Don’t forget! Our 2023 calendar, featuring some of our favorite “main characters of
paleontology” from classic Eons episodes, is available now at ComplexlyCalendars.com!
And exciting announcement, we also just released a puzzle!
For our first-ever puzzle, we’re celebrating the process of carcinization
with a reconstruction of what’s thought to be the oldest false crab: Platykotta.
It’s available now at DFTBA.com!
Nestled in the hills of Slovenia, just west of a small village, a stream has cut
a ravine down through the earth, revealing layers of rock and a rich array of fossils.
Tiny algae, mollusks, fish, and insects are preserved there,
along with plants like sea grasses, and leaves from trees like pines and willows.
These fossils date from the Miocene epoch, around 11.6 to 12.7 million
years ago, and paint a picture of a shallow, seaside bay near a forest.
And in the mid-2000s, researchers realized that this bay was once full of seahorses.
Paleontologists have unearthed more than two dozen
seahorse fossils at this site – the oldest ones known so far.
But, what were seahorses doing there, exactly? Most of Slovenia today isn’t exactly ocean-front
property. And recent genetic analysis suggests that seahorses didn’t originate there.
Today, they’re found in all of the world’s oceans.
But open-ocean swimming is a real challenge for an animal better suited
to clinging to a stalk of coral or a blade of seagrass than swimming long distances.
So how did one of the ocean’s worst swimmers spread around the globe? And where did they
come from in the first place? It turns out, it all comes down to plate tectonics.
Seahorses are pretty unique-looking fish, I'll give you that, with their upright posture,
long prehensile tails, and horsy-looking heads.
Yet, despite their weirdness, they are still fish. They belong to a group called syngnathids,
which includes seahorses, seadragons, and pipefishes.
And while fossil seahorses are rare, fossils of other syngnathids are more common,
especially around the Mediterranean Sea.
The oldest fossil from this group discovered so far is called Prosolenostomus. It dates
back to 48 to 50 million years ago, from the Eocene epoch of what’s now northeastern Italy.
But if we compare the genomes of living species and use the differences between
them as a kind of evolutionary stopwatch, then we know that syngnathids likely split
from other related groups much earlier, probably around 87 million years ago.
And this older genetic signal makes sense when we compare it to the fossils…
By the time Prosolenostomus appeared, we already
see some of the iconic adaptations of modern seahorses and pipefish.
These are things like fused jaws, no pelvic fins,
and a body covered in bony plates, rather than scales.
They had even probably already evolved
the male brooding behavior that seahorses are so famous for, too.
Male seahorses have a brood pouch, but all modern syngnathid dads actually carry their babies to
some extent, too – even if it’s just sticking eggs to their belly skin, like the snake pipefish does.
Turns out the ocean is full of cool fish dads.
And, the fact that so many species do this means that male brooding is likely
an ancestral condition, rather than something that independently cropped up multiple times.
So by the time of Prosolenostomus,
there were already species that shared a lot of the adaptations of seahorses,
but none had that classic seahorse shape yet… It’d take something else for that to happen.
By the Oligocene epoch, beginning 34 million years ago, the ancestors of seahorses were able to
spread from their home base in the Mediterranean region to other areas, including Southeast Asia.
And here’s where plate tectonics enters the evolutionary story of seahorses.
Throughout at least the last 55 million years, the ocean floor around Southeast Asia,
especially around Indonesia, has been an absolute mess of plate tectonics.
And maybe the most important shift happened around 25 million years ago,
near the end of the Oligocene.
As the Australian plate under what's now New Guinea pushed northwards,
it slammed into a mish-mash of plates under what’s roughly the modern Philippines,
as well as the stable, southeast tip of the massive Eurasian plate.
And this enormous geologic impact transformed what had been a deep
channel between the continents into many large, shallow seas.
And this may have been a big deal for the ecosystem there, as being closer to the
surface would bring a host of changes to the seabed, like exposing it to more sunlight.
Different types of life could thrive in these new conditions,
including corals, and especially important for seahorses, sea grasses.
