Homalozoa

Homalozoa

Scientific classification
Kingdom: Animalia
Phylum: Echinodermata
Included groups
  • Stylophora (mitrates and cornutes)
  • Homoiostelea (solutes)
  • Homostelea (cinctans)
  • Ctenocystoidea (ctenoid-bearing homalozoans)

Homalozoa is an extinct subphylum of Paleozoic echinoderms that lacked the typical pentamer body form. Instead all homalozoans were markedly asymmetric. Homalozoans are extremely variable in forms, but they all were sessile animals. 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.[1] In some forms the single ray (brachiole or aulacophore) possessed an ambulacral groove.[2] It has been claimed that some forms possessed gills and gill slits.[3]

Taxonomy

Homalozoans were traditionally considered to be stem-group echinoderms,[4] 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.[5]

They include the unusual stylophorans (mitrates and cornutes), Homoiostelea (solutes), the Homostelea (cinctans), and the Ctenocystoidea (ctenoid-bearing homalozoans).[6] 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.[6]

Solutes

Solutes are from the Paleozoic era. They are carpoid echinoderms. Unlike many other types of echinoderm, they lack radial symmetry (such as the five limbs of a starfish).[7] [8] They are the only representatives 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.[9]

References

  1. ↑ Lefebvre, Bertrand (2003). "Functional Morphology of Stylophoran Echinoderms". Palaeontology 46 (3): 511–555. doi:10.1111/1475-4983.00309.
  2. ↑ Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 1011. ISBN 0-03-056747-5.
  3. ↑ Dominguez, Patrício; Jacobson, Antone G.; Jefferies, Richard P. S. (2002). "Paired gill slits in a fossil with a calcite skeleton". Nature 417 (6891): 841–844. doi:10.1038/nature00805. PMID 12075349.
  4. ↑ James W. Valentine (2004). On the origin of phyla. University Chicago Press. 608 pp. Paperback. ISBN 0-226-84548-6. - pages 401-404
  5. ↑ UCMP Berkely, edu. "Echinodermata: Morphology". University of California Museum of Paleontology. Retrieved 21 March 2011.
  6. ↑ 6.0 6.1 David, Bruno; Lefebvre, Bertrand; Mooi, Rich; Parsley, Ronald (2000). "Are homalozoans echinoderms? An answer from the extraxial-axial theory". Paleobiol. 26 (4): 529–555. doi:10.1666/0094-8373(2000)026<0529:AHEAAF>2.0.CO;2.
  7. ↑ 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,
  8. ↑ Smith, A. B. (2005). "The pre-radial history of echinoderms". Geological Journal 40 (3): 255–280. doi:10.1002/gj.1018.
  9. ↑ Henry Gee Before the backbone: views on the origin of the vertebrates, Springer, 1996 ISBN 0-412-48300-9 page 204