Fossil of most primitive 4-legged
creature found
WASHINGTON (AP) — Scientists unearthed a skull of the most
primitive four-legged creature in Earth’s history, which should help them
better understand the evolution of fish to advanced animals that walk on land.
The 365 million-year-old fossil skull, shoulders and part of the
pelvis of the water-dweller, Ventastega curonica, were found in Latvia,
researchers report in a study published in the journal Nature. Even though
Ventastega is likely an evolutionary dead-end, the finding sheds new details on
the evolutionary transition from fish to tetrapods. Tetrapods are animals with
four limbs and include such descendants as amphibians, birds and mammals.
While an earlier discovery found a slightly older animal that was
more fish than tetrapod, Ventastega is more tetrapod than fish. The
fierce-looking creature probably swam through shallow brackish waters, measured
about three or four feet long and ate other fish. It likely had stubby limbs
with an unknown number of digits, scientists said.
“If you saw it from a distance, it would look like a small
alligator, but if you look closer you would find a fin in the back,” said lead
author Per Ahlberg, a professor of evolutionary biology at Uppsala University
in Sweden.
“I imagine this is an animal that could haul itself over sand
banks without any difficulty. Maybe it’s poking around in semi-tidal creeks
picking up fish that got stranded.”
This all happened more than 100 million years before the first
dinosaurs roamed Earth.
Scientists don’t think four-legged creatures are directly evolved
from Ventastega. It’s more likely that in the family tree of tetrapods,
Ventastega is an offshoot branch that eventually died off, not leading to the
animals we now know, Ahlberg said.
“At the time there were a lot of creatures around of varying
degrees of advancement,” Ahlberg said. They all seem to have similar
characteristics, so Ventastega’s find is helpful for evolutionary biologists.
Ventastega is the most primitive of these transition animals, but
there are older ones that are oddly more advanced, said Neil Shubin, professor
of biology and anatomy at the University of Chicago, who was not part of the
discovery team but helped find Tiktaalik, the fish that was one step earlier in
evolution.
“It’s sort of out of sequence in timing,” Shubin said of
Ventastega.
Ahlberg didn’t find the legs or toes of Ventastega, but was able
to deduce that it was four-limbed because key parts of its pelvis and its
shoulders were found. From the shape of those structures, scientists were able
to conclude that limbs, not fins were attached to Ventastega.
One question that scientists are trying to figure out is why fish
started to develop what would later become legs.
Edward
Daeschler, associate curator of vertebrate zoology at the Academy of Natural
Sciences in Philadelphia, theorizes that the water was so shallow that critters
like Ventastega had an evolutionary advantage by walking instead of swimming.