14 DAY TRIAL // University of Oregon professor of geological sciences Gregory J. Retallack says that the conclusions of his latest studies on how fish began turning into tetrapods (four-legged animals) indicate a picture different from commonly-accepted theories on how life moved out of the water.
The idea that most evolutionary biologists accept today is the one developed by University of Chicago/Harvard University paleontologist Alfred Romer. The expert, who passed away back in 1973, argued that fish learned to use stumpy limbs in order to move between drying ponds.
His scenario was one depicting the fish as experiencing a necessity to move out of their drying ponds, and into larger ones, filled with more water. Over time, the creatures' bodies adapted to travel on land, and legs evolved.
But Retallack believes that such an ancestor may have in fact had very little chance of surviving all the overwhelming odds that were stacked against it. In all likelihood, only a diminutive percentage of all fish trying to switch ponds would have survived the journey.
The fish-tetrapod transition occurred during the late Devonian period, which stretched from 390 to about 360 million years ago. Retallack does not dispute that, but his paper provides an entirely different way of looking at the “great escape” fish made from their ponds.
Details of the work appear in the May 2011 issue of the esteemed scientific Journal of Geology. The study leader is the co-director of paleontological collections at the UO Museum of Natural and Cultural History. He collected samples from numerous sites in Maryland, New York and Pennsylvania.
The focus of his studies were buried soils. The material is sometimes preserved in rock, and the science group was very interested in searching for small fossils and footprints or other tracks inside it. These soils provide a snapshot of lifeforms that can be cataloged as early transitional fossils between fish and amphibians.
“These transitional fossils were not associated with drying ponds or deserts, but consistently were found with humid woodland soils. Remains of drying ponds and desert soils also are known and are littered with fossil fish, but none of our distant ancestors,” Retallack explains.
“Judging from where their fossils were found, transitional forms between fish and amphibians lived in wooded floodplains. Our distant ancestors were not so much foolhardy, as opportunistic, taking advantage of floodplains and lakes choked with roots and logs for the first time in geological history,” he goes on to say.
The expert adds that stubby appendages on the bodies of evolved fish may have been used to climb or avoid woody obstacles, whereas flexible necks may have evolved to allow the creatures to feed more easily.
“Ancient soils and sediments at sites for transitional fossils around the world are critical for understanding when and under what conditions fish first walked. The Darwin fish of chrome adorning many car trunks represents a particular time and place in the long evolutionary history of life on Earth,” Retallack concludes.