Lungfish studies yield surprising insight into origin of terrestrial movement

 Jeffrey Serrill, Lungfish, Evolution of Substrate Dependent Locomotion

New insights into the evolution of quadrupedal movement in pre-tetrapodal species have been recently developed through behavioral and morphological analyses of a species of African lungfish. This research, conducted by the University of Chicago’s Organismal Biology and Anatomy Department, provides an important glimpse into the chronological progression from certain aquatic fish species to the first terrestrial four-limbed vertebrates.

The origin of terrestrial locomotion among tetrapods was one of the most important evolutionary achievements to date, and was a necessary precursor to the diversity of life that subsequently arose on land. It is widely agreed upon that this adaptation arose in the Devonian period (416 MYA-359 MYA), and that a significant number of morphological alterations to limb structures were required to eventually make this transition from water to land possible. Furthermore, certain behavioral characteristics must have evolved concurrently, such as the use of the underlying surface substrate as a propulsion mechanism, and alternation of limb movements to produce forward movement. However, the chronological order of these morphological changes is still a matter of debate, and the information gained from the fossil record is thus far not sufficient to address this question.

To more closely explore this transition, the researchers studied the locomotion of Protopterus annectans, one of the only finned sarcopterygians still in existence. As the sarcopterygians are widely believed to be the direct forerunner of tetrapods, an understanding of the way in which these fish alternate limb movements and propel themselves through their aquatic environments is of great importance in developing an understanding of the origins of terrestrial locomotion. These organisms (usually referred to as lungfish) are characterized by long slender fins, the presence of lungs, and “light-bodied” cartilaginous skeleton. When utilized in a buoyant aquatic environment, these characteristics may have helped promote the eventual shift from substrate-independent swimming to substrate-assisted ambulation on the aquatic floor.

"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."

It was shown that these lungfish do indeed seem to use their pelvic fins to “push” against the seafloor and propel themselves forward, and that this behavior was not observed with their pectoral fins, which appear to play no role in this method of locomotion. These movements were shown to be of several different forms, where alternating movements of their paired pelvic fins resulted in a “walking” movement, and synchronous movements resulted in “bounding” along the substrate. As tetrapods typically utilize their pelvic appendages in a similar fashion to propel themselves along the ground, this information suggests that this form of substrate-dependent locomotion initially arose in sarcopterygian species prior to the evolution of the tetrapodal class or the emergence of digited limbs.

The researchers also noted in their paper that these lungfish do not possess a sacrum, a skeletal feature which connects the pelvic limbs to the rest of the skeleton. Since this component is thought to be necessary for the transfer of propulsive force (and thus substrate-dependent movement) from the hind limbs, they suggest that the requirement for this characteristic may have been alleviated by the development of lungs in sarcopterygian species, which would allow additional buoyancy and thus require less effort to propel the organisms through an aquatic environment.

Collectively, these findings seem to shed light on the minimal morphological adaptations required for this form of movement to evolve in an underwater environment. As the sarcopterygians are thought to be closely related to the first tetrapodal species, this provides important insight into how terrestrial locomotion may have originated. Additionally, these findings suggest that fossil tracks in underwater substrates which were originally attributed to tetrapods may have actually been created by fish species, and that the scientific community may have to reevaluate its conception of where some of these early “footprints” actually originated.

"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."

Original Research Paper: http://www.pnas.org/content/early/2011/12/08/1118669109.abstract?sid=375...

Original Press Release: http://www.uchospitals.edu/news/2011/20111212-lungfish.html *Quotes were obtained from the original press release from the University of Chicago Medical Center.

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