This is a type of bony fish called a Coelacanth. Its fossilised skeletons have been found in rocks even older than those in Poland. It’s fins have an intriguing feature not seen in other kinds of fish. Their base is a rounded fleshy stump that looks tantalisingly like the beginnings of a leg. So scientists thought that this might well be the ancestor of all land-living vertebrates.

And then, a sensation. A living coelacanth was hauled up from the depths of the Indian Ocean and the museum has acquired several of them.

Coelacanth Fossil
Coelacanth Fossil

Here is the body of a baby coelacanth. The coelacanth female retains the egg in her body until it is fully developed. There is its yolk sac, and here is its fin and you can see this fleshy base to it here and then its fin rays. The question is, was that strong enough to enable a fish like this to haul itself out of the water and up onto land?

No, living coelacanths have been filmed in the depths of the sea. Its fleshy, muscular fins do certainly help it to manoeuvre its five-foot-long body. There is even the hint of a walking pattern, but detailed analysis has revealed that the fins are still a long way from being legs.

Ancient Coelacanth

The ancient coelacanth marked a crucial early stage in that transition, but some characteristics ruled it out as a direct ancestor of the land vertebrates.

Living Coelacanth
Living Coelacanth

All land-living backboned animals have limbs which have a basic similar bone structure. There is one bone at the top, then there are two bones and a group of bones, followed with digits. And the coelacanth’s didn’t have that structure. And then, recently, another fossil discovery was made.

Ellesmere Island lies in the icy waters between northern Canada and Greenland. The team of American palaeontologists, who shot this footage, believe that the rocks here were deposited in the right sort of environment for the vertebrates to move to land.

Dr Ted Daeschler “We learned about the sequence of rocks that formed in ancient stream systems. Our hypothesis was that it was in those sorts of environments, where limbs were being favoured over fins.”

Basic Limb Structure
Basic Limb Structure

The arrival of plants on land had stimulated a surge in life in and around freshwater swamps and this created new opportunities for the fish that lived here.

Dr Ted Daeschler “One of the niches that was being developed at the time, was for shallow-water predators. You know, which fish could find other fish that were living in the shallows, the swamps, the productive ecosystems that were just starting to appear on Earth at that time?”

Ted Daeschler and his colleagues believed that limb-like fins could have helped a fish to hunt in this kind of environment. And then, on the slopes of a barren valley, they made a thrilling discovery.

Dr Ted Daeschler investigates Coelacanth

Ted Daeschler “This was the fossil that got us really excited. We couldn’t have dreamt actually that we would find something as well preserved as this one. It’s about the front two-thirds or half of the body, as you can see, a very complete skull, and a large piece of the body, including parts of the fin.”

Dr Ted Daeschler
Dr Ted Daeschler

The team found features that matched the profile of a shallow-water predator. Eyes placed on the top of a flattened head… and ranks of sharp teeth. They gave it a local Inuit name – Tiktaalik. We can now work out from its bones how Tiktaalik moved around in those swamps and shallows.

In deep water, it must have swum like any other fish. But further examination of its bones showed that it could also move its body in a far more radical way.

Ted Daeschler “One of the really amazing aspects of Tiktaalik that we’ve noticed is this evolution of the neck. There was not a rigid connection between the skull and the rest of the body. Tiktaalik is the first vertebrate we see that has freed up the neck. And when you think about it, all limbed animals, including ourselves, would not be able to move our head independently of our shoulders if it were not for these innovations that were occurring in a form like Tiktaalik.”

A flexible neck allowed Tiktaalik to point its jaws at its prey when space was too cramped to manoeuvre its whole body. But it was the fins that provided the team with the most exciting evidence. Behind the spiny rays, there were lobe-like stumps, like those of the coelacanth. But Tiktaalik’s bones revealed a pattern that was much closer to the basic structure of limbs.

The fin of Tiktaalik

Ted Daeschler “We learned a lot about the fin of Tiktaalik from this specimen. Now, this is a cast of all the different bones that we found in association, including the shoulder girdle here. But that is the complete fin skeleton from the front fin so… I’m a lobe-fin fish, here is my front fin, we call it a limb now, but here is Tiktaalik’s front fin. We’ve got a shoulder joint and it’s very important that there’s a shoulder joint which is oriented a little bit laterally, a little bit down in Tiktaalik. Very different from an animal that’s just swimming with its fin and paddling along, this fin seemed to be oriented beneath the body.

Fossil to Excite
Fossil to Excite

So this is a humerus. We all have a humerus, that’s the first bone in the front appendage. We have an ulna and a radius. So do you and I, all limbed animals, have an ulna and radius. We have some wrist bones and we actually then have something which, like a wrist, could bend together and allow this fin to sit down and to contact the surface with a surface area. And so, when we see all of these features, we see a structure which is very much like our limbs. So here is a fish using its fin in a very limb-like way.”

Tiktaalik Fin
Tiktaalik Fin

Tiktaalik’s heavy-duty fin still helped it to swim. But if it hit the shallows, the bones and joints would help to push themselves up and punt around.

But this new limb didn’t just help mobility in the water. It became the driving force behind one of the most spectacular events in evolutionary history… the arrival of the first vertebrate animals on land.

Creatures Evolved

Over time, creatures evolved that spent most of their time out of water. They formed a new group we call amphibians. And to survive on land, they had to solve a new challenge. They had to be able to extract oxygen not from water, like their fish ancestors, but from the air.

Coming Ashore
Coming Ashore

Fish use gills to absorb oxygen into the body. In the air, gills quickly dry out and stop working.

China is the home of a rare and fascinating creature that can show us how the ancient amphibians overcame this problem.

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