Phosphatic fossilization
Encyclopedia
Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the scanning electron microscope
.
, are rare. In some cases their internal organs are replicated in phosphate. The phosphate mainly comes from the tissue itself, and may later be replaced by calcium carbonate. A low pH makes CaCO3 less likely to precipitate, clearing the way for phosphate to be laid down. This is facilitated by the absence of oxygen in the decaying tissue. Accordingly, (secondary) phosphate is generally only preserved in enclosed spaces, such as a tightly-closed bivalve shell.
Higher concentrations of phosphate in the sea water do not enhance phosphatization, as may seem natural; rather, it increases the rate at which the organism breaks up, perhaps because the mineral "fertilizes" the decay micro-organisms.
Phosphatization can happen quickly: The chitinous structures that support bivalve gills can be replaced by calcium phosphate, with a little help from co-occurring bacteria, in just two to six days. The gill axes and musculature of bivalves can also be preserved in phosphate.
The structures that most famously preserved in phosphate in the Burgess Shale are the midgut glands of Leanchoilia, perhaps on account of their central position and plausibly a low pH?
Phosphatization can be microbially mediated, especially in decay-resistant groups such as arthropods; or substrate-dominated, where phosphate-rich tissue leads the mineralization process (as in fish). Cephalopods fall somewhere between these two extremes.
(q.v.), a fossil-rich lagerstätte of the Ediacaran
period, about 590–565 Ma (megaannua; million years ago), display some of the most spectacular cellular-level preservation known from the geologic record. The fossils include what may be metazoan blastula
s, possibly animal embryos at an early stage in cell division.
The Doushantuo formation presents a classic example of phosphatic fossilization:
Careful acid baths etch away the limestone matrices, by slowly dissolving the carbonates, and reveal the phosphates that have replaced organic structures, in the manner that Dr. Chen describes. There are other means of fossilization represented in the Doushantuo formation as well.
A refinement to viewing the internal structure of fossilized embryos uses specialized microscopic three-dimensional x-ray computed tomography
, a kind of micro CATscan.
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
.
Phosphatic preservation in Burgess Shale-type fossils
Soft-tissue fossils, such as those found in the Burgess ShaleBurgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
, are rare. In some cases their internal organs are replicated in phosphate. The phosphate mainly comes from the tissue itself, and may later be replaced by calcium carbonate. A low pH makes CaCO3 less likely to precipitate, clearing the way for phosphate to be laid down. This is facilitated by the absence of oxygen in the decaying tissue. Accordingly, (secondary) phosphate is generally only preserved in enclosed spaces, such as a tightly-closed bivalve shell.
Higher concentrations of phosphate in the sea water do not enhance phosphatization, as may seem natural; rather, it increases the rate at which the organism breaks up, perhaps because the mineral "fertilizes" the decay micro-organisms.
Phosphatization can happen quickly: The chitinous structures that support bivalve gills can be replaced by calcium phosphate, with a little help from co-occurring bacteria, in just two to six days. The gill axes and musculature of bivalves can also be preserved in phosphate.
The structures that most famously preserved in phosphate in the Burgess Shale are the midgut glands of Leanchoilia, perhaps on account of their central position and plausibly a low pH?
Phosphatization can be microbially mediated, especially in decay-resistant groups such as arthropods; or substrate-dominated, where phosphate-rich tissue leads the mineralization process (as in fish). Cephalopods fall somewhere between these two extremes.
Phosphate-only fossils
In phosphatic fossils, the preservation is so fine that even some cellular structure has been preserved. The phosphatic microfossils of the Doushantuo FormationDoushantuo 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...
(q.v.), a fossil-rich lagerstätte 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...
period, about 590–565 Ma (megaannua; million years ago), display some of the most spectacular cellular-level preservation known from the geologic record. The fossils include what may be metazoan blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...
s, possibly animal embryos at an early stage in cell division.
The Doushantuo formation presents a classic example of phosphatic fossilization:
- 'This high-resolution fossil bed is about 30% phosphate, present as the mineral fluorapatiteFluorapatiteFluorapatite, often with the alternate spelling of fluoroapatite, is a mineral with the formula Ca53F . Fluorapatite is a hard crystalline solid. Although samples can have various color , the pure mineral is colorless as expected for a material lacking transition metals...
[Ca5(PO4)3F]. Phosphatic beds within this deposit are grainstones composed of 1- to 5-mm phosphoclasts. These derive from a phosphatic surface that formed on the sea floor, in the process recrystallizing existing surface sediments. In addition to replacing carbonate sediments, soft tissues of metazoan embryos, larvae, adults, and algae also appear to have been mineralized. The phosphatized sediment crust was then broken into small fragments by heavy current activity and then redeposited and mixed in with adjacent lime muds.
Careful acid baths etch away the limestone matrices, by slowly dissolving the carbonates, and reveal the phosphates that have replaced organic structures, in the manner that Dr. Chen describes. There are other means of fossilization represented in the Doushantuo formation as well.
A refinement to viewing the internal structure of fossilized embryos uses specialized microscopic three-dimensional x-ray computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
, a kind of micro CATscan.