that humans are clever, but can you find your friends in a crowd as well as a baby penguin? Drive a car as well as this rat? Sense Earth’s magnetic field like a fox? Head on over to PBS Terra to find out, and be sure to tell them that Blake sent you. And thanks to this month’s Eontologists,
who really croc our world: Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng,
and Patrick Seifert! By becoming an Eonite at patreon.com/eons
you can get fun perks like submitting a joke for us to read, like this one from Charles
Copley What do you call a dinosaur that only eats
the tastiest food? A connoisaur. [laughs] actually, ok. I give you points for that. That's good And as always thanks for joining me in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more evolutionary
escapades.
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