Fossil embryo

Fossil embryos are the preserved remains of organisms that have yet to hatch or be born. 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]

Fossilized embryos of the 635- to 551-million year old 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]

Ediacaran embryos

A helically coiled spheroidal fossils discovered in south china, which are related to the Ediacaran era [8] These fossils characterised by a three dimensional spiral tunnel that is punctured by a series of[9] pores, or holes. Scientists suggest that these fossils are similar to Megasphaera Ornata from many aspects such as the similarity in envelop, and in size, so there is an assumption said that these fossils represent postblastula embryos for the Megasphaera ornate. in addition, There was a remarkable discovery preserved an eggs, and embryos fossils which are believed to be the oldest metazoan fossils known in the fossils records. These fossils show the cleavage stage, and the blastula, in addition to the adult metazoans fossils that include somatic cells, and germ cells, but the identity of these adults not yet unknown. Also, there are a microfossils that considered as one of the largest fossils from Ediacaran period,[10] these fossils resemble the embryos, and larva of metazoan, and these embryo fossils are clearly shown the direct developing lecithotropic forms.

Cambrian embryos

Markuelia embryo fossils from Siberia , and Olivoodes embryo fossils from China considered as an example of embryo fossils from the earliest Cambrian (541-485.4 million years ago). Markuelia fossils from Aldan River in southern Yakutia, Siberia show a wormlike animals which have a chambers. The two ends are in on hemisphere which are connected to each other. There is a S-shape in the middle part of the fossil, in some specimen there are a rod like structure which suggested to be lateral nerve branches from a nerve cord that is longitudinal. Most Olivooides fossils observed the late embryonic stage, and eggs that contain an embryo which is looped; in addition, to earlier and later cleavage stages. The egg fossils show a smooth outer membrane that covered with nonannulated stellate tissue , most of these globules contain a pattern of a regular polygonal surface .Some fossils represent the early cleavage stages, and the others represent the gastrula.[11] There are some specimens show a hatched animal which is conical that has a star-shape surface, and stellae. There are a pertural in the peak. They are an addition of striated tissue which is indicates to a growth process. This fossils characterized by stellate cuticle which is cover the surface with a regular sharp protrusions. In general ,these fossils for (Markuelia , and Olivooides) are direct development, and they represent an early metazoan since of the egg size , which is generally larger in the direct developers , compared to eggs of indirect developers . stellate embryo, cleavage stage, embryo within fertilization envelope, and early and later post embryonic polyp stage clearly observed in these fossils. Some Markuelia of these fossils currently located in Swedish Museum of Natural History, while Olivooides fossils located in the National Geological Museum of China, and some of them deposit to Chinese Academy of Geological Sciences.

-In South China (Kuanchuaanpu Formation in Shaanxi Province) it was discovered a great embryos fossils from the lower Cambrian for Cubozoan embryos “ sea wasps” [12] (fig 1) one of the marina organisms a sister group of the Scyphozoa. The “ sea wasps” characterized by interradial tentacles “4 branches” , a box shape with four sided , and four complex eyes “well developed”, and from the enter these species contain four interradial pairs of gonads “a leaf shape” ,and four interradial septa & claustra.[13] Most fossils show the embryo in hemi-spherical shape, and the others are ellipsoid. Furthermore, there are two layers for the most specimens (exumbrella, and subumbrella )outer and inner in respectively. The gastric groove represents in between these two layers , and it is consists of set of lamellae, and gastric pockets. The main structures that are clear in these fossils are the tentacle which is “buds, or primordial”and the mouth cavity. The tentacles that appear in most specimen are between four or five, and they arise from the subumbrellar wall. The oral cavity has a lips in some fossils which are exist around the mouth opening, but in some specimen the oral lips represent in the perradii. There is a lumen in the end of oral cavity that represent in the esophagus which has an extend part of oral lips. There was a straight plates considers as interradial septa which is extend toward the stomach cavity, and there are a paired leaf like attached to it which is refers to gonad lamella.

Figure 1: Micro-CT photographs of microscopic cubozoan. Fossils.

Figure 1: Micro-CT photographs of microscopic cubozoan Fossils.

Moreover, in Mackenzie mountain, Northwest Territories, in northwestern Canada a metazoan fossils from early Cambrian discovered, these fossils appear simile to a small circular discs which represent a cnidarian, and some creeping bilaterians, and it was noted a clear presence of guts in bilaterians fossils.

Controversies

There are a famous fossils of embryo have been discovered in China, in the Ediacaran Doushantuo formation, Datang Quarry, Weng’s, Guizhou Province, southern China, most fossils characterized by appearance of cell cleavage(palintomic) for the nonmetazoa holozoa the sister group of fungi (fig 2), the study of these fossils has shown that specimens are related to Tianzhushania (ancestor of Spiralicellula, Megasphaera, and Parapandorina). There were a nucleus like bodies have a globular shape that clearly shown in all specimens, and their position was central to the cells. In another specimens the area surrounding the nucleus like bodies is homogeneous, it has some finely granular tissue. In the center of nucleus like bodies there was a spheroidal bodies that represent the nucleoli and it was surrounded by the same tissue. There was an amazing specimen that shows a stages of division because it contains an elongated, and dumbbell shape. In addition, there are some fossils that are likely to follow Spiralicellula which is similar to Tianzhushania in terms of pattern of cell division, and Spiralicellula characterized by an elongated and vermiform body. This study illustrate how most of these fossils seems to be similar to modern Mesomycetozoeans.

