Washington,
DC (9/30/98)- Trails left by worms under a shallow sea covering what
is now India suggest complex multi-celled organisms lived a half a billion
years earlier than previously believed.
A team of German, American and Indian researchers report the discovery
of fossilized worm tunnels in rock that can be dated at over a billion
years of age, twice as old as the oldest multi-cellular fossils on record.
The trace fossils are preserved in the mesoproterozoic Chorhat Sandstone,
which contains sand beds that built up during storms. The tops of many
sand beds were covered with a microbial mat that blanketed the floor periodically
and protected the sand below from any disturbances above. The ancient worm-like
animals may have migrated through the sand just below the mats, using them
as a source for food and oxygen, the researchers believe.
Fig. 1. Worm burrows
from the Mesoproterozoic Chorhat Sandstone of central India support the
claim of molecular biologists that triploblastic metazoans existed twice
as long ago as the assumed Cambrian
evolutionary explosion of modern animal phyla. A remnant of the original
sand veneer cover (arrow) shows that the burrows are not modern artifacts
(Yale specimen YPM 37665). (Photograph by W. Sacco)
Physical processes can create patterns in sedimentary rocks that look
very similar to tracks left behind by animals. The researchers note several
points in favor of the argument that these tunnels really were made
by ancient worms. For example, they note that the diameters vary from tunnel
to tunnel but remain constant along each individual tunnel. The tunnels
also do not resemble the structures commonly caused by physical processes
and are similar to younger trace fossils known to be produced by similar
animals in more recent times.
Hear: Study
Director Dr. Adolph Seilacher, University of Tübingen, Germany.
The new discovery is likely to stir debate about the evolution of organisms
prior to the 'Cambrian explosion', a period of time some 500-600 million
years ago, when animal life made a sudden evolutionary shift and diverged
into nearly all the major animal divisions we know from fossils. One argument
favoring this model of evolution has been that no multicellular organisms
had been found in rocks older than the Cambrian. The new findings provide
some evidence suggesting that the diversification of life may have begun
long before this evolutionary big bang. .
"The existence of worm-like animals so much earlier than the Cambrian
period would suggest that animal body plans changed very little before
the explosive emergence of new designs in the Precambrian/Cambrian transition
and the onset of an arms-race style of Darwinian evolution," notes Adolph
Seilacher, University of Tübingen, Germany. "They also suggest that
the diversification of animal designs proceeded very slowly before the
appearance of organisms with hard skeletons, which is probably the key
event in the Cambrian evolutionary explosion."
Fig.
2 The burrows become visible only after the top millimeter of
the sandstone bed has been weathered away. Fresh surfaces lack such burrows
except for less distinct collapse structures. Therefore, it is assumed
that the worm-like maker was an undermat miner (part of the burrow that
is still covered by the original sand veneer is indicated by the arrow,
see Fig 1).
The study appears in the October 2, 1998 issue of Science.
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