Earth’s oceans have changed a lot in the last half-a-billion years or so.
Diverse animal groups emerged, thrived, and vanished.
Entire ecosystems and seascapes have come and gone.
And nearly everything was killed off a couple of times, at least.
But through all that change, one thing has remained the same: almost as long as there
has been complex animal life in the oceans, there have been nautiloids.
This group of ancient cephalopods - squid and octopus relatives - have been a constant
presence for around 500 million years.
And we even still have species around today in the form of Nautiluses.
But around 30 million years ago, they suffered a mysterious catastrophe.
Something gradually began to push them from their former global range into a single remaining
refuge: the deep, tropical waters of the Indo-West Pacific.
But a refuge from what?
These hardy survivors had made it through everything the planet had thrown at them across
deep time, from a devastating asteroid impact to massive volcanic eruptions.
But this was a very different kind of threat: The evolution of a new group of predators
that would nearly finish them off…
Seals?
Nautiloids probably emerged in the Early Cambrian Period, but the oldest undisputed fossils
only date back around 490 million years.
And this was just 30 or 40 million years after the Cambrian explosion - when complex,
recognizably-animal fossils started becoming common in the fossil record.
And since then, nautiloids have endured and survived quite a lot - including all five
mass extinction events in this planet’s history.
Not to mention the emergence of pretty much every major group of marine predators - from
sharks, to mosasaurs and ichthyosaurs, to whales.
So, it’s fair to say this isn't their first rodeo.
And some of their most iconic traits are thought
to be ancient adaptations for dealing with these threats.
Like their famous coiled shells, for example.
These started out straighter and almost horn-shaped
in the late Cambrian.
The more predator-resistant coiled forms only evolved later, around the time that early
jawed fishes and predatory cephalopods arrived on the scene.
The heyday of nautiloids seems to have been the Ordovician Period, from around 485
to 444 million years ago, when they were at their most diverse – though they remained
pretty widespread for a long time after that.
And while their diversity has slowly declined since then, around 30 million years ago, Nautilus
fossils began to vanish from the fossil record of marine environments across the world.
This contraction of their range eventually left them confined to one small corner of
the oceans – the only place that they’re found
today: the deep waters of the tropical Indo-West Pacific.
And the exact reason for this sudden change
of fortunes has been tricky for scientists to explain.
Whatever pushed the nautiluses into their last refuge also seems to still be preventing them
from expanding out of it today.
But what could be keeping down a group with such a strong record of coping with
all kinds of environmental and ecological challenges?
Since at least 1979, the leading assumption has been that it was probably some complex
interplay of changing ocean circulation and temperature shifts over time.
But in 2019, a group of researchers began to wonder if it could be something much simpler…
Maybe even as simple as a new group of predators that the nautiluses just couldn't deal with.
Their approach to investigating this was pretty simple too.
Over the next few years, they gathered as much data as they could on the global distribution
of nautiluses - as well as their possible predators and other threats - throughout the
last 66 million years.
And when they published their analysis in 2022, they proposed that they had finally
found the culprit.
Only one key change seemed to match up with the disappearance of nautilus fossils
from any location, and that's the arrival of pinnipeds - which are seals and their relatives.
Now, pinnipeds evolved from terrestrial ancestors in the order Carnivora that became what we
call ‘secondarily aquatic.’
And the details of who exactly those ancestors were, and how their return to the water happened,
are kinda murky.
There are still some big gaps left to fill in that story.
But we do know that by about 28 million years ago, the earliest recognizable pinniped relatives
had begun splashing through the water.
Like Enaliarctos, for example, a genus that contained several extinct species with clear
adaptations for swimming, whose fossils have been discovered in coastal Oregon and California.
And as the researchers tracked the spread of pinnipeds to new coastal regions around
the world, they saw the same pattern again
and again: whenever they arrived, nautiluses vanished.
