How Plate Tectonics Gave Us Seahorses

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!