Phosphatization

Large quantities of phosphate are required, either from seawater or from the tissues of the decaying organism. In some cases microbes control the phosphatization, and the remains of the microbes that feed on the preserved tissue form the fossil. In others, the tissue itself is the source of phosphate and its phosphatized remains form the fossil. In the intermediate case the phosphatized tissue retains the impressions of the phosphatizing microbes.{{Cite journal | doi = 10.1016/S0016-6995(97)80056-3 | last1 = Wilby | first1 = P. | last2 = Briggs | first2 = D. | title = Taxonomic trends in the resolution of detail preserved in fossil phosphatized soft tissues | journal = Geobios | volume = 30 | pages = 493–502 | year = 1997| bibcode = 1997Geobi..30..493W }}

File:Gut tract and diverticula preservation in Mollisonia from the Cambrian (Wuliuan) Burgess Shale.png from the Burgess Shale]]

Soft-tissue fossils, such as those found in the Burgess Shale, 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.{{Cite journal | last1 = Briggs | first1 = Derek E. G. | last2 = Kear | first2 = Amanda J. | title = Decay and mineralization of shrimps | doi = 10.2307/3515135 | jstor=3515135 | journal = PALAIOS| volume = 9 | issue = 5 | pages = 431–456 | date = October 1994 | bibcode = 1994Palai...9..431B }} 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.{{Cite journal | doi = 10.1017/S001675680001147X| last1 = Wilby | first1 = P. R. | last2 = Whyte | first2 = M. A. | title = Phosphatized soft tissues in bivalves from the Portland Roach of Dorset (Upper Jurassic) | journal = Geological Magazine | volume = 132 | page = 117 | year = 1995| issue = 1 | bibcode = 1995GeoM..132..117W | s2cid = 140660499 }}

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,{{Cite journal | last1 = Klug | first1 = C. | last2 = Hagdorn | first2 = H. | last3 = Montenari | first3 = M. | title = Phosphatized Soft-Tissue in Triassic Bivalves | doi = 10.1111/j.1475-4983.2005.00485.x | journal = Palaeontology | volume = 48 | pages = 833–852 | year = 2005 | issue = 4 | bibcode = 2005Palgy..48..833K | doi-access = free }} with a little help from co-occurring bacteria, in just two to six days.{{Cite journal | last1 = Skawina | first1 = A. | doi = 10.2110/palo.2009.p09-081r | title = Experimental Decay of Gills in Freshwater Bivalves As a Key to Understanding Their Preservation in Upper Triassic Lacustrine Deposits | journal = PALAIOS | volume = 25 | pages = 215–220 | year = 2010 | issue = 3 | bibcode = 2010Palai..25..215S | s2cid = 129337648 }} The gill axes and musculature of bivalves can also be preserved in phosphate.{{Cite book |last1=Klug |first1=Christian |title=Cephalopods Present and Past: New Insights and Fresh Perspectives |last2=Montenari |first2=Michael |last3=Schulz |first3=Hartmut |last4=Urlichs |first4=Max |publisher=Springer |year=2007 |isbn=978-1-4020-6806-5 |editor-last=Landman |editor-first=Neil H. |location=Dordrecht |pages=205–220 |chapter=Soft-tissue Attachment of Middle Triassic Ceratitida from Germany |doi=10.1007/978-1-4020-6806-5_10 |editor-last2=Davis |editor-first2=Richard Arnold |editor-last3=Mapes |editor-first3=Royal H. |chapter-url=https://link.springer.com/chapter/10.1007/978-1-4020-6806-5_10}}

The structures that are most famously preserved in phosphate in the Burgess Shale are the midgut glands of Leanchoilia,{{cite journal| first1 = N. J. | title = Leanchoilia guts and the interpretation of three-dimensional structures in Burgess Shale-type fossils | journal = Paleobiology | volume = 28 | pages = 155–171 | year = 2002 | issn = 0094-8373| last1 = Butterfield | doi = 10.1666/0094-8373(2002)028<0155:LGATIO>2.0.CO;2 | s2cid = 85606166 }} 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.

In phosphatic fossils, the preservation is so fine that even some cellular structure has been preserved. The phosphatic microfossils of the Doushantuo Formation, 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 blastulas, possibly animal embryos at an early stage in cell division.

The Doushantuo Formation presents a classic example of phosphatic fossilization:{{long quote|date=February 2023}}

:'This high-resolution fossil bed is about 30% phosphate, present as the mineral fluorapatite [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 of 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.{{cite journal |vauthors=Chen JY, Oliveri P, Li CW, etal |title=Precambrian animal diversity: Putative phosphatized embryos from the Doushantuo Formation of China |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=9 |pages=4457–62 |date=April 2000 |pmid=10781044 |pmc=18256 |doi=10.1073/pnas.97.9.4457 |url=http://authors.library.caltech.edu/9385/1/CHEpnas00.pdf |doi-access=free }} 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 CAT scan.{{cite journal |vauthors=Donoghue PC, Bengtson S, Dong XP, etal |title=Synchrotron X-ray tomographic microscopy of fossil embryos |journal=Nature |volume=442 |issue=7103 |pages=680–3 |date=August 2006 |pmid=16900198 |doi=10.1038/nature04890 |bibcode=2006Natur.442..680D |s2cid=4411929 }}[http://www.spacedaily.com/news/life-02zs.html X-ray computerized tomography application to phosphatic microfossils.]

File:Sula figueroae holotype & paratype.jpg

File:Selected soft-bodied arthropods from the Spence Shale.jpg

File:Brachydelphis mazeasi specimen MUSM 887 (1).jpg

File:Aetomylaeus sp. from Bahía Inglesa Formation (4).jpg

{{reflist}}

• [https://archive.today/20150504/http://www.peripatus.gen.nz/Paleontology/lagDouPho.html A brief overview of Doushantuo formation's phosphatic microfossils]

Category:Fossilization