Until, by the Pliocene epoch, around 5 million years ago, nautilus fossils were gone from
everywhere apart from deep tropical waters of the Indo-West Pacific – an area notable
today for - you guessed it - not having any native pinnipeds.
Ok, but, it’s not like pinnipeds were the
first predatory group that nautiloids have had to contend with, right?
They weren't even the first mammalian marine predators.
Whales had already returned to the water millions of years earlier.
And the emergence of a few short-snouted and toothed ancient whale groups did match up
with the local disappearance of nautiloids from an area.
But that was a blip compared to the global trend that the researchers saw later with
the arrival of pinnipeds.
So why were whales less of a problem for the nautiloids than seals?
Well, the researchers proposed that it came down to their different eating strategies.
Some whales are – and were – much bigger than pinnipeds, so nautiluses were either
too small to be a worthwhile snack for them or they were filter feeders.
Other groups, like the basilosaurids, might have incorporated them into their diets, but
still didn't seem to have affected the nautiluses much upon arriving on the scene.
And we really don't see modern whales eating nautiluses today, either.
This is probably because getting them out of their shell takes a different kind of feeding
strategy than whales usually use.
Pinnipeds, on the other hand, excel at that task.
Their so-called ‘pierce-feeding technique’ allows them to catch prey with their sharp
teeth and then use suction or vigorous shaking to extract it from its shell.
Ancient seal fossils also have features of
their skulls and teeth that suggest they could do this, too.
So, it looks like they’d appeared with just
the right tools to end the reign of the nautiloids.
The researchers found that only one group seems to have been able to at least temporarily
hold out against the arrival of pinnipeds in the Oligocene epoch: a genus called Aturia.
They were the athletes of the nautiloid world,
with adaptations for especially fast and agile swimming.
Plus, they seem to have had an unusually rapid reproductive cycle.
And these traits seem to have allowed species of Aturia to survive, for a while at least,
even after the other species of nautilus had been pushed out.
Aturia stuck around until the end of the Miocene epoch, around 5 million years ago.
But eventually, their fossils vanished, too - not just from most coastal regions,
but globally.
So what happened to Aturia?
Well, the researchers proposed that they could run, but they couldn't hide.
See, Aturia was faster and more agile than your average nautiloid, sure, which gave them a
short term advantage in coping with the arrival of seals by simply being harder to catch.
But that also came with a trade-off.
Their thinner, light-weight shell meant that they couldn’t swim as deep as other groups.
So while the ancestors of modern nautiluses were able to retreat to greater depths, Aturia
just couldn’t follow them to safety.
And from the extinction of Aturia until now, the oceans have been almost totally nautiloid-free
for the first time in around 500 million years – save for the species that still thrive
in the last seal-free paradise…for now.
The marine realm is, after all, not so different from the terrestrial realm.
And like a flightless bird on an island having to deal with the sudden arrival of a new predator,
if pinnipeds were to expand their range, nautiluses
might find themselves with nowhere left to run.
This all goes to show that ancient doesn't mean invincible.
Changes of fortune can happen to any group, even one that's been a constant presence in
ecosystems for hundreds of millions of years.
Because nature is, at heart, pretty chaotic.
And from the nautilus’s point of view, that unprecedented chaos might just look a
lot like seals.
Hey so if you’re a fan of cephalopods, check
out our episode, “Where Are All The Squid Fossils?”
to find out why squids are the exception to the great cephalopod fossil record.
And thanks to this month’s Eontologists who get our seal of approval!
Gale Brown, Juan M, Jacksy Weiss, Melanie Lam Carnevale, Raphael Haase, Annie & Eric
Higgins, John Davison Ng, Jake Hart, and Colton.
By becoming an Eonite at patreon.com/eons
you can get fun perks like submitting a joke for us to read.
Here’s one from Dara.
Who is an Ediacaran's favorite poet? Emily Dickinsonia!
That was a good one, I like that one! That one's really good, ok.
Dickinsonia, hehe... ok [laughs]
And as always thanks for joining me in the Adam Lowe studio.
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