With more sunlight, it was easier for seagrass to photosynthesize.
And as plate tectonics gradually brought up the seafloor,
local seagrass beds could expand their range.
And it’s these seagrass meadows that we may have to thank for modern seahorses.
Some scientists have hypothesized that the conditions in these seagrass beds
could have pushed the syngnathids to transition from a long-and-straight
posture to the upright posture we see in seahorses today.
For one thing, the beds may have supported the
kind of ambush hunting that seahorses use to grab food.
It might be kind of funny to think of seahorses as predators, but they typically hunt by lying in
wait among corals, sea weeds, or sea grasses, and snapping out at small prey passing by.
Being upright actually gives seahorses a longer reach when striking, and this
posture may have helped them blend in and camouflage among the plants, too.
Also, losing their long fishy tail in favor of a prehensile one could’ve
allowed them to not just float near seagrass, but actually grab onto it.
And this might’ve helped them save energy instead of having to constantly
tread water to stay in place. While we don’t have fossils that
show this transition, seahorse DNA supports this seagrass hypothesis.
For example, the ancestors of seahorses seem to have split off from their closest relatives
around the same time as these seagrass beds appeared, roughly 25 million years ago.
And when we compare the genetics of species from around the world,
we see that the oldest and most diverse lineages come from the area around
what’s now Indonesia. This suggests that this is where they originated.
So it seems that early syngnathids, which looked more like pipefish,
may have evolved into seahorses thanks to an ecosystem created by plate tectonics.
But while it may be easy to think of the ocean as a single biome,
it’s actually made up of many different ones, like coastal shallows, reefs, and the open ocean.
And, like biomes on land, geologic activity can alter these areas in big ways, in this
case through things like changing light levels, altering currents, and generating new habitats.
And a new habitat can be enough to push an existing lineage to
take on new – sometimes strange – forms.
But while these Oligocene seagrass beds might explain how seahorses evolved in the first place,
the question remains: how did they get from Southeast Asia to the rest of the world?
Well, it looks like plate tectonics is behind this, too.
While seahorses are not great swimmers, they are champion rafters. They can hold
onto clumps of seagrass or seaweed traveling along in ocean currents with their tails.
And we can chart their expansion by analyzing the genomes of living species. Their genetics
suggest they first moved westward from Southeast Asia along with the current.
Today, this strategy would take them as far as the east coast of Africa before
they’d need to turn south and go around the Cape of Good Hope – which is possible,
and some later lineages might have done that, but it’s difficult.
It turns out, though, that early seahorses didn’t need to.
At this time, Africa hadn’t yet collided with Eurasia,
which left an opening that they could travel through.
This now-closed seaway is known as the Tethys – a
kind of a proto-Mediterranean – and it allowed seahorses to return back
to the syngnathid homeland as well as travel straight into the Atlantic Ocean.
They could then raft across the Atlantic, and through the gap between North and South
America to enter the eastern Pacific, in a series of seahorse shortcuts.
From their origins in the seagrass beds of Southeast Asia, to their journey around the globe,
the evolution of seahorses has been shaped by the movements of our planet’s tectonic plates.
And while we often think about their effect on the lives of terrestrial animals,
they play just as big a role in creating the patterns of diversity that we see in our oceans.
So, if it wasn’t for plate tectonics, seahorses would never have been able to take over the world.
Why the long face? We’re here to thank this month’s Eontologists!
Jacksy Weiss, Colton, Melanie Lam Carnevale, Chase Archambault, Annie & Eric Higgins,
John Davison Ng, and Jake Hart. Become an Eonite at patreon.com/eons
and you’ll get fun perks - like submitting a joke for me to read. Here’s one from Hosni.
Why does a moon rock taste better than an Earth rock?
Because it's a little meteor.
We were overdue for a good joke, I feel like. That's a good one.
And as always thanks for joining me in the Adam Lowe studio. Subscribe
at youtube.com/eons for more ancient adventures.
Turns out the ocean is full of cool fish dads.
I'm just so happy I get to say that!