| The Fish That
(almost) Walked Away
New Fossils Show Ancestor Fish Preparing to Leave the Wate
What does a fish with legs look
like? If any one image springs to mind, it’s probably
the Darwin fish: the car-bumper ornament that represents
our fishy ancestors who evolved
limbs to crawl from water to land. The icon has been around
for decades and the evolutionary theory behind it since the
nineteenth century. But nobody has ever seen a fish with
legs—nobody, that
is, until Ted Daeschler.
Daeschler, Gr’98, assistant
curator and chair of vertebrate zoology at Philadelphia’s
Academy of Natural Sciences, is quick to point out that “a
fish with limbs is actually an oxymoron. By definition, if
it has limbs, it’s a tetrapod.” Semantics aside,
the distinction hints at a challenge paleontologists face—documenting
the largely unexplained transition from fish to terrestrial
animals.
The beginnings of limb formation can be traced back to the
Devonian era, 400 million years ago, a period that Daeschler
has mined since his days as a grad student.
His fieldwork
began in the most unusual of places: alongside highway road
cuts in north-central Pennsylvania. A professor
had suggested the location, home of Devonian-era red rock,
but was less than optimistic. “He said we’d only
find maybe one good fossil a year.” But thanks to a
PennDot demolition team, “within two weeks of being
at the Red Hill site I found an early tetrapod. It was as
rare as hen’s teeth.”
The road cuts proved so
fertile that Daeschler and colleagues have returned ever
since. The fossil hunters swoop into action
on Sundays, scouring fresh rock surfaces while construction
crews take the day off. “They’re moving more
rock than every paleontologist could move in 100 years,” he
notes. “We’re recovering 15 to 20 good fossils
a year, on average, making our work the most productive,
in that time frame, in the world.”
In 1993, Daeschler
made his first major discovery: the shoulder girdle of a
creature he dubbed Hynerpeton bassetti (after
his science-loving grandfather). Hynerpeton was remarkable
because it probably had enough musculature to walk on streambeds. “The
animal could have done push-ups,” says Daeschler. “Now,
granted, push-ups are easier in water.”
Five years
later, he struck gold again, this time with Sauripterus taylorii,
also known as “fish fingers.” The specimen
has eight fingerlike structures, which Daeschler thinks may
have been used to palm the muddy floor of a Devonian swamp. “These
fish were not using their fins in the way we know of fins
being used,” he says. “They could have used them
for crawling [in shallow water] or other things.”
His
most recent discovery is a 370-million-year-old humerus (upper
arm) bone. Daeschler suggests the fish could have
been a predator, using its permanently bent “elbow” to
hold steady in shallow water, waiting for prey to swim by.
His groundbreaking work has been featured in the New York
Times, National Geographic, and Science. “Fish
fingers” earned him a date on Nightline. While media
stories usually interpret fossils as points along an evolutionary
time line, Daeschler proposes that, “instead of a line,
what we’ve got is a complex bush of diversity.” He
adds: “We still don’t know which lobe-finned
fish
gave rise to the first terrestrial animals.” The answer
just might lie somewhere in that Devonian red rock, along
a Pennsylvania highway that could go anywhere.
—
Ted Mann, C’00 |
