Doushantuo Formation
Encyclopedia
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. The formation is of particular interest because it appears to cover the boundary between the problematic organisms of the Ediacaran
geological period and the more familiar fauna of the Cambrian explosion
of lifeforms whose descendents are recognizable. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, predating by perhaps five million years (Ma) the earliest of the 'classical' Ediacaran faunas from Mistaken Point, Newfoundland, and recording conditions a good forty to fifty million years before the Cambrian explosion.
with the underlying Liantuo Formation, which is free of fossils, an unconformity usually being interpreted as a period of erosion. On that unconformity lie tillites, the Nantuo Formation of cemented glacial till formed of glacial deposits of cobbles and gravel laid down at the end of the Cryogenian
glaciation ('Snowball Earth
'). This glacial level is tentatively dated ca 654 — 635 Ma.
The Doushantuo formation itself has three layers representing aquatic sediments that formed as sea levels rose with the melting of worldwide glaciation. Biomarkers indicate highly saline conditions, such as might be found in a lagoon, low oxygen levels, and very little sediment that had been washed off land surfaces.
The richest finds (the Lagerstätte itself) lie at the bottom of the middle stratum, with a date about 570 Ma.
, a common feature in many modern-day animals which is usually assumed to have evolved later, during the Cambrian Explosion. A nearly microscopic fossil animal, Vernanimalcula
("springtime micro-animal") was announced in October 2005, with the claim that it was the oldest known bilateral animal. However, the absence of adult forms of almost all animal types in the Doushantuo (there are microscopic adult sponge
s and coral
s) makes these claims difficult to prove: some argue that their lack suggests these finds are not larval and embryonic forms at all; supporters contend that some unidentified process "filtered out" all but the smallest forms from fossilization. An alternative interpretation suggests that it was created by non-biological rock-forming processes
. The team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found ten specimens (not illustrated) of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike.
The discovery was made when the rich phosphate deposits were being mined, and was first reported in 1998. The finds offer direct evidence that confirms expectations that major evolutionary diversification of animals already had occurred before the onset of the Cambrian period, with its apparent 'explosion' of metazoan life-forms and, therefore, that more remote ancestral forms of the phyla
recognizable in Cambrian macrofossils must have existed previously.
The documented biota
now includes phosphatized microfossils
of algae
, multicellular thallophyte
s (seaweeds), acritarch
s, ciliate
s, and cyanophytes, besides adult sponges and adult cnidaria
ns, which may include early forms of tabulate coral
s. There also seem to be what scientists cautiously report as bilateral animal embryos, termed Parapandorina, and eggs (Megasphaera). Some of the possible animal embryos are in an early stage of cellular division that was first interpreted as spore
s or algal cells, including eggs and embryos which are most probably of sponges or cnidarians, as well as adult sponges, a variety of adult cnidarians.
An alternative possibility is that the "embryos" and "eggs" are in fact fossils of giant sulfur bacteria
resembling Thiomargarita, a bacterium so large that it is visible to the naked eye. The interpretation would also provide a mechanism for phosphatic fossilization
through microbially mediated phosphate precipitation
by the bacteria, which has been observed in modern environments. If dark spots in the fossil transpire to be fossilised nucleii - an unverified claim - this would refute the Thiomargarita hypothesis.
Guizhou
' is a province of the People's Republic of China located in the southwestern part of the country. Its provincial capital city is Guiyang.- History :...
Province, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
that is notable for being one of the oldest fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
beds to contain highly preserved fossils. The formation is of particular interest because it appears to cover the boundary between the problematic organisms of the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
geological period and the more familiar fauna of the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
of lifeforms whose descendents are recognizable. Taken as a whole, the Doushantuo Formation ranges from about 635 Ma at its base to about 551 Ma at its top, predating by perhaps five million years (Ma) the earliest of the 'classical' Ediacaran faunas from Mistaken Point, Newfoundland, and recording conditions a good forty to fifty million years before the Cambrian explosion.
