December 13, 2011
African Lungfish Research Hints At New Evolutionary Step
Researchers have revealed that the African lungfish can use its thin pelvic limbs to propel itself forward.
The team's discovery reshuffles the order of evolutionary events leading up to creatures being able to walk, and also suggests that fossil tracks long thought to be the work of early tetrapods could have been produced instead by lobe-finned ancestors of the lungfish.
"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," Melina Hale, PhD, associate professor of Organismal Biology and Anatomy at the University of Chicago Medical Center, said in a press release "We've found that aquatic animals with fundamentally different morphologies and that aren't tetrapods could potentially make very similar track patterns."
Lungfish are a popular pet in the paleontological community and have a unique evolutionary heritage.
"The lungfish is in a really great and unique position in terms of how it is related to fishes and to tetrapods," Heather King, a graduate student and lead author of the study, said in a press release. "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."
The researchers 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 use their hind limbs to elevate their body off the surface and propel themselves forward. Although the forelimbs look similar to the hind limbs, they were not involved in the movement.
"This is all information we can only get from a living animal," King said in a press release. "Because if you were just to look at the bones, like you would with a fossil, you might not ever know these motions could occur."
The fish could also demonstrate both "bounding" motions and "walking" motions. 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," study co-author Michael Coates, professor of Organismal Biology and Anatomy, said in a press release. "The fin or limb use thought to be unique to tetrapods is actually more general."
The team said the creature's ability to use its limbs to support its body may be helped by the reduced demands of gravity underwater.
The lungfish may increase the buoyancy of its front end by filling its lungs with air. This enables it to use its hind limbs to lift the entire body off the ground.
"If you showed me the skeleton of this creature and asked me to make a bet on whether it walks or not, I would have bet it couldn't," co-author Neil Shubin said in a press release. "Their fins seem like the furthest thing from walking appendages possible. But it shows what's possible in an aquatic medium where you don't have to support yourself with gravity."
The discovery suggests that many of the developments necessary for the transition from water to land could have occurred before early tetrapods.
"This shows us – pardon the pun – the steps that are involved in the origin of walking," Shubin said. "What we're seeing in lungfish is a very nice example of how bottom-walking in fish living in water can easily come about in a very tetrapod-like pattern."
The research was published in the Proceedings of the National Academy of Sciences (PNAS).
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