Embryo fossils are the preserved remains of unhatched or unborn organisms. Many

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s of the

Doushantuo Formation The Doushantuo Formation (formerly transcribed as Toushantuo or Toushantou, from ) is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerst ...

have been interpreted as embryos;

embryos An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

are also common throughout the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

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. 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. 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.

Affinities

Some embryos have been interpreted as colonies of sulfur-reducing bacteria, a claim that cannot be upheld in all cases. 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 The blastocoel (), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammal ...

formation or the organization of blastomeres into

epithelia Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

– epithelialization should be underway in modern embryos with >100 cells. Features preserved on Doushantuo embryos are compatible with

metazoans Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

(animals), but the absence of epithelialization is consistent only with a stem-metazoan affinity. It is not until the Cambrian that embryos with demonstrable eumetazoan features occur;

gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. ...

and a pentaradial symmetry are expressed in some lower Cambrian fossils. Some fossil embryos are considered to belong to

cnidarians Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that the ...

and

ecdysozoans Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo ''et al.'' in 1997, based mainly on phylogenetic tre ...

, if they even fall into the metazoan crown group. No

deuterostome Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...

lophotrochozoan Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, brachiopo ...

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. 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. Some have also been interpreted as algal.

Dinosaurs

Some

hadrosaur Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which includ ...

fossils recovered from the

Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 76. ...

may represent embryos.

Titanosauria Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thr ...

n and tyrannosaurid embryos are also known.

Preservation

Affinities

Dinosaurs

References

Further reading