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