Episode 77 – Fins to Feet: The Fish-Tetrapod Transition

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Over 350 million years ago, vertebrate animals made one of the most dramatic evolutionary transitions in the history of life on Earth. A full-body anatomical makeover allowed one branch of the fish family tree to conquer the continents, setting the stage for a 300-million-year reign of vertebrates on land. In this episode, we discuss the Fish-Tetrapod Transition.

In the news
In New Zealand: a really old penguin hints at how these odd birds got started
A new fossil whale and an extinct form of whale locomotion
In amber: lice feeding on dino-feathers, 100 million years ago
Cambrian arthropods preserve what appears to be brains and nerve tissue

Tetrapods: the weirdest fishes
All amphibians, reptiles, birds, and mammals are tetrapods. All of these animals are descended from the earliest four-legged vertebrates; indeed, the word tetrapod means “four feet” (note that plenty of tetrapods no longer have four feet – snakes, whales, etc. – but are still tetrapods by ancestry).

But the first vertebrates were fish. They evolved over 500 million years ago and diversified into an incredible variety. Most of the fish in our modern world are either cartilaginous fish (including sharks and rays) or bony fish (including pretty much everything else). Most of those bony fish are Actinopterygii, the ray-finned fish. As the name implies, these fish’s fins are supported by thin bony rays.

But there is another group of bony fish: the Sarcopterygii, or lobe-finned fish. These fish have fleshy fins with a single bone attaching the fin to the rest of the skeleton – evolutionarily comparable to your own humerus and femur! These fish first evolved by the Early Devonian, over 400 million years ago, and they have three groups of living descendants: coelacanths, lungfish, and tetrapods.

Sarcopterygii: the lobe-finned fish. Represented in the modern world by coelacanths (top, image by Alberto Fernandez Fernandez), lungfish (bottom left, image by George Berringer Jr.) and tetrapods (bat, bottom right, image by PD-USGov). Images from Wikimedia Commons.

The transition from early lobe-finned fish to true tetrapods occurred during the Middle to Late Devonian. Plants and arthropods had already built early land-based ecosystems (not to mention microbes), and there were resources to exploit for intrepid vertebrates. From around 390 to 360 million years ago, the fossil record shows the incredible transition from fins to limbs, from fish-like faces to salamander-like snouts, and from water-bound fish to land-worthy tetrapods.

The fish-tetrapod transition, in phylogenetic view. Image from Clack, 2009, Springer Science + Business Media LLC.

The most famous aspect of the transition: fins to feet! From the disorganized mess of bones in a lobe-finned fish fin to the simplified and specialized foot of a tetrapod. Image from Clack, 2009, Springer Science + Business Media LLC.

Tetrapod-like fish

The earliest sarcopterygians had arisen by the Early Devonian, over 400 million years ago. And by the Late Devonian, this group included a diversity of fish with suspiciously tetrapod-like features. Here are a few famous examples.

Eusthenopteron, from Canada, was one of the first tetrapod-like fish discovered. In addition to some unusual skull features, it had paired bones in its fins that appear to be the evolutionary precursors of the radius-ulna and tibia-fibula!

Panderichthys, from Europe, is one of the earliest known tetrapod-like fish to have a flatter, longer snout, similar to what we’ll see in the earliest true tetrapods.

Tiktaalik, from Ellesmere Island, is the famous “fishapod” discovered in 2004. Exceptionally well-preserved fossil remains reveal that it had a generally fish-like body (complete with true fins and fishy scales) combined with very tetrapod-like features including front limbs strong enough to do a push-up! Also unlike fish, it’s shoulder was detached from its head – it had a neck!

The famous “fishapod” Tiktaalik roseae. Pictured here coming out of the water, though how much time this animal actually spent on land is unknown. Image from Zina Deretsky, NSF via Wikimedia Commons.

Fish-like tetrapods

By the latest Devonian, those not-quite-tetrapod fish had given rise to true limbed tetrapods. Animals like Elginerpeton, Ventastega, Tulerpeton, and the very recently named Parmastega have limbs for walking, complete with strong shoulders and hips at one end and toes at the other. They also have heads shaped like salamanders or lizards, with long snouts and large eyes.

The most famous of the early tetrapods are Acanthostega and Ichthyostega, both from Greenland, both having lived right at the end of the Devonian, both known from well-preserved fossils, and both discovered in the early to mid-1900s. They had muscular limbs with lots of toes (7 per foot in Ichthyostega and 8 in Acanthostega!).

A reconstruction of the skeleton of Acanthostega. Image by Ryan Somma via Flickr.

But these early tetrapods were still seemingly aquatic. Most of them are found in ancient coastal environments. The limbs of Acanthostega appear to have been paddle-like, while Ichthyostega had an inner ear specialized for underwater hearing. From their well-developed bony limbs to their salamander-like faces, these were true tetrapods, but they hadn’t left the oceans yet.

The Devonian ends catastrophically, but in the Early to Middle Carboniferous Period, we finally see the earliest truly terrestrial vertebrates in animals like Casineria. Soon, the land is covered in early amphibians – temnospondyls, anthracosaurs, and more – the tetrapod takeover had begun.

And here’s that SciShow video we mentioned about How Tongues Helped Vertebrates Conquer Land.

Learn More

The origin of tetrapods at Understanding Evolution is a great, quick, non-technical overview of the transition.

The Fish-Tetrapod Transition: New Fossils and Interpretations, by Jennifer Clack – a long technical paper that provides an excellent review of the topic. Open Access!

The Tiktaalik homepage

Early tetrapod trackways, a non-technical article about recent discoveries

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 83 – Coelacanths
• Episode 65 – The Late Devonian Extinction(s)
• Episode 91 – Frogs

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