Tiktaalik: The Fish That Took the First Steps Toward Land

Tiktaalik roseae is an extinct fish that lived about 375 million years ago in the Late Devonian, and one of the most famous transitional fossils ever found. Discovered in 2004 on Ellesmere Island in Arctic Canada, it combined the scales, fins and gills of a fish with a flat head, a neck and sturdy fins containing bones equivalent to those in our arms and wrists. Its discovery gave scientists a remarkably clear picture of how vertebrates began the move from water to land.

Background: from fins to limbs

All four-limbed vertebrates, or tetrapods, including amphibians, reptiles, birds and mammals, descend from lobe-finned fish. Unlike most modern fish, lobe-finned fish have fleshy fins supported by a series of bones. Living examples include the coelacanths and the lungfishes.

By the late 20th century palaeontologists knew of several stages in this transition. Eusthenopteron, a fish about 385 million years old, had fin bones comparable to the upper arm and forearm. Panderichthys, from Latvia, had a flatter head. At the other end were early tetrapods such as Acanthostega and Ichthyostega from East Greenland, about 365 million years old, which had limbs with digits but still lived largely in water.

There was a gap in the record between fish such as Panderichthys and the first tetrapods. Researchers expected that animals filling the gap would be found in rocks of roughly 380 to 365 million years old, laid down in shallow freshwater environments.

The search in the Arctic

The American palaeontologists Neil Shubin of the University of Chicago and Edward (Ted) Daeschler of the Academy of Natural Sciences in Philadelphia, with Farish Jenkins of Harvard University, decided to look for such a fossil deliberately. They searched geological maps for exposed sedimentary rocks of the right age and type.

One of the most promising areas was the Canadian Arctic, where Late Devonian river and delta deposits of the Fram Formation are exposed on Ellesmere Island. With little vegetation and soil, the rocks were easy to examine, but the location was remote and the field season short.

The team made several expeditions from 1999 onwards, working in cooperation with the authorities of Nunavut. In July 2004, at a site they had already explored on earlier trips, they uncovered a flat-headed fish skull emerging from the rock. Further specimens followed, including several individuals of different sizes.

Preparation of the fossils took around two years. On 6 April 2006 the team published two papers in Nature describing the animal and its pectoral fin.

Naming the animal

Because the fossils were found in Nunavut, the researchers asked the Nunavut Elders Council to suggest a name in Inuktitut, the language of the Inuit. They proposed Tiktaalik, a word for a large freshwater fish, the burbot.

The species name roseae honours an anonymous donor who helped fund the expeditions. The fossils remain the property of the people of Nunavut, and casts are displayed in museums around the world.

Anatomy: a fish with tetrapod features

Tiktaalik looked something like a cross between a fish and a crocodile. Known specimens suggest lengths of roughly 1.25 to 2.75 metres.

Head and neck. Its head was broad and flat, with the eyes on top, suited to looking upwards from shallow water. Unlike most fish, it had lost the bony gill covers that link the head to the shoulder girdle. This gave it a mobile neck, allowing it to move its head independently of its body.

Breathing. It had gills but also large openings called spiracles on top of its head, which some researchers interpret as an aid to breathing air. Like other lobe-finned fish of its time, it probably had lungs as well. A 2008 study of the skull suggested its feeding and breathing were shifting towards those of tetrapods.

Ribs and body. Robust, overlapping ribs may have helped support the body out of water or in very shallow water, and assisted breathing.

Pectoral fins. The front fins contained a shoulder, elbow and wrist-like joints. Bones equivalent to the humerus, radius and ulna were present, along with smaller bones resembling those of the wrist. The fin could bend at the elbow and wrist, allowing it to prop up the front of its body, rather like a push-up. Yet the fins still carried rays, like those of a fish, rather than digits.

Pelvis and hind fins. In 2014 Shubin, Daeschler and Jenkins described the pelvis and hind fin, showing that the pelvic girdle was large and robust and the hind fins well developed. This suggested that the shift to rear-wheel-drive locomotion, typical of tetrapods, had already begun in fish.

Where and how it lived

About 375 million years ago, the rocks of Ellesmere Island lay near the equator, on the continent of Laurussia, which had formed during the Caledonian orogeny. The Fram Formation was deposited by rivers on a broad, warm floodplain.

