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.