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.
Preservation
Taphonomic studies indicate that embryos are preserved for longest in reducing, anoxic conditions - a length of time similar to that required for bacteria to mineralise the cells and permit their preservation.[1] However, phosphatisation was very rapid in the Doushantuo, so it's possible that earlier preservation allowed embryos in different regimes to be preserved.
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.[2]
Alternative explanations
Some embryos have been interpreted as colonies of sulfur-reducing bacteria, a claim that cannot be upheld in all cases.[2]
Affinities
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]
References
- ^ a b Gostling, Neil J.; Thomas, Ceri-Wyn; Greenwood, Jenny M.; Dong, Xiping; Bengtson, Stefan; Raff, Elizabeth C.; Raff, Rudolf A.; Degnan, Bernard M.; Stampanoni, Marco; Donoghue, Philip C. J. (June 2008). "Deciphering the fossil record of early bilaterian embryonic development in light of experimental taphonomy". Evolution & Development 3 (11): 339–349. doi:10.1111%2Fj.1525-142X.2008.00242.x.
- ^ a b Gostling, N.J.; Donoghue, P.C.J.; Bengtson, S (2007). "The Earliest Fossil Embryos Begin to Mature". Evolution & Development 9 (3): 206–7. doi:10.1111/j.1525-142X.2007.00152.x.
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