A recent fossil discovery in South China, combined with fossils
found in 1998 and 2000 could reveal how the earliest known
egg-laying organism developed from embryo to adulthood.
Researches discovered thousands of 600-million-year-old fossilized
embryos in the Doushantuo Formation nine years ago. Two years
later, the same team unearthed fossils of a tubular coral-like
animal, named Megasphaera ornata, which appeared to be adult
versions of the embryos discovered earlier.
The case for a relationship between the two fossil types grew
stronger following the recent discovery of about 80
intermediate-stage fossils that have traits in common with both
groups.
The finding, to be detailed in the February issue of the journal
Geology, could provide the missing link between egg and
adult versions of one of Earth's earliest animals.
"The new fossils provide some suggestive evidence that these two
groups of fossils are linked developmentally," said study team
member Shuhai Xiao of Virginia Tech.
On the outside, the early and intermediate stage embryos look
very similar. They are about the same size -- about 0.02 inches
wide or about as big as a grain of sand -- and both have similar
outer coverings, or embryonic envelopes.
It's what's inside the egg that is different. Using an imaging
technique called microfocus X-ray computed tomography (microCT),
the researchers virtually peeled away the envelope to reveal the
new fossils' innards.
What the researchers found were three-dimensional spiral
structures that look like grooves on a screw.
The only signs of these coils on the outside of the embryos are
tiny holes arranged in a pattern resembling stitches on a baseball.
Traces of these coils are also found on the external coverings of
the adult fossils.
Some of the intermediate embryos also appeared to be unfurling,
encouraging the speculation that if the process had continued, the
embryos would distend like a stretched slinky or a flattened
fuselli noodle into the tubular adult form.
The new embryos could help shed light on how ancient animals
developed and whether the process was similar to that of living
organisms. Ancient embryos and embryos from modern day animals are
remarkably similar, but the developmental journey from egg to adult
for ancient organisms is still cloaked in mystery.
"Now we have isolated dots. We need to connect them and make a
complete story before we can say anything about their evolutionary
relationship," Xiao told LiveScience.
The researchers are hopeful that they can find later stages of
embryo development for M. ornata.
"I think this is an encouraging sign that later embryonic stages
may still be preserved in this rock," Xiao said. "If we keep
looking, we may even find a developmental sequence."
(Agencies via Xinhua News Agency January 24, 2007)
