Homalozoa is an obsolete extinct
subphylum
In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum.
The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used th ...
of
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
era
An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth.
Comp ...
echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
s, prehistoric
marine invertebrate
Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some hav ...
s. They are also referred to as carpoids.
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
In biology, a theca (plural thecae) is a sheath or a covering.
Botany
In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a comm ...
) 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
Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which 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 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 ...
groove.
It has been claimed that some forms possessed
gills
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
and
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 and rays, as well as deep-branching vertebrates such as lampreys. In con ...
s.
Taxonomy
Homalozoans were traditionally considered to be stem-group
echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
s,
[ - pages 401-404] but had also been considered to lie in the stem lineage of the
chordates (
calcichordates
The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornutes and mitrates. It is synonymous with the subphylum Calcichordata. Their unusual appearances have led to a v ...
). 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
Mitrates are an extinct group of stem group echinoderms, which may be closely related to the hemichordates. Along with the cornuta, cornutes, they form one half of the Stylophora.
Morphology
The organisms were a few millimetres long. Like the e ...
s and
cornute
Cornuta is an extinct order of echinoderms. Along with the mitrate
Mitrates are an extinct group of stem group echinoderms, which may be closely related to the hemichordates. Along with the cornuta, cornutes, they form one half of the Stylop ...
s), Homoiostelea (
solute
In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
s), the Homostelea (cinctans), and the Ctenocystoidea (ctenoid-bearing homalozoans).
They have recently been recognised as a
polyphyletic
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
group. The
stylophorans are now classified as a clade of the
Crinozoa
Crinozoa is a subphylum of mostly sessile echinoderms, of which the crinoids, or sea lilies, are the only extant members. Crinozoans have an extremely extensive fossil history, which may or may not extend into the Precambrian (provided the enigm ...
, whereas the other three are classified as clades of the
Blastozoa
Blastozoa is a subphylum of extinct animals belonging to Phylum Echinodermata. This subphylum is characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. This subphylum ranged from the Cambrian to ...
.
[
]
Solutes
Unlike many other types of echinoderm, solute homalozoans lack radial symmetry (such as the five limbs of a starfish).
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
Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on br ...
at its end, and a "stele" at the other, which may have been used by the animal to propel itself along the sea floor.[Henry Gee '' Before the backbone: views on the origin of the vertebrates'', Springer, 1996 page 204]
See also
*
**Blastozoa
Blastozoa is a subphylum of extinct animals belonging to Phylum Echinodermata. This subphylum is characterized by the presence of specialized respiratory structures and brachiole plates used for feeding. This subphylum ranged from the Cambrian to ...
** Cystoidea
*
References
{{Taxonbar, from=Q138052
Paleozoic echinoderms
Animal subphyla