Irregularia is an
extant
Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to:
* Extant hereditary titles
* Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English
* Exta ...
infraclass of
sea urchin
Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...
s that first appeared in the Lower
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
.
Description and characteristics
These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often even the mouth are at the two poles of the test, creating a bilateral symmetry instead of the classical 5-fold symmetry of
echinoderm
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s. The group includes the well known
heart urchins, as well as flattened
sand dollars,
sea biscuits and some other forms. Most of them live inside the sediment, moving in thanks to their particular spines, and feed on its organic fraction.
Image:Heart urchin Spatangoida 1380049 Nevit.jpg,
Image:Echinocyamus pusillus.jpg,
Image:Keyhole sand dollar 01.jpg,
Image:Live Sand Dollar trying to bury itself in beach sand.jpg,
Image:Clypeaster reticulatus both sides.jpg,
Image:Encope emarginata (Leske, 1778) derivate 2013.jpg,
Image :Clypeaster rosaceus (Linnaeus, 1758) derivate 2013.jpg,
Image:Clypeaster aegypticus (inside).JPG,
Taxonomy
* super-order
Atelostomata
** fossil family
Acrolusiidae Mintz, 1968 †
** fossil family
Collyritidae d'Orbigny, 1853 †
** fossil family
Disasteridae Gras, 1848 †
** order
Holasteroida
** order
Spatangoida
** fossil family
Tithoniidae Mintz, 1968 †
* fossil genus ''
Atlasaster''
Lambert, 1931 †
* fossil family
Desorellidae Lambert, 1911a †
* order
Echinoneoida
* fossil family
Galeropygidae Lambert, 1911a †
* fossil genus ''
Grasia''
Michelin, 1854 †
* fossil order
Holectypoida †
* fossil family
Menopygidae Lambert, 1911 †
* fossil genus ''
Mesodiadema''
Neumayr, 1889 †
* super-order
Neognathostomata
** familia
Apatopygidae Kier, 1962
** fossil family
Archiaciidae Cotteau & Triger, 1869 †
** order
Cassiduloida
** order
Clypeasteroida
Sand dollars (also known as sea cookies or snapper biscuits in New Zealand and Brazil, or pansy shells in South Africa) are species of flat, burrowing sea urchins belonging to the Order (biology), order Clypeasteroida. Some species within the or ...
** fossil family
Clypeidae Lambert, 1898 †
** fossil family
Clypeolampadidae Kier, 1962 †
** order
Echinolampadoida
** fossil family
Nucleolitidae L. Agassiz & Desor, 1847 †
** fossil family
Pygaulidae Lambert, 1905 †
** fossil genus ''
Pygolampas''
Saucède, Dudicourt & Courville, 2012 †
* fossil order
Oligopygoida Kier, 1967 †
* fossil family
Pygasteridae Lambert, 1900 †
* fossil family
Pygorhytidae Lambert, 1909b †
References
*
*
Echinoidea
{{echinoidea-stub