Paleo Profiles: Tiktaalik

A reconstruction and skull, from National Geographic

It is rare in palaeontology that transitional fossils are found - these are the fossils which show how organisms evolve over time. Among some of the famous ones include the Archaeopteryx, which showed how dinosaurs evolved into birds, and Darwinius, which helped show where apes branched off from lemurs. For a long time scientists have known that there must be transitional animals between fish and amphibians, but the question was what would they look like? Then, in 2004, one transitional fossil was found: Tiktaalik. 

Discovery and Fossils

Tiktaalik was first discovered in 2004 way in the frozen north of Canada on Ellesmere Island. Modern day Ellesmere Island is not a place you would imagine to find a transitional fish fossil - the geomagnetic north pole can be found on the island. However, a group of palaeontologists discovered this important fossil and fully described it in 2006. This group, (comprising of Neil Shubin, Edward Daeschler, and Farish Jenkins), named it Tiktaalik roseae; Tiktaalik comes from an Inuktitut word roughly meaning 'large freshwater fish', and was actually suggested by elders from the local Inuit Council, and roseae was chosen to honour an anonymous donor. After the initial discovery this alerted palaeontologists to the importance of Ellesmere Island, and successive digs have found other Tiktaalik remains allowing this strange animal to be better understood.

Biology

This animal has been described as a mixture of a salamander and a fish. Although gills do not fossilise the bony structures which support them do, and on the crocodile-like head of the Tiktaalik they took the form of holes called 'spiracles'. This gives us the indication that it could live underwater, but the layout of the ribs, and a secondary use of the spiracles, gives an indication that it also had lungs. Fish have a swim bladder which is full of air to keep them buoyant, but these swim bladders can evolve to become larger becoming a lung over millions of years. Modern day lungfish have this adaptation - they can exist a long time out of water thanks to their swim bladders evolving to become more like a lung which land vertebrates (tetrapods) have. Tiktaalik could then go on land and the water, and its fins helped it do so. Unlike the fish we know today fish of the Devonian, when Tiktaalik lived, had bones in their fins which form a 'hand' you might see in modern whales or the mosasaurs. These were the lobe-finned fish, today there are only eight species of them - the two coelacanths and the eight lungfish species.

A West Indian coelacanth

These lobe-fins will help answer why the Tiktaalik could move onto land. Later fossil finds have managed to unveil the fish's pelvis and tail, unlike its other fish cousins Tiktaalik had a larger pelvis and tail allowing greater movement. This allowed it to move onto land, but the pelvis was not too strong, so it was largely confined to the water side. Shubin, Daeschler, and Jenkins have advocated for a 'front wheel drive hypothesis' - like modern mudskippers it would use its front fins as a way to prop itself up while on land. The ribs were strong for this reason as it required thicker ribs to support its organs outside of the water. Finally, we have the neck. The spiracles allowed Tiktaalik to lose the bony structure which normally protected the gills, so this gave the Tiktaalik a neck with the ability to look around without completely moving its entire body. Although primitive, Tiktaalik and similar animals, one similar fish can also be found in Poland, set the stage for the body plans of amphibians, reptiles, birds, and mammals. This could be used as a way for it to search for prey, or possibly spot predators. At a metre long it was sizeable but was dwarfed by other fish as seen below.

A seize chart from Prehistoric-Wildlife.com

When and Where

Ellesmere Island today

As we've already mentioned, Tiktaalik has so far only been found on Ellesmere Island in the very north of Canada. The North East of North America is well known for Devonian lobe-finned fish and the first amphibians. In Quebec there is the closely related Eusthenapteron, which possibly was an ancestor to Tiktaalik, in Pennsylvania there was the giant lobe-finned fish Hyneria and the salamander-like Hynerpeton, and in Greenland there was a possible descendant of Tiktaalik, another salamander-like amphibian called Icthyostega. Tiktaalik has currently been found from fossil sites dating to 375 million years ago during the Devonian period. During this time the first large plants grew on the land, and invertebrates had already conquered the land. The Devonian has been known as 'The Age of the Fishes' for the explosion in the diversity of fish: sharks became common, there were the lobe-finned fish, and armoured fish called placoderms. Oxygen was also a lot less compared to present-day air - possibly around 75% less than today. This possibly explains why Tiktaalik moved onto land. The diversity of plants on the land would die, get washed into the ocean, and start decomposing which increasingly stripped the oceans of oxygen. As a result, any fish which could breathe on land and in water had an advantage. Eusthenopteron had spiracles on the top of its head instead of on the side of its head like other fish (even today), so it could go to the surface and breathe. Consequently, Tiktaalik evolved from lobe-finned fish like Eusthenopteron to take advantage of this.

Habitat

While it could go on land, the Tiktaalik was still very dependent on the water to survive. If you travelled back 375 million years to spot a Tiktaalik you would have to search riverbeds. Shubin and Daeschler have theorised that it would live in the first swamps, as well as streams and ponds, resting on edge of the water. Most of its time would be spent in the water - quite possibly it would be an ambush predator. Like crocodiles, it would lay on the water's surface, or the riverbed, waiting for prey, and then lunge with its powerful tail and pelvis after an unfortunate fish or bug. As other, larger fish were constrained to the water Tiktaalik would have gone to the shore as a way of effortlessly hiding from them. This fish's ancestors, such as Icthyostega, would start evolving to be more and more on land until the Carboniferous period when the first reptiles would permanently leave the water.

The sources I have used are as follows:

-'Tiktaalik', Prehistoric-Wildlife.com, [Accessed 20/02/2020]

-Dan Vergano, 'Our Fishy Ancestors Had Fins made for Walking', National Geographic, (14/01/2014), [Accessed 20/02/2020]

-Jason P. Downs, Edward B. Daeschler, Farish A. Jenkins, Jr. and Neil H. Shubin, 'The Cranial Endoskeleton of Tiktaalik roseae', Nature, 455/7215, (2008)

-Jennifer Clack, 'The Fish-Tetrapod Transition: New Fossils and Interpretations', Evolution: Education and Outreach, 2, (2009), 213-223

-Neil Shubin, Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body, (New York: 2008)

-Neil H. Shubin, Edward B. Daeschler, and Farish A. Jenkins Jr, 'Pelvic girdle and fin of Tiktaalik roseae', PNAS, 111:3, (2014), 893-899

-PBS Eons, 'When Fish First Breathed Air', YouTube.com, (19/06/2018), [Accessed 20/02/2020]

Thank you for reading, for our other Paleo Profiles we have a list here. For other blog updates please see our Facebook or catch me on Twitter @LewisTwiby.