Editing Sea urchin (section)

=== Fossil history ===
[[File:Cidaridae - radiola (cropped).JPG|thumb|upright=1.7|The thick spines (radiola) of [[Cidaridae]] were used for walking on the soft seabed.]]

The earliest echinoid [[fossil]]s date to the [[Middle Ordovician]] period (''circa'' 465 [[million years ago|Mya]]).<ref name="BottingAndMuir2012">{{cite journal |last1=Botting |first1=Joseph P. |last2=Muir |first2=Lucy A. |date=March 2012 |title=Fauna and ecology of the holothurian bed, Llandrindod, Wales, UK (Darriwilian, Middle Ordovician), and the oldest articulated holothurian |journal=[[Palaeontologia Electronica]] |volume=15 |issue=1 |pages=1–28 |doi=10.26879/272 |s2cid=55716313 |doi-access=free }}</ref><ref name="ThompsonEtAl2022">{{cite journal |last1=Thompson |first1=Jeffrey R. |last2=Cotton |first2=Laura J. |last3=Candela |first3=Yves |last4=Kutscher |first4=Manfred |last5=Reich |first5=Mike |last6=Bottjer |first6=David J. |date=14 April 2022 |title=The Ordovician diversification of sea urchins: systematics of the Bothriocidaroida (Echinodermata: Echinoidea) |url=https://www.tandfonline.com/doi/abs/10.1080/14772019.2022.2042408 |journal=[[Journal of Systematic Palaeontology]] |volume=19 |issue=20 |pages=1395–1448 |doi=10.1080/14772019.2022.2042408 |s2cid=248192052 |access-date=29 October 2022}}</ref><ref name="BGS">{{cite web |title=Echinoids |url=http://www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/echinoid.html |publisher=British Geological Survey |access-date=14 March 2018 |date=2017 |archive-date=15 March 2018 |archive-url=https://web.archive.org/web/20180315133241/http://www.bgs.ac.uk/discoveringGeology/time/Fossilfocus/echinoid.html |url-status=live }}</ref> There is a rich fossil record, their hard tests made of [[calcite]] plates surviving in rocks from every period since then.<ref>{{cite web |title=The Echinoid Directory {{!}} Introduction |url=http://www.nhm.ac.uk/our-science/data/echinoid-directory/intro/introduction.html |publisher=Natural History Museum |access-date=16 March 2018 |archive-date=25 August 2024 |archive-url=https://web.archive.org/web/20240825043212/https://www.nhm.ac.uk/our-science/data/echinoid-directory/intro/introduction.html |url-status=live }}</ref>
Spines are present in some well-preserved specimens, but usually only the test remains. Isolated spines are common as fossils. Some [[Jurassic]] and [[Cretaceous]] [[Cidaroida]] had very heavy, club-shaped spines.<ref>{{cite web |title=The Echinoid Directory {{!}} Spines |url=http://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/regulars/spine.html |publisher=Natural History Museum |access-date=16 March 2018}}</ref>

Most fossil echinoids from the [[Paleozoic]] era are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals, mostly in [[Devonian]] and [[Carboniferous]] rocks. The shallow-water [[limestone]]s from the Ordovician and [[Silurian]] periods of [[Estonia]] are famous for echinoids.<ref name="Kirkaldy 1967">{{cite book|last1=Kirkaldy|first1=J. F. <!--Prof. Geology Queen Mary College London-->|title=Fossils in Colour|date=1967|publisher=Blandford Press|location=London|pages=161–163}}</ref> Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin tests, they would certainly not have survived in the wave-battered coastal waters inhabited by many modern echinoids.<ref name="Kirkaldy 1967"/> Echinoids declined to near extinction at the end of the Paleozoic era, with just six species known from the [[Permian]] period. Only two lineages survived this period's massive extinction and into the [[Triassic]]: the genus [[Miocidaris]], which gave rise to modern [[cidaroida]] (pencil urchins), and the ancestor that gave rise to the [[euechinoidea|euechinoids]]. By the upper Triassic, their numbers increased again. Cidaroids have changed very little since the Late [[Triassic]], and are the only Paleozoic echinoid group to have survived.<ref name="Kirkaldy 1967"/>

The euechinoids diversified into new lineages in the [[Jurassic]] and [[Cretaceous]] periods, and from them emerged the first irregular echinoids (the [[Atelostomata]]) during the early Jurassic.<ref>{{cite book|last=Schultz|first=Heinke A.G.|title=Echinoidea: with pentameral symmetry|url=https://books.google.com/books?id=Ff4-CwAAQBAJ&pg=PT36|year=2015|publisher=Walter de Gruyter|isbn=978-3-11-038601-1|pages=36 ff, section 2.4|access-date=2018-03-16|archive-date=2024-08-25|archive-url=https://web.archive.org/web/20240825043211/https://books.google.com/books?id=Ff4-CwAAQBAJ&pg=PT36#v=onepage&q&f=false|url-status=live}}</ref>

Some echinoids, such as ''[[Micraster]]'' in the chalk of the Cretaceous period, serve as zone or [[Index fossil|index]] fossils. Because they are abundant and evolved rapidly, they enable geologists to date the surrounding rocks.<ref>{{cite book |title=The Science of life |first1=H. G. |last1=Wells |author1-link=H. G. Wells |first2=Julian |last2=Huxley |author2-link=Julian Huxley |first3=G. P. |last3=Wells |year=1931 |pages=346–348 |url=https://archive.org/details/in.ernet.dli.2015.221951/mode/2up}}</ref>

In the [[Paleogene]] and [[Neogene]] periods (''circa'' 66 to 2.6 Mya), [[sand dollar]]s (Clypeasteroida) arose. Their distinctive, flattened tests and tiny spines were adapted to life on or under loose sand in shallow water, and they are abundant as fossils in southern European limestones and sandstones.<ref name="Kirkaldy 1967"/>

<gallery mode="packed">
File:Archaeocidaris brownwoodensis MHNT.jpg|''[[Archaeocidaris]] brownwoodensis'', [[Cidaroida]], [[Carboniferous]], c. 300 mya
File:Miocidaris coaeva MHNT.PAL.2006.94.jpg|''[[Miocidaris]] coaeva'', [[Cidaroida]], Middle [[Triassic]], c. 240 mya
File:Clypeus plotti, echinoid, Middle Jurassic, Gloucester, Gloucestershire, England - Houston Museum of Natural Science - DSC01800.JPG|''[[Clypeus (echinoderm)|Clypeus]] plotti'', [[Irregularia]], Middle [[Jurassic]], c. 162 mya
File:Fossil Echinoid Echinocorys.jpg|''[[Echinocorys]]'', [[Holasteroida]], Upper [[Cretaceous]], c. 80 mya
File:Echinolampas ovalis M Eocene Civrac-en-Médoc France.JPG|''[[Echinolampas]] ovalis'', [[Cassiduloida]], Middle [[Eocene]], c. 40 mya
File:Clypeaster portentosus.jpg|''[[Clypeaster]] portentosus'', [[Clypeasteroida]], [[Miocene]], c. 10 mya
File:Clypeus plotii.JPG|''Clypeus ploti'' gives its name to the [[Clypeus Grit]] of Western England, part of the Oolite.<ref name="BGS - CG">{{cite web |title=Clypeus Grit Member |url=https://webapps.bgs.ac.uk/lexicon/lexicon.cfm?pub=CG |publisher=British Geological Survey |access-date=21 February 2025 |ref=CG |date=2025 |quote=Parent Unit: Salperton Limestone Formation}}</ref>
</gallery>