Many fossils of the 580 million year old Doushantuo formation have been interpreted as fossil embryos; embryos are also common throughout the Cambrian fossil record.

[edit] Preservation

Taphonomic studies indicate that embryos are preserved for longest in reducing, anoxic conditions - these conditions can keep the embryos intact for long enough for bacteria to mineralise the cells and permit their preservation.^[1] However, phosphatisation was very rapid in the Doushantuo, so it's possible that faster preservation allowed embryos in different regimes to be preserved.

Preservation is mediated by bacterial biofilms. The cells of the embryo break down within hours of their death, under the effect of its own enzymes; bacteria invade the dacaying embryo before its cells can collapse, and biofilms taking on and preserving the three dimensional structure of the cells. The activity of these bacteria promotes mineralisation.^[2]

Currently, only fossils representing very early stages in embryonic development have been preserved - no embryos have been interpreted as having undergone a large number cleavage stages.^[3]

[edit] Affinities

Some embryos have been interpreted as colonies of sulfur-reducing bacteria, a claim that cannot be upheld in all cases.^[3]

Most fossil embryos are considered to belong to cnidarians and ecdysozoans, if they even fall into the metazoan crown group. No deuterostome or lophotrochozoan embryos have yet been reported, despite their similar preservation potential; this may be a result of different egg laying behaviour, as ecdysozoans lay eggs in the sediment rather than releasing them into the open - enhancing the chance of them becoming mineralised.^[1]

[edit] References

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