Today, lungfish live only in Africa, South America and Australia. Lungfish are freshwater fish. Lungfish are best known for their ability to breathe air, and the presence of lobed fins with a well-developed internal skeleton. The eel-like body and scrawny limbs of the African lungfish would appear to make it an unlikely innovator for locomotion. But its improbable walking behavior, newly described by University of Chicago scientists, redraws the evolutionary route of life on Earth from water to land. Extensive video analysis, published in the Proceedings of the National Academy of Sciences, reveal that the African lungfish can use its thin pelvic limbs to not only lift its body off the bottom surface but also propel itself forward. Both abilities were previously thought to originate in the early tetrapods, the limbed original land-dwellers that appeared later than the lungfish's ancestors.
The observation reshuffles the order of evolutionary events leading up to terrestrial adaptation and living on land. It also suggests that fossil tracks long believed to be the work of early tetrapods could have been produced instead by lobe-finned ancestors of the lungfish.
Tetrapods, by the way, are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods had been believed to be evolved from the lobe-finned fishes in the Devonian.
"In a number of these trackways, the animals alternate their limbs, which suggested that they must have been made by tetrapods walking on a solid substrate," said Melina Hale, PhD, associate professor of Organismal Biology and Anatomy. "We've found that aquatic animals with fundamentally different morphologies and that aren't tetrapods could potentially make very similar track patterns."
"The lungfish is in a really great and unique position in terms of how it is related to fishes and to tetrapods," said Heather King, a graduate student and lead author of the study. "Lungfish are very closely related to the animals that were able to evolve and come out of the water and onto land, but that was so long ago that almost everything except the lungfish has gone extinct."
King and her colleagues designed a special tank in which the motions of lungfish could be videotaped from the side and below for in-depth analysis. The videos revealed that lungfish commonly use their hind, or pelvic, limbs to elevate their body off the surface and propel themselves forward. Though the forelimbs look similar to the hind limbs, they were not involved in locomotion, the authors found.
Lungfish also demonstrated both bounding motions, where both limbs moved at once, and walking, marked by alternating limbs. Coupled with the ability of the lungfish to fully rotate the limb and place each subsequent footfall in front of the joint, the motion suggests that similar creatures would have been capable of producing some of the fossil tracks credited to tetrapods.
"It's tempting to attribute alternating impressions to something like the footfalls of an early tetrapod with digits, and yet here we've got good evidence that living lungfish can leave similar sequences of similar gait," said Coates, PhD, professor of Organismal Biology and Anatomy.
The lungfish's ability to use its thin limbs to support its body may be helped by the reduced demands of gravity underwater, the authors proposed. By filling its lungs with air, the lungfish may increase the buoyancy of its front end, enabling the scrawny hindlimbs to lift the entire body off the ground.
The discovery suggests that many of the developments necessary for the transition from water to land could have occurred long before early tetrapods took their first steps on shore. Lobe-finned ancestors of the lungfishes as well as tetrapods could have evolved hind limb propulsion and the ability to walk on the substrate at the bottom of a lake or marsh millions of years before limbs with digits and land-dwelling animals appeared.
For further information: http://www.uchospitals.edu/news/2011/20111212-lungfish.html