Editing Echinoderm (section)

===Fossil history===

Echinoderms have a rich fossil record due to their mineralized endoskeletons.<ref>{{harvnb|Rahman|Zamora|2024|p=296}}</ref><ref name="Waggoner">{{harvnb|Waggoner|1995}}</ref>

====Possible early echinoderms====
The three oldest known candidate echinoderms all lack [[stereom]] and other echinoderm [[apomorphy|apomorphies]], making their inclusion in the phylum controversial.<ref name=origins>{{harvnb|Rahman|Zamora|2024|pp=299–303}}</ref>

[[File:Arkarua adami pennetta.png|thumb|Arkarua adami illustration by Pennetta]]

The oldest potential echinoderm [[fossil]] is ''[[Arkarua]]'' from the late [[Ediacaran]] of Australia ''circa'' 555 [[megaannum|Ma]]. These fossils are disc-like, with radial ridges on the rim and a five-pointed central depression marked with radial lines. However, the fossils have no [[stereom]] or internal structure indicating a water vascular system, so they cannot be conclusively identified.<ref>{{harvnb|Smith|2005}}</ref>  Additionally, all known early pentaradial echinoderms are pseudo-pentaradial in a 2-1-2 pattern, with true pentaradiality as seen in ''Arkarua'' not seen until the emergence of the [[Eleutherozoa]].<ref name=origins />

The next possible echinoderms are the [[Vetulocystida|vetulocystids]], which date to the early to mid [[Cambrian]], 541–501 Ma.  While the youngest vetulocystid, ''Thylacocercus'', displays some characteristics that could be interemediate between older vetulocystids and ''[[Yanjiahella]]'', its discoverers consider vetulocystids more likely to be stem ambulacrarians than stem echinoderms.<ref name=vetulocystids />

[[File:Yanjiahella concept.png|thumb|Artist's conception of ''Y. biscarpa'']]

''[[Yanjiahella]]'', from the [[Fortunian]] (''circa'' 539–529 Ma), is unlike the older fossils in that it has a plated theca, albeit one without evidence of stereom.  To some, this is a reason to place it as a stem ambulacrarian or stem hemichordate.<ref>{{harvnb|Zamora|Wright|Mooi|Lefebvre|2020}}</ref>  Others argue that absence of evidence for stereom is not evidence of absence, and consider a stem echinoderm position more likely.<ref>{{harvnb|Topper|Guo|Clausen|Skovsted|2020}}</ref>

====Echinoderms in the Cambrian and Ordovician====

The first universally accepted echinoderms appear in the [[Lower Cambrian]] period; asterozoans appeared in the [[Ordovician]], while the crinoids were a dominant group in the [[Paleozoic]].

It is hypothesised that the ancestor of all echinoderms was a simple, motile, bilaterally symmetrical animal with a mouth, gut and anus. This ancestral organism adopted an attached mode of life with suspension feeding, and developed radial symmetry. Even so, the larvae of all echinoderms are bilaterally symmetrical, and all develop radial symmetry at metamorphosis. Like their ancestor, the starfish and crinoids still attach themselves to the seabed while changing to their adult form.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=792–793}}</ref>

The first known echinoderms were non-motile,<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=792–793}}</ref><ref name="Dornbos Bottjer 2000">{{harvnb|Dornbos|Bottjer|2000}}</ref> but evolved into animals able to move freely. These soon developed endoskeletal plates with stereom structure, and external ciliary grooves for feeding.<ref>{{harvnb|UCMP Berkeley|loc=Echinodermata: Morphology}}</ref> The Paleozoic echinoderms were globular, attached to the [[Substrate (biology)|substrate]] and were orientated with their oral surfaces facing upwards. These early echinoderms had [[ambulacral groove]]s extending down the side of the body, fringed on either side by brachioles, like the pinnules of a modern crinoid. Eventually, the mobile [[eleutherozoa]]ns reversed their orientation to become mouth-downward. Before this happened, the podia probably had a feeding function, as they do in the crinoids today. The locomotor function of the podia came later, when the re-orientation of the mouth brought the podia into contact with the substrate for the first time.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=792–793}}</ref>

<gallery mode="packed">
File:EarlyEchinoderms NT.jpg|Early echinoderms ''[[Ctenoimbricata]]'', ''[[Ctenocystoidea|Ctenocystis]]'', ''[[Gogia]]'', ''[[Protocinctus]]'' and ''[[Rhenocystis]]''
File:Echinosphaerites.JPG|The Ordovician cystoid ''[[Echinosphaerites]]'' from northeastern Estonia
File:fossile-seelilie.jpg|Fossil [[crinoid]] crowns
File:Hyperoblastus.jpg|Calyx of ''Hyperoblastus'', a [[blastoid]] from the [[Devonian]] of [[Wisconsin]]
</gallery>