In 1984, an amateur paleontologist in Scotland found a remarkable specimen: a nearly complete fossil of what looked to be a lizard or salamander. Rather small in size at 20 centimeters, it would turn out to be a crucial piece in the puzzle of animal evolution.
This creature, called Westlothiana lizziae, is one of the earliest examples of a four-legged animal that had evolved from living underwater to dwelling on earth. It, and other stem tetrapods like it, are common ancestors of the amphibians, birds, reptiles and mammals that exist today, including humans.
Despite its significance, researchers had never determined an accurate age of the fossil. But thanks to new research out of The University of Texas at Austin, scientists now know that the Westlothiana lizziae, along with similar salamander-like creatures from the same spot in Scotland, are potentially 14 million years older than previously thought.
The new age — dating back to 346 million years ago — adds to the significance of the find because it places the specimens in a mysterious hole in the fossil record called Romer's Gap.
The research, published recently in the journal PLOS One , was led by Hector Garza, who just graduated with his doctoral degree from the Department of Earth and Planetary Sciences at the UT Jackson School of Geosciences. Garza took a risk when he embarked on his mission to date the ancient fossils using a geochemical technique called radiometric dating.
That's because while geoscientists can use zircon crystals to determine how long ago a rock was formed, not all rock types are amenable to this type of analysis. And the site in Scotland where the fossils were discovered was near ancient volcanoes whose lava flows had long hardened into basalt rock, where zircons do not typically form. Fellow scientists warned Garza that chemically dating the rocks might be fruitless.
"I think that was one of the reasons why no one tried to go into them before," Garza said. "Because of all the time and effort that it takes to obtain the zircons and then taking the risk of not finding any."
But he got lucky. As mud cascaded down from the volcanoes, the flowing lava and debris eroded sediment that contained zircons, which got swept into a lake where limestone was forming, entombing these early tetrapod creatures.
Garza X-rayed 11 of the rock samples at the Jackson School and was able to extract zircons from the rock surrounding six of the fossils. He then conducted uranium-lead laser dating on the zircons at the University of Houston to determine their oldest possible age.
Before Garza's gamble, scientists had figured the fossils were as old as similar fossils from around the world — about 331 million years old.
The more accurate, older maximum age of 346 million years is significant because it places the specimens in Romer's Gap. This is a time period from 360 to 345 million years ago where, for reasons scientists are not exactly sure of, very few fossils have been discovered. It is during this crucial point in history that water-dwelling fish took an evolutionary leap, growing lungs and four legs to become land animals. This is one of the most pivotal milestones in the history of animal evolution.
"I can't overstate the importance of the iconic East Kirkland tetrapods," said Julia Clarke, professor at the Jackson School and co-author of this paper. "Better constraining the age of these fossils is key to understanding the timing of the emergence of vertebrates on to land. Timing in turn is key to assessing why this transition occurs when it does and what factors in the environment may be linked to this event."
The site in Scotland where the fossils were found is the East Kirkton Quarry, a veritable treasure trove of early tetrapod records. Seven stem tetrapod fossils, including the Westlothiana lizziae, have been found there. Hundreds of millions of years ago when these early four-legged creatures roamed, this site was a tropical forest with nearby active volcanoes, a toxic lake, and a diverse plant and animal community.
The National Museum of Scotland provided Garza with bits of rock that surrounded the fossils to use for the sampling. Other study co-authors are Associate Professor Elizabeth Catlos and Michael Brookfield, both of the Department of Earth and Planetary Sciences at the Jackson School, and Thomas Lapen, professor and chair of the Department of Earth and Atmospheric Sciences at the University of Houston.
