Homalozoa
Encyclopedia
The Homalozoa are extinct, 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 (crinoid
s), 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. In some forms the single ray (brachiole or aulacophore) possessed an ambulacral
groove. It has been claimed that some forms possessed gills and gill slit
s.
s, 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.
They include the unusual stylophorans (mitrate
s and cornutes), Homoiostelea (solutes), the Homostelea (cinctans), and the Ctenocystoidea (ctenoid-bearing homalozoans).
They have recently been recognised as a paraphyletic group. The stylophorans
are now classified as a clade of the Crinozoa, whereas the other three are classified as clades of the Blastozoa
.
era. They are carpoid echinoderm
s. Unlike many other types of echinoderm, they lack radial symmetry (such as the five limbs of a starfish).
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.
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
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
Theca
A theca refers to any case, covering, or sheath.In botany, the theca of an angiosperm consists a pair of microsporangia that are adjacent to each other and share a common area of dehiscence called the stomium. Any part of a microsporophyll that bears microsporangia is called an anther. Most...
) was covered with calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
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 (crinoid
Crinoid
Crinoids are marine animals that make up the class Crinoidea of the echinoderms . Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They live both in shallow water and in depths as great as 6,000 meters. Sea lilies refer to the crinoids which, in their adult form, are...
s), 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. In some forms the single ray (brachiole or aulacophore) possessed an ambulacral
Ambulacral
Ambulacral is a term typically used in the context of anatomical parts of the phylum Echinodermata or class Asteroidea and Edrioasteroidea. Echinoderms can have ambulacral parts that include ossicles, plates, spines, and suckers. For example, sea stars or "star fish" have an ambulacral groove on...
groove. It has been claimed that some forms possessed gills and gill slit
Gill slit
Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of Cartilaginous fish such as sharks, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s.
Taxonomy
Homalozoans were traditionally considered to be stem-group echinodermEchinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, 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.
They include the unusual stylophorans (mitrate
Mitrate
Mitrates are a group of stem group Echinoderms, which may be closely related to the hemichordates.-Morphology:The organisms were a few millimetres long. Like the echinoderms, they are covered in armour plates, each of which comprises a single crystal of calcite...
s and cornutes), Homoiostelea (solutes), the Homostelea (cinctans), and the Ctenocystoidea (ctenoid-bearing homalozoans).
They have recently been recognised as a paraphyletic group. The stylophorans
Stylophora
The stylophorans are an extinct, possibly polyphyletic group allied to the echinoderms, comprising the cornutes and mitrates. It is synonymous with the subphylum Calcichordata....
are now classified as a clade of the Crinozoa, whereas the other three are classified as clades of the Blastozoa
Blastozoa
Blastozoa is a subphylum of extinct animals belonging to Phylum Echinodermata. This subphylum is characterized by the presence of hydrospires.-External links:*...
.
Solutes
Solutes are from the PaleozoicPaleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era. They are carpoid echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. Unlike many other types of echinoderm, they lack radial symmetry (such as the five limbs of a starfish).
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
Tube feet
Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...
at its end, and a "stele" at the other, which may have been used by the animal to propel itself along the sea floor.