Next Wednesday, October  12th is National Fossil Day,   and we’ll be doing a livestream to  celebrate and encourage folks to   support us on Patreon.

See you then for four  hours of games, Q&As, and special guests!

In 2004, a team of paleontologists was on  an expedition in the remote Canadian Arctic.

Their mission?

To scour the Late Devonian rocks  for clues about one of the most important events   in natural history: the transition of  vertebrate life from water to land.

They were looking for fossils that could bridge  the gap between lobe-finned fish that lived   around 380 million years ago, and four-limbed  tetrapods that appeared 365 million years ago.

And, after years of searching with little success,   one day they finally found  what they were looking for.

Several partially complete skeletons from  around 375 million years ago that belonged   to a large species halfway between  a fish and a tetrapod - a fishapod!

It had scales, gills, and fins like  a fish.

But, like early tetrapods,   it had primitive lungs and a flat head  with a neck.

And its stubby pectoral fins   contained bones that matched the  arm and wrist bones of tetrapods.

It was named Tiktaalik, and overnight  it became an icon of natural history   for its place at the cusp of  transition from water to land.

But evolution rarely happens in a simple straight  line – and nothing about it is inevitable.

See, there was another fossil collected  earlier on that same trip that would lie   forgotten for years while Tiktaalik got all  the attention – one that would show just that.

Because, around the time that some of  our fishapod relatives were crawling out   of the water, others were turning  around and diving right back in.

Tiktaalik and the other fishapods  are members of a group called the   Elpistostegalians that lived in  the mid to late Devonian Period.

They were flat, crocodile-shaped predators  that lived and hunted in shallow waters and,   in some cases, may have even  ventured onto land at times.

You may remember these guys from our old  episode, When Fish First Breathed Air.

And they were part of a larger group of fishes  called the Sarcopterygians, who differ from other   bony fish in having pairs of fleshy, lobed fins  connected to their bodies with a single bone.

Today, this ancient group of fish is represented  only by coelacanths, lungfish, and technically,   us, tetrapods.

Yes, from a certain point of  view, we're a type of Sarcopterygian land-fish!

Because this is the group of fishes that we can  trace our ancestry to, via the Elpistostegalians.

But exactly which ones we have to thank - or blame  - for the transition to land is still unclear.

For example, we don’t know whether  Tiktaalik is a direct ancestor of   modern tetrapods, or just a close relative.

We might actually trace our lineage to   a different fishapod lineage that  we haven't found fossils of yet.

When you’re trying to trace  a family tree back nearly 400   million years, exact relationships  become…pretty hard to pin down.

But Tiktaalik was exactly the kind of  transitional animal that we expected to   find - an almost 50/50 split of fish and tetrapod.

So even if it’s not our direct ancestor and  is more like a cousin instead, it still lets   us glimpse what our fishapod ancestors were  probably like in morphology and lifestyle.

Tiktaalik was found in  sediment from a river delta,   and while it was clearly a swimmer,  its fins were also made for walking.

Or at least, for propping itself up and pulling  its body around when the situation required.

It probably did this mostly  in the shallows underwater,   but it may have even occasionally  ventured on to land for periods of time.

Now, Tiktaalik got pretty  big at up to 2.7 meters long,   so propping itself up was an impressive feat that  it only achieved thanks to… its impressive feet.

Well, technically its pectoral fins.

These had the  beginnings of the tetrapod limb structure - with   bones corresponding to the upper arm bone,  a lower arm bone, and even parts of a wrist.

This was an important moment in the transition  from paddle-like fins to weight-bearing limbs.

And it was thought for a while that  Tiktaalik operated in front-wheel   drive mode, mostly using its new  stubby front fins to get around.

But more recently described fossils of its back   half suggest that its pelvis also  had some tetrapod-like features.

Many parts seemed to have  become bigger, more robust,   and more mobile - including the pelvic girdle,  the ball and socket hip joint, and the pelvic fin.

