Editing Microfossil (section)

==Cloudinids==
The [[cloudinid]]s were an early [[metazoan]] [[Family (biology)|family]] that lived in the late [[Ediacaran]] [[Period (geology)|period]] about 550 million years ago,<ref name="NYT-20200110">{{cite news|url=https://www.nytimes.com/2020/01/10/science/fossil-guts-intestines.html|title=Fossil Reveals Earth's Oldest Known Animal Guts - The find in a Nevada desert revealed an intestine inside a creature that looks like a worm made of a stack of ice cream cones.|last=Joel|first=Lucas|date=10 January 2020|work=[[The New York Times]]|access-date=10 January 2020}}</ref><ref name="NAT-20200110" /> and became extinct at the base of the [[Cambrian]].<ref name="Yang2016">{{cite journal|doi=10.1016/j.precamres.2016.09.016|title=Transitional Ediacaran–Cambrian small skeletal fossil assemblages from South China and Kazakhstan: Implications for chronostratigraphy and metazoan evolution|journal=Precambrian Research|volume=285|pages=202–215|year=2016|last1=Yang|first1=Ben|last2=Steiner|first2=Michael|last3=Zhu|first3=Maoyan|last4=Li|first4=Guoxiang|last5=Liu|first5=Jianni|last6=Liu|first6=Pengju|bibcode=2016PreR..285..202Y}}</ref> They formed small millimetre size conical fossils consisting of [[calcareous]] cones nested within one another; the appearance of the organism itself remains unknown. The name ''Cloudina'' honors [[Preston Cloud]].<ref name=Description>{{cite journal| doi=10.2475/ajs.272.8.752| author=Germs, G.J.B. | title=New shelly fossils from Nama Group, South West Africa| journal=American Journal of Science |date=October 1972 | volume=272 | pages=752–761| issue=8| bibcode=1972AmJS..272..752G}}</ref>  Fossils consist of a series of stacked vase-like [[calcite]] tubes, whose original mineral composition is unknown,<ref name="Porter2007">{{cite journal| journal=Science | date= 1 June 2007 |volume= 316 |issue=5829 |doi= 10.1126/science.1137284| title= Seawater Chemistry and Early Carbonate Biomineralization |author=Porter, S.M.| pages = 1302 | pmid= 17540895 | bibcode=2007Sci...316.1302P| s2cid= 27418253 }}</ref> Cloudinids comprise two genera: ''Cloudina'' itself is mineralized, whereas ''Conotubus'' is at best weakly mineralized, whilst sharing the same "funnel-in-funnel" construction.<ref>{{cite journal|doi=10.1130/G38157.1|title=The end of the Ediacaran: Two new exceptionally preserved body fossil assemblages from Mount Dunfee, Nevada, USA|journal=Geology|volume=44|issue=11|pages=911|year=2016|last1=Smith|first1=E.F.|last2=Nelson|first2=L.L.|last3=Strange|first3=M.A.|last4=Eyster|first4=A.E.|last5=Rowland|first5=S.M.|last6=Schrag|first6=D.P.|last7=MacDonald|first7=F.A.|bibcode=2016Geo....44..911S}}</ref>

Cloudinids had a wide geographic range, reflected in the present distribution of localities in which their fossils are found, and are an abundant component of some deposits. ''Cloudina'' is usually found in association with microbial [[stromatolites]], which are limited to shallow water, and it has been suggested that cloudinids lived embedded in the [[microbial mat]]s, growing new cones to avoid being buried by silt. However no specimens have been found embedded in mats, and their mode of life is still an unresolved question.

The [[Taxonomy (biology)|classification]] of the cloudinids has proved difficult: they were initially regarded as [[polychaete]] worms, and then as coral-like [[cnidarian]]s on the basis of what look like [[Budding|buds]] on some specimens. Current scientific opinion is divided between classifying them as polychaetes and regarding it as unsafe to classify them as members of any broader grouping. In 2020, a new study showed the presence of [[Nephrozoa]]n type [[Gastrointestinal tract|guts]], the oldest on record, supporting the [[bilateria]]n interpretation.<ref name="NAT-20200110">{{cite journal |author=Schiffbauer, James D. |display-authors=et al. |title=Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period |date=10 January 2020 |journal=[[Nature Communications]] |volume=11 |number=205 |pages=205 |doi=10.1038/s41467-019-13882-z |pmid=31924764 |pmc=6954273 |bibcode=2020NatCo..11..205S }}</ref>

Cloudinids are important in the history of animal evolution for two reasons. They are among the earliest and most abundant of the [[small shelly fossils]] with [[Mineralization (biology)|mineralized]] [[skeleton]]s, and therefore feature in the debate about why such skeletons first appeared in the Late Ediacaran. The most widely supported answer is that their shells are a defense against predators, as some ''[[Cloudina]]'' specimens from China bear the marks of multiple attacks, which suggests they survived at least a few of them. The holes made by predators are approximately proportional to the size of the ''Cloudina'' specimens, and ''[[Sinotubulites]]'' fossils, which are often found in the same beds, have so far shown no such holes. These two points suggest that predators attacked in a selective manner, and the [[evolutionary arms race]] which this indicates is commonly cited as a cause of the [[Cambrian explosion]] of animal [[Biodiversity|diversity]] and complexity.