The Fish That Almost Walked

The first fossil of Tiktaalik discovered.

In 2004, Neil Shubin and Ted Daeschler discovered a remarkable 375 million year old fossil of a fish, Tiktaalik roseae, which appeared to bridge the gap between the aquatic and terrestrial worlds, showing exactly where the first amphibians, and indeed all land-dwelling vertebrates, came from anatomically.

Alongside its flattened body and complex skull, its most remarkable feature was its front legs. They were not like the thin cartilaginous rods found in most fish, these were composed of strong, thick bones and tips, which were beginning to differentiate into digits. This was a limb designed to support weight.

The fossil was well preserved and in three dimensions, but parts were missing from the hips down, including its back fins. Now, the previously unknown portion of Tiktaalik has been recovered from the same site in the frozen waste of Ellesmere Island, by Shubin and Daeschler. Simply put, it is a remarkable specimen which shows just how far removed Tiktaalik was from its water-bound cousins. While the pelvis hints at its primitive past, it has a ball and socket joint - something seen in just one fish, the lungfish - and is incredibly robust, with pronounced ridges for muscle attachments.

The near complete fossil of Tiktaalik from Ellesmere Island. The orange square marks out the newly found pelvic region.

This would have given Tiktaalik a range of motion far beyond what ordinary fish could achieve, allowing it to carefully position its limbs in order to support its weight, and then apply force in order to move. 'This is an amazing pelvis, particularly the hip socket, which is very different from anything that we knew of in the lineage leading up to limbed vertebrates,' said Daeschler. 'Tiktaalik was a combination of primitive and advanced features.'

African lungfish have similar large pelvises, with the ability to walk underwater on the bottom. 'Regardless of the gait Tiktaalik used, it's clear that the emphasis on hind appendages and pelvic-propelled locomotion is a trend that began in fish, and was later exaggerated during the origin of tetrapods,' Shubin said.

The exodus from the oceans is one of the most important, most complex and most mysterious events in the history of life. The changes involved were anatomically and physiologically vast, but the fossil record has been reluctant to divulge its secrets. The discovery of Tiktaalik brings us closer to understanding the origins of limbs, lungs, skulls and indeed our complete terrestrial anatomy. Focus on the environments where transition between aquatic and terrestrial life occurred, over 350 million years ago, will yield rich pickings about this complex revolution.