Sedimentology
The whole sequence sits on an unconformityUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
with the underlying Liantuo Formation, which is free of fossils, an unconformity usually being interpreted as a period of erosion. On that unconformity lie tillites, the Nantuo Formation of cemented glacial till formed of glacial deposits of cobbles and gravel laid down at the end of the Cryogenian
Cryogenian
The Cryogenian is a geologic period that lasted from . It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran...
glaciation ('Snowball Earth
Snowball Earth
The Snowball Earth hypothesis posits that the Earth's surface became entirely or nearly entirely frozen at least once, some time earlier than 650 Ma . Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical...
'). This glacial level is tentatively dated ca 654 — 635 Ma.
The Doushantuo formation itself has three layers representing aquatic sediments that formed as sea levels rose with the melting of worldwide glaciation. Biomarkers indicate highly saline conditions, such as might be found in a lagoon, low oxygen levels, and very little sediment that had been washed off land surfaces.
The richest finds (the Lagerstätte itself) lie at the bottom of the middle stratum, with a date about 570 Ma.
Fossils
Doushantuo fossils are all aquatic, microscopic, and preserved to a great degree of detail. The latter two characteristics mean that the structure of the organisms that made them can be studied at the cellular level, and considerable insight has been gained into the embryonic and larval stages of many early creatures. One contentious claim is that many of the fossils show signs of bilateral symmetrySymmetry (biology)
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry .In nature and biology,...
, a common feature in many modern-day animals which is usually assumed to have evolved later, during the Cambrian Explosion. A nearly microscopic fossil animal, Vernanimalcula
Vernanimalcula
Vernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria . It is known from deposits dating to . The fossils are between 0.1 and 0.2 mm across...
("springtime micro-animal") was announced in October 2005, with the claim that it was the oldest known bilateral animal. However, the absence of adult forms of almost all animal types in the Doushantuo (there are microscopic adult sponge
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...
s and coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s) makes these claims difficult to prove: some argue that their lack suggests these finds are not larval and embryonic forms at all; supporters contend that some unidentified process "filtered out" all but the smallest forms from fossilization. An alternative interpretation suggests that it was created by non-biological rock-forming processes
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
. The team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found ten specimens (not illustrated) of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike.
The discovery was made when the rich phosphate deposits were being mined, and was first reported in 1998. The finds offer direct evidence that confirms expectations that major evolutionary diversification of animals already had occurred before the onset of the Cambrian period, with its apparent 'explosion' of metazoan life-forms and, therefore, that more remote ancestral forms of the phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
recognizable in Cambrian macrofossils must have existed previously.
The documented biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...
now includes phosphatized microfossils
Phosphatic fossilization
Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures...
of algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, multicellular thallophyte
Thallophyte
The thallophytes are a polyphyletic group of non-mobile organisms traditionally described as "relatively simple plants" or "lower plants" with undifferentiated bodies...
s (seaweeds), acritarch
Acritarch
Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
s, ciliate
Ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...
s, and cyanophytes, besides adult sponges and adult cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns, which may include early forms of tabulate coral
Tabulate coral
The tabulate corals, forming the order Tabulata, are an extinct form of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to a honeycomb. Adjacent cells are joined by small pores...
s. There also seem to be what scientists cautiously report as bilateral animal embryos, termed Parapandorina, and eggs (Megasphaera). Some of the possible animal embryos are in an early stage of cellular division that was first interpreted as spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s or algal cells, including eggs and embryos which are most probably of sponges or cnidarians, as well as adult sponges, a variety of adult cnidarians.
An alternative possibility is that the "embryos" and "eggs" are in fact fossils of giant sulfur bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
resembling Thiomargarita, a bacterium so large that it is visible to the naked eye. The interpretation would also provide a mechanism for phosphatic fossilization
Phosphatic fossilization
Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures...
through microbially mediated phosphate precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
by the bacteria, which has been observed in modern environments. If dark spots in the fossil transpire to be fossilised nucleii - an unverified claim - this would refute the Thiomargarita hypothesis.