3. Jan, 2020

Tiktaalik: a leap from water to land?

Neil Shubin a paleontologist at the University of Chicago, and his colleagues recently described in the PNAS journal the anatomy of a fossil that may provide the ‘missing link’ between tetrapods  (four-legged animals)  and finned fishes. 

Left:  Reconstruction of the skeleton of Tiktaalik roseae,  with a pelvic girdle at the back, suggesting  early stage of  hind-fin driven locomotion

The fossil, called Tiktaalik represents a  fish species that must have lived around 375 million years ago. In more official terms their conclusion was that:  ‘the mosaic of primitive and derived features in Tiktaalik reveals that the enhancement of the pelvic appendage of tetrapods and, indeed, a trend toward hind limb-based propulsion have antecedents in the fins of their closest relatives’.

Fossils of their close finned relatives of the tetrapods often have a large pectoral appendage but only tiny pelvic appendages. This gave rise to the hypothesis of ‘front-wheel-drive’  early locomotion.  That is,  that primitive fishes were probably able to move on land using their strong pectoral fins. The discovery of Shubin and his team suggested that in species like Tiktaalik the hip joint could have been the start to the development of  ‘four-wheel drive’ locomotion,  such as animals that walk on land using four limbs. Looking closely at Tiktaalik’s hip joint (figure above) you will notice it has a deep socket, similar to the corresponding human socket, which allows us to move our legs in many directions.

Indeed, the big surprise  (discovered only recently in a more refined analysis of the back part of the fossil,  described already in 2006 ) was the sheer size of Tiktaalik’s pelvic girdle and hind fin relative to its pectoral girdle.  In that respect suggesting that hind-fin-driven locomotion probably began before the tetrapods. That notion is further supported by a 2011 PNAS report of an African lungfish, a living cousin of Tiktaalik, that also used its hind fin to “walk” underwater, very much like a tetrapod. This intermediate link between fish and amphibians probably represented features that foreboded a leap from water to land.

Hind limb walking gives an animal—especially a creature with heavy, air-filled lungs in the front of the body—incredible ability to maneuver in complex aquatic environments, such as swamps, streams, and estuaries. An unanswered question is still the timing of onset of the attachment of the pelvic girdle to the vertebral column: did that occur in finned or limbed creatures? Answers to these questions can only come from the fossils yet to be discovered.