Fossils reveal how fish made transition from water to land

Kalliopi Monoyios/University of Chicago

Since the discovery of Tiktaalik roseae a decade ago, it has been hailed as an important link in the evolution of terrestrial animals. New analysis of the Tiktaalik fossils explains how animals were able to make the transition from water to land hundreds of millions of years ago. The study was led by Neil Shubin of the University of Chicago and was published in the Proceedings of the National Academy of Sciences.

Tiktaalik roseae was a fish which resembled a crocodile and lived 417-354 million years ago. It was discovered in 2004 by Shubin and his team during an expedition to Ellesmere Island, Canada. Despite the age, the specimen was very well preserved. The researchers were able to determine that Tiktaalik had many features that resembled an intermediate between fish and land animals and represents the transition from aquatic to terrestrial life. A true transitory animal, Tiktaalik was able to breathe using a combination of gills and lungs that were even protected by a rib cage, and the head resembled a crocodile more than a fish.

The latest report comes from analysis of the pelvic girdle and fin. Because early vertebrates with limbs (known as tetrapods) had relatively large hind limbs, the team focused on trying to understand how this trait emerged. The analysis revealed that Tiktaalik had a widened pelvic girdle, which would have allowed the pelvic fins to be strong enough to push around muddy riverbeds, not just swim through water.

The size of the pelvic girdle was unexpected, as it was believed that powerhouse hind limbs would have evolved later when animals were spending more time on land, as a response to the new environment. The findings indicate that the morphological changes actually predated the actual transition from water to land. Despite their relative width,  the pelvis was still more like a fish than a tetrapod, though it does represent a very important start.

Even with all of the amazing discoveries surrounding Tiktaalik to date, the researchers are not done yet. There are still bones missing from the skeleton, including the bones of the pelvic fin. These bones could give clues about features of modern feet, such as toes. If and when these fossils are discovered, researchers will get yet another piece of the puzzle for one of the most amazing evolutionary feats of all time.

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