In Depth
In 2004 the discovery of three fossils would help bridge one of the most important evolutionary gaps in the fossil record. These fossils were the first known representations of Tiktaalik, a creature that was not only a fish but also possessed the rudimentary features of the later tetrapods. Unlike other creatures which are usually named using ancient Greek, Tiktaalik has been named using the Inuktitut word for Burbot.
As a transitional form, one of the key features of Tiktaalik is the construction of the front fins. Instead of simple rays of bone as seen in most other fish, these have a bone structure that forms a very basic arm structure that includes a wrist and fingers. While these limbs where not suitable for walking in the classical sense as animals do today, they would have allowed Tiktaalik to lift its body up off the ground. They may also have been used to push its way through the shallows where the water was not deep enough to support its body. This would have enabled Tiktaalik to reach other areas of water that would not have been accessible to other species enabling it to hunt in areas of low competition, save for others of the same ability.
Another important feature of Tiktaalik is the neck in that it actually had one. Earlier fish had their gills reinforced by bones, which means that they could not turn their heads independently from their bodies. Tiktaalik however, while still retaining gills, had no bone structure supporting them, meaning its head had a non-rigid attachment to its body. This enabled Tiktaalik to move its head, without having to move its body as well, a very important function for a predator of the shallows where body movement would have been restricted. The rib cage is quite sturdy especially when compared to other fish, with the individual ribs slightly overlapping one another. This is important as without the buoyancy of water to support its mass, Tiktaalik would need extra support from its skeleton to protect its internal organs from its own body weight.
On top of the head of Tiktaalik are holes which suggest the presence of spiracles, air passages that would have connected to what may have been lungs. This is another key evolutionary step that would have allowed Tiktaalik to exploit the oxygen content of the air, allowing it live for longer periods out of the water, as well as providing a boost for it to live in waters with poor oxygen content.
What can be said about the lifestyle of Tiktaalik is that it was a predator that may have lain in ambush for its prey. The eyes are situated on top of its skull meaning that it could either approach its prey from below in the water, or like a crocodile, lain near the surface with just its eyes above the water. When its prey approached close enough it could then have raised itself up for extra reach and turned its head for prey capture. The teeth inside the jaw are small and many, adaptations that would have been better suited to capturing small and nimble prey like insects and fish.
Further Reading
– A Devonian tetrapod-like fish and the evolution of the tetrapod body plan – Edward B. Daeschler, Neil H. Shubin & Farish A. Jenkins Jr – 2006. – The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb – Neil H. Shubin, Edward B. Daeschler and Farish A. Jenkins Jr – 2006. – Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes – Heather M. King, Neil H. Shubin, Michael I. Coates & Melina E. Hale – 2011. – Pelvic girdle and fin of Tiktaalik roseae. – Proceedings of the National Academy of Sciences. 111 (3): 893–899. – N. H. Shubin & E. B. Daeschler & F. A. Jenkins – 2014.
