Clues sealed in stone
A road cut in rural Pennsylvania offers fossil evidence of how fish became land animals.
NORTH BEND, Pa. - The guy at the equipment-rental place didn't quite understand at first why Ted Daeschler needed the brute force of an electric demolition hammer.
The 1,400-watt power tool is the sort of thing a contractor uses to break through a concrete wall. But Daeschler would be using it to dig into an ancient formation of red sandstone in north-central Pennsylvania.
"Is there buried treasure up there?" the salesman asked.
The very next morning, the answer would be yes. But it was a kind of treasure that Daeschler valued more than jewels or gold.
Inch-long teeth. Armor-like scales. Bony bits of fins and skulls. Evidence of creatures that were buried in sediment 365 million years ago, long before dinosaurs ever rumbled across the landscape.
Daeschler is a paleontologist, at Philadelphia's Academy of Natural Sciences, and the demolition hammer was a new aid in his career-long quest: to decode a pivotal chapter in the history of life on Earth.
The fossils he seeks are from fish - but fish on the cusp of an extraordinary transition. Over millions of years their fins began to look more limb-like, their gills less gill-like, and their scales began to melt away.
Sure, dinosaurs are nice, but they represent a small chapter in the rich story of evolution. Just one subset of fish, on the other hand - the bony critters called Osteichthyes - eventually gave rise to dinosaurs, modern fish and humans, not to mention every other land animal with a spine.
"Bony fish are a very successful group," says Daeschler, 48.
Today, through a combination of luck and the inexorable forces of geology, there are just a handful of places in the world where he and other sleuths have found fossils from the early stages of this transition.
One is in Australia, another in Canada. A third is here in northern Pennsylvania, the epicenter of which is Red Hill - a rocky slope laid bare by an ordinary highway road cut, courtesy of the Pennsylvania Department of Transportation.
'Pages in a book'
The battering force of the electric hammer was just the thing to open a crack in the Clinton County hillside, as trucks whizzed by 20 feet below on Route 120.
Once they got a crack started, Daeschler and his colleagues - postdoctoral fellow Jason Downs and amateur paleontologist Doug Rowe - switched to chisels and pry-bars.
They removed chunks of rock one by one, throwing most to one side. A few that looked intriguing were placed carefully in a box for the trip back to Daeschler's lab in Philadelphia, where fossils could be extracted with surgical precision.
The scientists find something almost every time they come to Red Hill, and their trip two weeks ago was no exception.
Some rocks contained the clear outlines of ancient plants. Others held bits of extinct fish such as Hyneria lindae, an 8- to 12-foot-long predator with muscular "lobe" fins.
Embedded in several rocks were unusual pebbly scales that the team has seen on previous trips - the remains, they believe, of a new species. But more bones are needed to see just where the unnamed fish fits in.
Often, the researchers were able to break off flat sections of rock in their hands.
"Like pages in a book," says Rowe, who lives in nearby Renovo and has helped Daeschler for years.
"A history book," Daeschler adds.
About 365 million years before history books, actually. The rocks were from the latter part of the Devonian period, often called the Age of Fishes.
A sense of scale: 365 million is the distance from Philadelphia to San Francisco and back.
Scientists calculate prehistoric dates by measuring the decay in radioactive elements within the rocks. Volcanic rock, for example, contains an isotope of potassium when it first hardens but that decays steadily into argon over millions of years.
By measuring how much argon has accumulated and how much potassium is left, scientists can construct a sort of chemical clock.
Because some fossils at Red Hill match those from other sites that have been dated with radioisotopes, researchers can get a good idea of the time frame.
Paleontologists first looked for fossils at the roadside site in the 1950s and again in the 1960s, with occasional success. Then, in the early 1990s, highway crews widened the roadbed, cutting away tons of rock and exposing a wealth of fossils.
It was more than enough for Daeschler to get his Ph.D. in 1998. He has been coming back to Red Hill ever since, often for just a day or two at a time.
Besides Pennsylvania, Daeschler returns periodically to another rich source of Devonian fossils, the Canadian Arctic. Last year, he and colleagues gained worldwide acclaim for their discovery of a new species there: Tiktaalik roseae, a fish with limb-like fins and a very un-fishlike neck, is a key illustration of the transition from water to land.
Daeschler became interested in science in grade school in northern New Jersey, especially the hands-on stuff like tracing leaves and looking at algae in water samples.
Three relatives were geologists, so that was a natural direction for him to pursue. As an undergraduate at Franklin & Marshall College, he became especially interested in the ancient history of the landscape and the things that lived in it.
"When you add time to what you see on Earth," he says, "then you're developing this historical narrative of why things are the way they are."
Pennsylvania, conveniently, is a treasure trove. Daeschler is always on the lookout for more places to dig, as sedimentary deposits from the Devonian period cover the northern part of the state.
Prowling road projects
For a while, he subscribed to a PennDot newsletter to keep abreast of new road projects that might expose fossils. Lately, he simply drives around with Downs, the post-doc, and keeps his eyes open.
Hillsides that have been stripped of trees are a good way to tell where the earth-moving equipment is likely to strike next.
Actual trucks are good, too. Daeschler copies down the names of contractors from vehicle doors, then calls up the owners later.
And nature is an ally, weathering away the Devonian sandstone little by little. Ice, for example, is a great widener of cracks.
There was no ice when the 12-foot Hyneria and its watery cousins inhabited what is now North America. Most of it was underwater, and Pennsylvania was a swampy, subtropical hothouse.
But the landscape changed, and changed again. It is always changing, even today, so a shrewd fossil-hunter knows to come back every month or two. Did that big slab of rock fall down yet? Has that crack gotten wider?
For Ted Daeschler, that's how the story of the Age of Fishes will be revealed.
History in Brief
4.5 billion years ago: The Earth is formed.
3.5 billion years ago: The first single-celled organisms appear.
650 million years ago: Multi- cellular, soft-bodied organisms arise during the Vendian period.
417 to 354 million years ago: Devonian period, or "Age of Fishes." Limbed animals evolve from bony fish. Vascular plants emerge, including the first trees.
225 to 65 million years ago: Rise and fall of the dinosaurs.
130,000 years ago: The first humans - Homo sapiens.
A slide show of photos from the dig and a timeline of Devonian evolution: http://go.philly.com/scienceEndText