Some 365 million years ago, a four-limbed vertebrate known as a tetrapod hauled itself out of the marsh water in what is now north-central Pennsylvania. Since then, the creature's fossils remained embedded in the cliffs of Clinton County, unseen and unrecognized - until a team of researchers from Penn and the Academy of Natural Sciences discovered a number of fragments.
According to Edward Daeschler, a graduate student in Penn's Department of Geology and a paleontologist at the academy, "The earliest amphibians are the link between fish and all other back-boned animals." The fossils of the tetrapod he discovered represent some of the earliest and best evidence of the transition from one class of animal to the other.
What Daeschler and his colleagues found are bits of skull and a complete shoulder that is amazingly well-preserved for its age. The fossils, which predate the earliest dinosaurs by 140 million years, belonged to the oldest amphibian ever found in North America and the second oldest in the world.
This newly discovered amphibian has been named Hynerpeton
bassetti (hi-NER-pe-tahn BAH-se-tie). Hyner is the nearest town to the
site of the discovery; the "-peton" suffix indicates a crawling animal;
and Edward Bassett was a grandfather of Daeschler who, he notes, helped
pay his graduate tuition.
The importance of the shoulder
Hynerpeton's shoulder clearly shows large areas for attachment
of muscles that were capable of supporting the body and moving the
forelimb back and forth. "This animal could do push-ups!" says
Daeschler.
According to Dr. Neil Shubin, assistant professor of biology at Penn and another of the researchers involved, "If there is a single bone that tells a lot about these animals, it's the shoulder."
The mode of locomotion suggested by this fossil differs significantly from the flexing of the backbone and sideways movement of the tail that characterizes the way fish swim. Daeschler points out that the researchers have not found enough parts of the tetrapod to say if it did or did not have a tail to propel itself through water, but they do know Hynerpeton's forelimbs were well developed.
Daeschler and Shubin, along with Keith Thomson, president of the
Academy of Natural Sciences, and William Amaral, fossil preparator at
Harvard University, described their find in the July 29 issue of
Science magazine. Since then, their discovery has been reported around
the nation and around the world.
"Storage vaults" for fossils?
According to Daeschler, the rock formations in northern
Pennsylvania are perfect storage vaults for prehistoric fossils. "Around
365 million years ago, this area was a tropical, muddy marshland - an
ideal place for preserving these kind of fossils," he explains. "Our
search depended on finding places where these rocks are well exposed, so
we spent much of our time searching cliffs along major roadways." In
this case, they had an unexpected help from the Pennsylvania Department
of Transportation, which was widening roads in the region: that work
exposed the fossil beds.
"We expected to find evidence of tetrapods with weak limbs and
gills, best suited for life in water," says Daeschler. "Instead we found
an animal without gills, and with shoulders more developed than anyone
would have expected."
Raising new questions
Shubin reports that the age of these fossils, combined with the
fact that the shoulder of Hynerpeton is so strongly built, raises new
questions for paleobiologists. For example, he asks, "Did the earliest
amphibians evolve quickly to exploit the unprecedented opportunities
open to them on land, or are we missing fossils that would document a
slow evolution of the specialized features found in Hynerpeton?"
Scientists still do not know many details of how and why some fishes adapted to life on land while others remained under water. But they have found that land pioneers like Hynerpeton seemed to live in streams and swamps. According to one of the prevalent current theories, legs were first a way to maneuver in shallow, plant-choked water, and lungs were for gulping in air in the oxygen-depleted water. And because land had already been "colonized" by plants and insects, it might have offered tempting grounds for early amphibians.
Shubin gives another theory: "It was a fish-eat-fish world at the time. Fish were evolving bigger teeth for attack and bigger body size or tougher armor for protection. These ancient amphibians revolutionized the whole ball game by walking away from the fray."
Shubin, who is Daeschler's Ph.D. advisor, belongs to both the Biology and the Geology Graduate Group. For him, part of the attraction of his field of study is the fun in reconstructing the world of 365 million years ago. "We usually forget that life on earth has a long history. To understand diversity in today's life forms, we have to look at how life has evolved from the past."
In the not-so-distant past of Joseph Leidy, Penn and the Academy of Natural Sciences began to collaborate on research. Leidy, after whom the Leidy Laboratories that house the Department of Biology are named, is considered the father of American vertebrate paleontology. The current collaboration, says Shubin, "brought together the academy's superior skills in handling the fossils and Penn's human power and library resources."