Strong-arm tactics drove creatures from the pond
Fossil find shows our underwater ancestors could do
press-ups.
2 April 2004
LAURA NELSON
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| Artist's impression of a tetrapod. |
| ©
Kalliopi Monoyios |
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The discovery of an ancient arm bone has helped scientists understand
what happened as water-dwelling creatures evolved into land animals.
The three-inch arm bone, found in Pennsylvania and belonging to a 370
million-year-old freshwater animal, has characteristics that indicate it was
part of a much stronger limb than previously thought.
The discovery suggests that early animals used their arms for more than
just splashing around in the water; they could have been doing press-ups
instead.
"Scientists had thought the limbs were for paddling, rather than
walking," says Jennifer Clack, a zoologist at the University of Cambridge. Now
they think the animals were doing something between swimming and walking while
still living in the water.
These early creatures had gills and tails like fish, but looked more
like salamanders. Many of their bones have been discovered and dated by
studying the rocks in which they were fossilized.
The new bone has a unique texture not seen on other such finds: a
pattern of ridges indicating that it was once attached to strong chest muscles.
"The pectoral muscles were probably well developed," says Neil Shubin, an
evolution expert at the University of Chicago and main author of the study in
Science1.
“Upper arm bones are often seen in fish, but this one is unique,” he
says.
As well as indicating strength, the orientation of the ridge means that
the main movement of the arm was up and down, not side to side, Clack says.
The creatures would eventually have used their strong limbs to push
themselves out of the water as they evolved into land animals, roughly 350
million years ago. Details are sketchy, however. We do not know, for example,
how developed these creatures became underwater before crawling ashore, but the
new find should add to the current picture.
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