Editing Phosphatization (section)

== Phosphatic preservation in Burgess Shale-type fossils ==
[[File:Gut tract and diverticula preservation in Mollisonia from the Cambrian (Wuliuan) Burgess Shale.png|thumb|left|Phosphatized gut diverticula of ''[[Mollisonia]]'' 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.<ref name="ref_b">{{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 }}</ref> A low pH makes CaCO<sub>3</sub> less likely to precipitate, clearing the way for phosphate to be laid down.<ref name="ref_b" /> 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.<ref name="Wilby2009">{{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 }}</ref>

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.<ref name="ref_b" />

Phosphatization can happen quickly: The chitinous structures that support bivalve gills can be replaced by calcium phosphate,<ref name="ref_">{{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 }}</ref> with a little help from co-occurring bacteria, in just two to six days.<ref name="ref_a">{{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 }}</ref> The gill axes and musculature of bivalves can also be preserved in phosphate.<ref name="ref_"/><ref name=":0">{{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}}</ref>
The structures that are most famously preserved in phosphate in the Burgess Shale are the midgut glands of ''[[Leanchoilia]]'',<ref name="Butterfield2002">{{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 }}</ref> perhaps on account of their central position and plausibly a low pH.

Phosphatization can be microbially mediated, especially in decay-resistant groups such as [[arthropod]]s; or substrate-dominated, where phosphate-rich tissue leads the mineralization process (as in fish). [[Cephalopod]]s fall somewhere between these two extremes.<ref name="Wilby1997a" /><ref name="ref_" /><ref name=":0" />