Biology:Embryo fossil
Embryo fossils are the preserved remains of unhatched or unborn organisms. Many fossils of the 580 million year old Doushantuo Formation have been interpreted as embryos; embryos are also common throughout the Cambrian fossil record.
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 is 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 decaying embryo before its cells can collapse, and biofilms take on and preserve the three-dimensional structure of the cells. The activity of these bacteria promote 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]
Affinities
Some embryos have been interpreted as colonies of sulfur-reducing bacteria, a claim that cannot be upheld in all cases.[3]
Embryo fossils found in Doushantuo Formation of southern China exhibit occasional asynchronous cell division, common in modern embryos, implying that sophisticated mechanisms for differential cell division timing and embryonic cell lineage differentiation evolved before 551 million years ago. However, embryos composed of hundreds to more than ~1000 cells still show no evidence of blastocoel formation or the organization of blastomeres into epithelia – epithelialization should be underway in modern embryos with >100 cells. Features preserved on Doushantuo embryos are compatible with metazoans (animals), but the absence of epithelialization is consistent only with a stem-metazoan affinity.[4] It is not until the Cambrian that embryos with demonstrable eumetazoan features occur; gastrulation and a pentaradial symmetry are expressed in some lower Cambrian fossils.[5]
Some 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]
An alternative interpretation of the embryos appears to discount an animal – or indeed embryonic – interpretation altogether. Cell division without enlargement appears to continue beyond what it would in embryos, and without other embryonic traits becoming apparent. Such division is found in a wide range of eukaryotes, including some that are not truly multicellular, and this more conservative interpretation looks to be more parsimonious than embryonic claims.[6]
Some have also been interpreted as algal.[7]
Dinosaurs
Some hadrosaur fossils recovered from the Dinosaur Park Formation may represent embryos.[8] Titanosaurian and tyrannosaurid embryos are also known.
References
- ↑ 1.0 1.1 Gostling, N. J.; Thomas, C. W.; Greenwood, J. M.; Dong, X.; Bengtson, S.; Raff, E. C.; Raff, R. A.; Degnan, B. M. et al. (2008). "Deciphering the fossil record of early bilaterian embryonic development in light of experimental taphonomy". Evolution & Development 10 (3): 339–349. doi:10.1111/j.1525-142X.2008.00242.x. PMID 18460095.
- ↑ Raff, E. C.; Schollaert, K. L.; Nelson, D. E.; Donoghue, P. C. J.; Thomas, C. -W.; Turner, F. R.; Stein, B. D.; Dong, X. et al. (2008). "Embryo fossilization is a biological process mediated by microbial biofilms". Proceedings of the National Academy of Sciences 105 (49): 19360–19365. doi:10.1073/pnas.0810106105. PMID 19047625. Bibcode: 2008PNAS..10519360R.
- ↑ 3.0 3.1 Gostling, N. J.; Donoghue, P. C. J.; Bengtson, S. (2007). "The earliest fossil embryos begin to mature". Evolution & Development 9 (3): 206–207. doi:10.1111/j.1525-142X.2007.00152.x. PMID 17501743.
- ↑ Hagadorn, J. W.; Xiao, S.; Donoghue, P. C. J.; Bengtson, S.; Gostling, N. J.; Pawlowska, M.; Raff, E. C.; Raff, R. A. et al. (2006). "Cellular and Subcellular Structure of Neoproterozoic Animal Embryos". Science 314 (5797): 291–294. doi:10.1126/science.1133129. PMID 17038620. Bibcode: 2006Sci...314..291H.
- ↑ Yao, X.; Han, J.; Jiao, G. (2011). "Early Cambrian epibolic gastrulation: A perspective from the Kuanchuanpu Member, Dengying Formation, Ningqiang, Shaanxi, South China". Gondwana Research 20 (4): 844–851. doi:10.1016/j.gr.2011.04.003. Bibcode: 2011GondR..20..844Y.
- ↑ Huldtgren, T.; Cunningham, J. A.; Yin, C.; Stampanoni, M.; Marone, F.; Donoghue, P. C. J.; Bengtson, S. (2011). "Fossilized Nuclei and Germination Structures Identify Ediacaran "Animal Embryos" as Encysting Protists". Science 334 (6063): 1696–1699. doi:10.1126/science.1209537. PMID 22194575. Bibcode: 2011Sci...334Q1696H.
- ↑ Zhang, X. G.; Pratt, B. R. (2014). "Possible Algal Origin and Life Cycle of Ediacaran Doushantuo Microfossils with Dextral Spiral Structure". Journal of Paleontology 88: 92–98. doi:10.1666/13-014.
- ↑ Tanke, D.H.; Brett-Surman, M.K. (2001). "Introduction". Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206-218.. Mesozoic Vertebrate Life—New Research Inspired by the Paleontology of Philip J. Currie. Bloomington: Indiana University Press. pp. xviii + 577 pp.
Further reading
- Summary of perspectives up to 2012 (subscription required): Butterfield, N. J. (2011). "Terminal Developments in Ediacaran Embryology". Science 334 (6063): 1655–1656. doi:10.1126/science.1216125. PMID 22194566. Bibcode: 2011Sci...334.1655B.
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