So its hind-fins had become pretty powerful,  which may have given it more of a four-wheel   drive mode while paddling, propping itself  up, and occasionally scooting around.

Now, it’s not every day that you find an  extinct species that fills an evolutionary   gap so perfectly, so it makes sense that it  was instantly recognized as a very big deal.

But it’s easy to look at an animal like Tiktaalik   and come away with an idea of  evolution that’s too simple.

The development of limbs from  fins and the transition to land   from water can seem like a sign of  inevitable evolutionary ‘progress’.

But the fishapods weren’t ‘leveling up’  into four-limbed tetrapods because that   was simply their evolutionary destiny – the  inexorable next advance from life as fish... Because evolution doesn't work  like that at all.

It would just   take another fishapod to show just  how messy the transition really was.

See, during that 2004 expedition, just  a few days before finding Tiktaalik,   the researchers had collected some other  fossils from a site 1.5 kilometers away.

They’d noticed a few small jaw fragments, teeth,   and scales embedded in some rocks,  which they collected to study later.

But when Tiktaalik was discovered  and captured everyone’s attention,   those other fossils were all but forgotten about.

In fact, it wasn’t until 2020  that the researchers actually   got around to scanning the material  and found that, to their surprise,   embedded in one of the rocks was a complete  pectoral fin from a new species of fishapod.

They named it Qikiqtania wakei, and  while it was both smaller and older   than Tiktaalik, it was clearly a close relative.

But it did have one other major difference.

While Tiktaalik had fins that seemed to be   evolving into limbs, Qikiqtania had fins  that seemed to be evolved from limbs.

It had an upper arm bone like Tiktaalik’s,  but it was boomerang shaped and smooth,   without the attachment sites  for muscles that Tiktaalik had.

So it wasn’t capable of propping itself up or  taking steps.

This was a full-time swimmer,   but its fin structure was also  unlike any other swimming fish.

In fact, it was so unusual, that the  researchers concluded that it was the   result of this fishapod doing an evolutionary 180.

Like Tiktaalik, its ancestors had fins  built solely for swimming that were   then remodeled into stubby weight-bearing fins.

But at that point, right when Qikiqtania’s  ancestors were on the cusp of transitioning   to land, they just…didn’t.

They  returned to the water, instead.

Their weight-bearing fins were then remodeled back   into slender fins.

This gave them  different morphology from the fins   of their ancestors - a legacy of having  gone through the stubby limb-like stage.

After all, once a fin has become more like a limb,  natural selection has to work with what’s there.

And, in this case, that meant the development  of a fin that was good enough for swimming,   but not exactly like the fins of its ancestors.

They once again became open water fish, but  their fins still had structures from a time   when they lived at the water’s  edge as something tetrapod-like.

Now, exactly what it was about life on land that   didn’t vibe with Qikiqtania’s  ancestors is still a mystery.

But its discovery shows us how  evolution really works – not in simple,   ‘March of Progress’-style linear  journeys towards a single ‘end goal’.

Instead, it’s more like a messy, branching  tree of species heading off in any number   of evolutionary directions, propelled  by all kinds of different pressures.

Some species undergo major transitions and never  look back, while others return to ways of life   from their past… But that doesn't mean that some  are ‘more evolved’ or more advanced than others.

It just means that species - even closely-related  ones - can settle on wildly different ways of   making a living.

And one fishapod’s exciting  ecological future is another’s disappointing past.

I’d like to o-fish-ally thank  this month’s Eontologists!

Annie & Eric Higgins, Chase Archambault, Colton,   Jake Hart, John Davison Ng,  and Melanie Lam Carnevale.

Become an Eonite at patreon.com/eons and  you can get fun perks like submitting a   joke for us to read.

Here’s one from Katherine G. Which dinosaur has the best  manners?

The plesiosaurus.

And as always thanks for joining me  in the Adam Lowe studio.

Subscribe   at youtube.com/eons for more primordial ordeals.

We're a type of Sarcopterygian land-fish.

This fish crawled out of the ocean,  now I have to pay rent and taxes... Ok...