Fossil embryos
Encyclopedia
Fossil embryos are the preserved remains of organisms that have yet to hatch or be born. Many fossil
s of the Doushantuo formation
have been interpreted as embryos; embryos are also common throughout the Cambrian
fossil record.
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.
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.
Fossilized embryos of the 635- to 551-million year old Doushatuo 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.
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.
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.
might actually be from embryos.
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s of the Doushantuo formation
Doushantuo Formation
The Doushantuo Formation is a Lagerstätte in Guizhou Province, China that is notable for being one of the oldest fossil beds to contain highly preserved fossils...
have been interpreted as embryos; embryos are also common throughout the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
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'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.
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.Fossilized embryos of the 635- to 551-million year old Doushatuo 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.
It is not until the Cambrian that embryos with demonstrable eumetazoan features occur; gastrulation
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...
and a pentaradial symmetry are expressed in some lower Cambrian fossils.
Some fossil embryos are considered to belong to cnidarians and ecdysozoans, if they even fall into the metazoan crown group. No deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
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.
Dinosaurs
Some hadrosaur fossils recovered from the Dinosaur Park FormationDinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...
might actually be from embryos.