Homalozoa

{{Short description|Extinct historic group of marine invertebrates}} {{paraphyletic group | image = Paleoart reconstructing carpoids in life position.jpg | image_caption = Reconstructions of various homalozoans in life position. From the left to the right of each row and from top to bottom: ''Coleicarpus'' (a solute), ''Chinianocarpus'' (a mitrate), ''Ceratocystis'' (a basal stylophoran), ''Trochocystites'' (a cinctan), and ''Courtessolea'' (a ctenocystoid) | auto = yes | taxon = Homalozoa | authority = Whitehouse, 1941 | extinct = yes | includes = *Stylophora (mitrates and cornutes) *Soluta *Cincta *Ctenocystoidea (ctenoid-bearing homalozoans) | excludes =

}}

'''Homalozoa''' is an obsolete extinct subphylum of Paleozoic era echinoderms, prehistoric marine invertebrates. They are also referred to as '''carpoids'''.<ref>{{cite journal |author=Imran Rahman |title=Making sense of carpoids |journal=Geology Today |volume=25 |issue=1 |date=January–February 2009 |pages=34–38 |url=https://www.academia.edu/350507 |doi=10.1111/j.1365-2451.2009.00703.x|bibcode=2009GeolT..25...34R |author-link=Imran Rahman }}</ref>

==Description== The Homalozoa lacked the typical pentamer body form of other echinoderms, but all were sessile animals. Instead all Homalozoans were markedly asymmetric, and were extremely variable in forms.

The body (theca) was covered with calcite plates with a number of openings. Their form is in some cases so unusual that it is unclear which openings are to be considered as mouth and anus. Many of them were stalked, similar to sea lilies (crinoids), but often their bodies were bent over, so that the mouth and anus projected forwards rather than upwards. Some forms, especially stylophorans, rested flat on the sea floor.<ref name=Lefebvre2003>{{cite journal|last=Lefebvre|first=Bertrand|title=Functional Morphology of Stylophoran Echinoderms|journal=Palaeontology|year=2003|volume=46|issue=3|pages=511–555|doi=10.1111/1475-4983.00309|doi-access=free|bibcode=2003Palgy..46..511L }}</ref>

In some forms the single ray (brachiole or aulacophore) possessed an ambulacral groove.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|page= 1011|isbn= 978-0-03-056747-6}}</ref>

It has been claimed that some forms possessed gills and gill slits.<ref name=Dominguez2002>{{cite journal|last=Dominguez|first=Patrício|author2=Jacobson, Antone G. |author3=Jefferies, Richard P. S. |title=Paired gill slits in a fossil with a calcite skeleton|journal=Nature|year=2002|volume=417|issue=6891|pages=841–844|doi=10.1038/nature00805|pmid=12075349|bibcode=2002Natur.417..841D}}</ref>

The first homalozoans found in the fossil record are in lower Middle Cambrian rocks of northern Utah.<ref>{{Cite journal|title=Ctenocystoidea: new class of primitive echinoderms|first1=R. A.|last1=Robison|first2=J.|last2=Sprinkle|date=December 19, 1969|journal=Science |volume=166|issue=3912|pages=1512–1514|doi=10.1126/science.166.3912.1512|pmid=17742852}}</ref>

==Taxonomy== Homalozoans were traditionally considered to be stem-group echinoderms,<ref name=Valentine2004>{{cite book|title=On the origin of phyla|year=2004|author = James W. Valentine| publisher = University Chicago Press. 608 pp. Paperback | isbn =978-0-226-84548-7}} - pages 401-404</ref> but had also been considered to lie in the stem lineage of the chordates (calcichordates). However, it is now generally accepted that homalozoans were echinoderms because their calcite skeleton was composed of the typical stereom crystalline structure.<ref>{{cite web|last=UCMP Berkeley|first=edu|title=Echinodermata: Morphology|url=http://www.ucmp.berkeley.edu/echinodermata/echinomm.html|work=University of California Museum of Paleontology|accessdate=21 March 2011}}</ref>

They include the unusual stylophorans (mitrates and cornutes), Homoiostelea (solutes), the Homostelea (cinctans), and the Ctenocystoidea (ctenoid-bearing homalozoans).<ref name=David2000>{{cite journal|last=David|first=Bruno|author2=Lefebvre, Bertrand |author3=Mooi, Rich |author4= Parsley, Ronald |title=Are homalozoans echinoderms? An answer from the extraxial-axial theory|journal=Paleobiology|year=2000|volume=26|issue=4|pages=529–555|doi=10.1666/0094-8373(2000)026<0529:AHEAAF>2.0.CO;2|bibcode=2000Pbio...26..529D }}</ref> They have recently been recognised as a polyphyletic group. The stylophorans are now classified as a clade of the Crinozoa, whereas the other three are classified as clades of the Blastozoa.<ref name=David2000/>

===Solutes=== Unlike many other types of echinoderm, solute homalozoans lack radial symmetry (such as the five limbs of a starfish).<ref>A. B. Smith ''Deuterostome phylogeny and the interpretation of problematic fossil echinoderms'', page 543-544 in Thomas Heinzeller, James H. Nebelsick ''Echinoderms: München'', CRC Press, 2004 {{ISBN|0-415-36481-7}},</ref> <ref>{{Cite journal| doi = 10.1002/gj.1018| title = The pre-radial history of echinoderms| year = 2005| last1 = Smith | first1 = A. B.| journal = Geological Journal| volume = 40| issue = 3| pages = 255–280| bibcode = 2005GeolJ..40..255S}}</ref> Solutes are the sole order of the class Homoiostelea.

Solute fossils have an irregularly shaped flattened body covered in calcite plates, and are up to about 10 cm long. The body has two appendages, interpreted as a "feeding arm" at one end, bearing tube feet at its end, and a "stele" at the other, which may have been used by the animal to propel itself along the sea floor.<ref>Henry Gee '' Before the backbone: views on the origin of the vertebrates'', Springer, 1996 {{ISBN|0-412-48300-9}} page 204</ref>

==See also== *{{C|Prehistoric echinoderms}} **Blastozoa **Cystoidea *{{C|Paleozoic echinoderms}}

==References== {{reflist}}

Category:Homalozoa Category:Paleozoic echinoderms

Category:Animal subphyla

Category:Cambrian animals