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Fossil may be earliest arm bone
Fossil may be earliest arm bone
By Paul Rincon
BBC News Online science staff
A tiny fossil found by a road cutting in Pennsylvania, US, could be the earliest
example of an arm bone.
The 360-370-million-year-old humerus, or upper arm bone, indicates that limbs
may have evolved for use in water and not to get around on land.
It suggests the earliest limbed animals were fish navigating shallow rivers, but
its place in the evolutionary tree is the subject of some controversy.
US biologists have published details of the discovery in the journal Science.
The evolutionary process that transformed fins into limbs is poorly understood,
despite being a key transition in evolution.
It led to the emergence of tetrapods, four-legged vertebrates that mostly live
on land. Before tetrapods, vertebrates were confined to water.
This grade of animal was part of the evolutionary history of all limbed animals,
Dr Ted Daeschler, Academy of Natural Sciences
"It's like a Rosetta stone, it helps us translate between these two different
forms; the fish fin and the tetrapod limb," Dr Ted Daeschler, of the Academy of
Natural Sciences, Philadelphia, US, told BBC News Online.
Limb by limb
"This grade of animal was part of the evolutionary history of all limbed animals
including ourselves," he explained.
In the first half of the 20th Century, researchers postulated that the evolution
of limbs was driven by the need to support and move the body on land.
The humerus evolved from being a mobile bone, as it is in the fish, to one that
was stationary and acted as prop to support the body. But this had nothing to do
with walking on land, the authors of this paper contend.
"It clearly shows that the lower limb was very muscular and directed towards the
bottom, and the elbow permanently bent," Dr Daeschler said.
The authors propose that the animal, still known only by the number ANSP 21350,
was an ambush predator that used its limbs to hold itself steady in the current
of a shallow stream until prey came along.
Alternatively, the limb may have been used to move the animal through the
The specimen will be an important addition to our understanding of the processes
that allowed tetrapods to use their limbs for moving about on land.
"It can tell us how some of the structures that became useful on land first
developed. Evolution seems to work by finding new functions for old structures,"
Dr Daeschler commented.
"Those features became useful when the animals had to hold themselves against
gravity on land; they were already in the toolkit."
The authors have constructed a scheme to show relationships between some key
fossils in the transition between lobe-finned fish and tetrapods.
But they admit they have left some important specimens such as Ichthyostega and
Elginerpeton out of the scheme.
Jenny Clack, senior assistant curator at the University Museum of Zoology in
Cambridge, UK, said there were still many unresolved questions with regards to
what kind of creature ANSP 21350 was and what exactly it used its arms for.
"The humerus has got some very peculiar features all of its own and those help
us with sequences for acquisition of certain characters. But how it will fit in
the long run I'm not sure," said Dr Clack.
Professor Robert Carroll, of the Redpath Museum, Montreal, Canada, told BBC News
Online: "This individual bone is clearly that of an animal that is a very
primitive tetrapod or a primitive lobe-finned fish or perhaps something
"The problem is that it isn't like any of the later humeri that you encounter in
the later Carboniferous."
Story from BBC NEWS:
Published: 2004/04/02 05:02:45 GMT
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