Tiktaalik probably lived in shallow streams and ponds, hunting fish and other prey. Shallow water could be warm and low in oxygen, which may have favoured animals able to breathe air and to support themselves in the shallows. The flat head with upward-facing eyes suggests it may have waited near the surface, like a modern crocodile.

It shared its world with other fish, including large placoderms related to Dunkleosteus. On land, plants were spreading, a process traced in the article on the first land plants, and early forests were forming.

Key people

Neil Shubin is a palaeontologist and anatomist at the University of Chicago. His 2008 book Your Inner Fish used Tiktaalik to explain how the human body reflects its evolutionary history, and it became a three-part PBS television series in 2014.

Edward B. Daeschler is a palaeontologist at the Academy of Natural Sciences of Drexel University in Philadelphia, specialising in Devonian vertebrates.

Farish A. Jenkins Jr. was a Harvard University palaeontologist and anatomist known for his work on early vertebrates. He died in 2012.

Myths and facts

Tiktaalik is often called a “missing link” or “the first animal to walk on land”. Both labels need care.

Scientists prefer “transitional fossil”, meaning a species showing a mix of features of an ancestral group and a descendant group. Tiktaalik fits this definition well, but it is not necessarily the direct ancestor of tetrapods.

In 2010 a team led by Grzegorz Niedźwiedzki reported tetrapod-like trackways from Zachełmie in Poland, about 395 million years old. If these are correctly interpreted, animals with limbs existed some 18 to 20 million years before Tiktaalik. This would mean Tiktaalik was a late-surviving member of a lineage close to tetrapods, rather than the actual ancestor. Its value as a model of the transition is unchanged.

Tiktaalik also did not live on land. It was an aquatic fish that could probably support itself in shallow water and perhaps make short excursions onto mudflats.

Later discoveries

Research on Tiktaalik and its relatives has continued. A 2022 study described Qikiqtania wakei, a closely related fish from the same area, whose front fins appear better suited to swimming. This suggests that some relatives of tetrapods returned to a more fully aquatic lifestyle.

Other fossils, such as Elpistostege from Quebec, described in more detail in 2020, show that similar animals lived elsewhere in the Late Devonian.

A test of evolutionary prediction

The discovery of Tiktaalik is often used to show how evolutionary theory makes testable predictions. The team did not stumble on the fossil by chance. They predicted that a fish intermediate between Panderichthys and the earliest tetrapods should be found in rocks of a particular age, laid down in a particular environment, and they went looking where such rocks were exposed.

The find was announced at a time of public debate about the teaching of evolution in some American schools, and Tiktaalik quickly became a prominent example in that discussion. It appeared in newspapers and magazines worldwide and became a familiar image in textbooks and museum displays.

The fossil also illustrates how evolution modifies existing structures. The bones of Tiktaalik’s fins correspond to the humerus, radius and ulna in the human arm. Studies of the genes that control fin and limb development in living fish and mammals have since shown that the same genetic tools help shape both fins and limbs.

Timeline

• c. 395 million years ago: Possible tetrapod trackways at Zachełmie, Poland.
• c. 385-380 million years ago: Eusthenopteron and Panderichthys live in Devonian waters.
• c. 375 million years ago: Tiktaalik roseae lives in rivers on what is now Ellesmere Island.
• c. 365 million years ago: Acanthostega and Ichthyostega live in what is now East Greenland.
• 1999: First expedition to Ellesmere Island by Shubin, Daeschler and Jenkins.
• July 2004: The first Tiktaalik skull is found.
• 6 April 2006: Tiktaalik is announced in two papers in Nature.
• 2008: Neil Shubin publishes Your Inner Fish.
• 2010: Older tetrapod trackways from Poland are reported.
• 2014: The pelvis and hind fin of Tiktaalik are described.

Why it matters

Tiktaalik is one of the clearest illustrations of a major evolutionary transition. It was found by testing a scientific prediction about the age and type of rocks where such a fossil should appear, a powerful example of how science works. Its fins contain the beginnings of the bones in human arms and wrists, linking us directly to the fish that first explored the edges of the water.