Figure 2: Fossilized Nuclei and Germination Structures Identify Ediacaran “Animal Embryos” as Encysting Protists.

. These fossils are still under discussion about their identity, Therese Huldtgren with his colleagues said these fossils incompatible with metazoan embryos “multicellular” as some scientists assume because the developmental pattern is resemble a nonmetazoan holozoans,[14] but on the another side, some scientists like Xiao publish an article to comment about these fossils, and as he said these fossils doesn’t resemble a mesomycetozoeans[15] because the structures that Huldgren said there are nuclei isn’t nuclei since the size is too large to be nuclei .Huldtgren responds to this article and said that he depends on the number of morphological features when he identify the nuclei ;such as, the position with cells,the shape volumetric between the cells, and the nuclei. He stresses that these fossils didn’t represent Prokaryotes ,multicellular stem, or crown metazoans as some believe.so in general there are a strong controversies, and opposed concerning the identity of these fossils, and each scientist try to prove his theory.

Fossils and evolution

Records of fossils that discovered so far contain a set examples of the contribution of fossils in people’s understanding of the process of biological evolution, also help us to answer some questions about their evolutionary steps, and give us a unique view into a history of earth .Hoatzin chicks; for instance, has claws on their wings, and this is a big evidence that refers to that birds ancestors had clawed hands .Moreover, some animals like frogs, rabbits , and lizards have different forelimbs , although of the differences , they have the same set of bones with the same arrangement, and these bones have the same organisation in the fossil of their common ancestor (Eusthenopteron). In addition, fossils provide us with evidence about some process that happened in our plant, such as, the process of transition from one environment to another, for instance, when some scientists in 1988 discovered a special fossil (370 million years old) refers to a transition from sea creatures to land animals (tetrapods). ”,and the transition process from one species to another like a one reptiles group (Cynodonts) appears 260 million years ago as reptiles, but by the time it appears in some fossils as mammals (245 million years ago).Evolution and transition process are very clear in the bone structure of ear, so all fossils that are record until today they provide us with sufficient clarity to understand more about the evolution history.

In general, there is a clear relationship between the evolution(a theory about history of life), and the paleontology (studying of fossils) that provide us with information about the evolution , and about ourselves as humans, and this relationship is that evolution development from a palaeontology “the historic”. [16] evolution history.

Dinosaurs

Some hadrosaur fossils recovered from the Dinosaur Park Formation may represent embryos.[17]

References

  1. 1 2 Gostling, N. J.; Thomas, C. W.; Greenwood, J. M.; Dong, X.; Bengtson, S.; Raff, E. C.; Raff, R. A.; Degnan, B. M.; Stampanoni, M.; Donoghue, P. C. J. (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.
  2. ↑ Raff, E. C.; Schollaert, K. L.; Nelson, D. E.; Donoghue, P. C. J.; Thomas, C. -W.; Turner, F. R.; Stein, B. D.; Dong, X.; Bengtson, S.; Huldtgren, T.; Stampanoni, M.; Chongyu, Y.; Raff, R. A. (2008). "Embryo fossilization is a biological process mediated by microbial biofilms". Proceedings of the National Academy of Sciences 105 (49): 19360–19365. Bibcode:2008PNAS..10519360R. doi:10.1073/pnas.0810106105.
  3. 1 2 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.
  4. ↑ Hagadorn, J. W.; Xiao, S.; Donoghue, P. C. J.; Bengtson, S.; Gostling, N. J.; Pawlowska, M.; Raff, E. C.; Raff, R. A.; Turner, F. R.; Chongyu, Y.; Zhou, C.; Yuan, X.; McFeely, M. B.; Stampanoni, M.; Nealson, K. H. (2006). "Cellular and Subcellular Structure of Neoproterozoic Animal Embryos". Science 314 (5797): 291–294. Bibcode:2006Sci...314..291H. doi:10.1126/science.1133129. PMID 17038620.
  5. ↑ 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. doi:10.1016/j.gr.2011.04.003.
  6. ↑ 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. doi:10.1126/science.1209537.
  7. ↑ 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. doi:10.1666/13-014.
  8. ↑ http://www.ucmp.berkeley.edu/vendian/ediacaran.php
  9. ↑ Xiao, S. ".Rare helical spheroidal fossils from the Doushantuo Lagerstatte: Ediacaran animal embryos come of age" 35 ((2)): 115–118. PMID 23277.
  10. ↑ Raff, E.C. Villinski, J.T. Turner, F.R. Donoghue, P.C.J. and Raff, R.A.2005.Experimental taphonomy shows the feasibility of fossil embryos .PNAS Journal.103 (15):5846-5851.
  11. ↑ Bengtson, S. "Fossilized Metazoan Embryos from the Earliest Cambrian" 227 (5332). PMID 1126.
  12. ↑ http://www.ucmp.berkeley.edu/cnidaria/cubozoa.html
  13. ↑ Han, J. "Early Cambrian Pentamerous Cubozoan Embryos from South China". PMID 1371.
  14. ↑ Huldtgren, T. ".Fossilized Nuclei and Germination Structures Identify Ediacaran "Animal Embryos" as Encysting Protists" .334. PMID 1209537.
  15. ↑ Xiao, S. "Comment on "Fossilized Nuclei and Germination Structures Identify Ediacaran "Animal Embryos" as encysting protist" 335. PMID 1218814.
  16. ↑ http://evolution.berkeley.edu/evolibrary/article/evo_01
  17. ↑ Tanke, D.H. and Brett-Surman, M.K (2001). "Introduction". In D.H. Tanke and K. Carpenter. 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

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