Homalozoa is an obsolete extinct
subphylum of
Paleozoic era echinoderms, prehistoric
marine invertebrates. 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) was covered with
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
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.
In some forms the single ray (
brachiole or
aulacophore) possessed an
ambulacral groove.
It has been claimed that some forms possessed
gills and
gill slits.
Taxonomy
Homalozoans were traditionally considered to be stem-group
echinoderms,
[ - pages 401-404] but had also been considered to lie in the stem lineage of the
chordates
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...
(
calcichordates). However, it is now generally accepted that homalozoans were echinoderms because their calcite skeleton was composed of the typical
stereom
Stereom is a calcium carbonate material that makes up the internal skeletons found in all echinoderms, both living and fossilized forms. It is a sponge-like porous structure which, in a sea urchin may be 50% by volume living cells, and the rest b ...
crystalline structure.
They include the unusual
stylophoran
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 va ...
s (
mitrates and
cornute
Cornuta is an extinct order of echinoderms. Along with the mitrates, they form one half of the Stylophora
The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cor ...
s), Homoiostelea (
solutes), the Homostelea (cinctans), and the Ctenocystoidea (ctenoid-bearing homalozoans).
They have recently been recognised as a
polyphyletic group. The
stylophora
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 va ...
ns 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 eni ...
, whereas the other three are classified as clades of the
Blastozoa.
[
]
Solutes
Unlike many other types of echinoderm, solute homalozoans lack radial symmetry (such as the five limbs of a starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. 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 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
**Cystoidea
Cystoidea is a class of extinct crinozoan echinoderms, termed cystoids, that lived attached to the sea floor by stalks. They existed during the Paleozoic Era, in the Middle Ordovician and Silurian Periods, until their extinction in the Devonian ...
*
References
{{Taxonbar, from=Q138052
Paleozoic echinoderms
Animal subphyla